The role of digital literacy in augmented, virtual, and mixed reality in popular science education: a review study and an educational framework development

This study aims to bridge the gap between extended reality (XR) and digital literacy (DL) in popular science education and further develop a DL–XR framework. XR includes augmented, virtual, and mixed reality (AR, VR, and MR), which has received increased attention and has been used for educational purposes in recent years. However, the studies of XR in popular science education and its impact on students are scant. It is also challenging to find studies entailing XR and DL in education. This study not only offers an overview of the status quo of XR education but also is the first research presenting a referential framework that systematically integrates the many dimensions of XR and DL for future research and educational practices. XR has been extensively used in museums, benefiting users with immersive, authentic, hands-on, and interactive experiences. In the DL–XR framework, based on the variations of “individual-group” and “passive consumption-active creation”, eight dimensions of DL linked to XR are proposed, including “access and understanding”, “evaluation”, “ethics and well-being”, “interaction”, “collaboration”, “creation”, “problem-solving”, and “civic engagement and responsibility”. In the nurturing of DL, evidence revealed that XR is mostly used for learners to access knowledge/information and interact with virtual items; nonetheless, its applications for active creation, problem-solving, and collaboration are seldom prioritised. This study further proposes integrating project-based learning into XR pedagogical practices, which can maximise its impact on learning and empower the learners to achieve advanced levels of DL.


Introduction
The ubiquity and development of technology have urged an increasing number of educators and scholars to advocate using digital devices in educational settings to support teaching and learning. Many countries and governments have thus launched educational technology-relevant policies, programmes, and initiatives intending to enhance learning quality. Technology innovation has the potential to contribute to modern teaching and learning practices (Chauhan 2017). However, the premise is that the users (normally educators and students) know how to utilise technologies in education timely, adequately, and appropriately. That is, there is an emerging call to nurture an individual's digital literacy (DL), which maps out essential knowledge, skills, and attitudes required to thrive in the digital era (Bawden 2008;Ferrari 2012). Nowadays, students are regarded as digital natives who are accustomed to using various information and communications technologies. However, digital natives are not meant to be naturally equipped with DL (Ferrari 2012;Redecker and Punie 2017), as students tend to overestimate their digital literacy, while their actual performance is inadequate (Porat et al. 2018). Similarly, Sánchez-Cruzado et al. (2021) found that educators' selfperception of digital literacy is low and considered that their digital skills should be strengthened. It is imperative to equip educators and learners with digital literacy so that they can efficiently use digital devices to support their teaching and learning. "Learning to learn" and "learning to teach" with advanced technologies are more important than technological development. To accentuate the indispensable role of DL in education, several institutions and organisations have developed DL frameworks to map out the critical dimensions that individuals should possess to function well in the digital society (Feerrar 2019;Redecker and Punie 2017). Individuals' DL plays a paramount role herein, which also makes an emergent call for the discussion and conduction of the current study-to develop a framework for integrating DL and advanced educational technologies.
Among the various new technologies, extended reality (XR), which includes augmented, virtual, and mixed reality (AR, VR, and MR), has received increased attention and has been used for educational purposes in recent years. XR motivates users to immerse, engage, and see something unseeable in real life (e.g. microorganisms and ancient animals). The offering of hands-on experience can facilitate the learners to retain knowledge and grasp abstract concepts, especially mathematics and science (Akçayır and Akçayır 2017;Erbas and Demirer 2019;Ibáñez and Delgado-Kloos 2018). However, the studies of XR in popular science education and its impact on students are scant. Popular science refers to the dissemination of scientific knowledge to the public in an understandable way, educating the public and students about the latest scientific knowledge, which becomes crucial during the pandemic when people urgently need the newest knowledge about COVID-19 (Kitsa 2021). It is very challenging to find studies platforming XR and DL in education. Although, with the assistance of XR, students can have nascent learning experiences, the role of DL is seldom discussed. Educators and learners with a high level of DL can be beneficial from XR technology much more than those who do not possess much DL (Maas and Hughes 2020). That is the reason why the pedagogical potential of XR is still untapped, which also limits students' learning gains (Zhou et al. 2022).
The current study aims to explore the status quo of XRembedded popular science education, organise and determine the dimensions of digital literacy, and then develop a DL-XR framework. The framework has its theoretical and practical values because most prior scholarly works focused on how XR can be used in classrooms while not proposing a framework to offer design guidelines and untapped potential (Maas and Hughes 2020). The framework serves as a foundation for debate, improvement, and refinement, opening a new field in XR-embedded popular science education or even in other disciplines. In the DL-XR framework, based on the variations of "individual"-group" and "passive consumption-active creation", eight dimensions of DL linked to XR are proposed, including access and understanding, evaluation, ethics and well-being, interaction, collaboration, creation, problem-solving, and civic engagement and responsibility. Students usually are consumers who receive knowledge and teaching materials from XR instead of creators (Hsu et al. 2019). Although high-order thinking is prioritised in DL, such as creation, problem-solving, and collaboration (Ferrari 2013), scholars and educators rarely design XR-embedded teaching approaches that particularly cultivate these thinking skills (Zhou et al. 2022). The many dimensions of DL can be regarded as a reference for educators and learners to use XR effectively in classrooms to enhance the learning quality and users' thinking skills. This study not only offers an overview of the status quo of XR education but also can be regarded as the first research presenting a referential framework that systematically integrates the dimensions of DL for future XR research and educational practices. The current study addresses the following three research objectives: (a) To explore the status quo of XR-enhanced popular science education. (b) To identify the definition and sub-dimensions of DL in XR-enhanced learning. (c) To establish the DL-XR framework that systematically integrates the dimensions of DL for future XR research and educational practices.

Extended reality and popular science education
Augmented reality (AR) is a technology that overlays virtual elements onto a real environment. It enables users to view the actual surroundings while computer-generated objects are superimposed onto them, using mobile devices, computers, or projectors. In his work published in 1997, Azuma identified three key characteristics of AR: (a) the blending of real and virtual elements, (b) real-time interaction, and (c) three-dimensional presentation virtual reality (VR), on the other hand, refers to a technology that creates a fully simulated environment in which users can interact with and manipulate virtual objects. VR can be classified into two main types: fully immersive VR and non-immersive VR. Recent studies by Fu et al. (2022) and Maas and Hughes (2020) have discussed these distinctions. Fully immersive VR necessitates the use of head-mounted displays (HMDs), which immerse users in a completely virtual environment. Non-immersive VR, also known as desktop/projector VR, allows users to experience three-dimensional or 360-degree environments through screens or monitors. Building upon the immersive capabilities of VR, the concept of the metaverse has gained significant attention in recent years. The metaverse refers to a virtual universe where users can engage in various activities and interactions, with the potential to profoundly transform human life. Consequently, there has been growing interest in fully immersive VR, which has become a prevailing trend. Researchers have been investigating its practical applications and educational value.
Mixed reality (MR) is a technology that combines elements of both AR and VR. It enables users not only to visualise virtual objects but also to interact with and manipulate them within a hybrid environment, blending the real and virtual worlds (Maas and Hughes 2020) (Fig. 1).
MR is the newest among the three technologies and requires the highest computing power. The application of MR in education is still in its infancy, which merits future exploration to evaluate its pedagogical value (Pellas et al. 2020). Constructivism is the most referred theory that explains why XR can contribute to students' learning (Eisenlauer 2020). XR creates authentically simulated environments and objects, allowing students to interact with these virtual items, find knowledge behind the content, and then construct their knowledge (Loscos et al. 2003). XR (especially MR) also offers the possibility of virtually social constructivist/constructionist learning, enabling students to collaborate in a real (AR/MR) or virtual (MR/VR) space and construct knowledge through social interaction (Bekele et al. 2021;Cheng and Tsai 2013). MR has great potential that allows students to interact with people and content and further collaborate with each other to complete tasks in virtual or hybrid settings (Bekele and Champion 2019). The interaction and manipulation with virtual items in simulated environments are one of the most powerful strengths of XR technology, providing students with novel and authentic learning experiences (Eisenlauer 2020;Pellas et al. 2020).
By using the keywords used by the prior review studies (Maas and Hughes 2020;Zhou et al. 2022), including augmented reality (AR), virtual reality (VR), mixed reality (MR), extended reality (XR), and popular science in the search for literature through online databases, including EBSCO, Science Direct, Google Scholar, and ERIC, in total, 79 journal articles were found. To narrow down the scope of analysis, we invited three scholars specialising in educational technology and discussed the criteria with them. The authors and all three scholars agreed with the selection criteria used in the inclusion/exclusion of articles, including (a). Empirical study: The study must collect and analyse empirical data; (b). Use of technologies: XR, AR, VR, or MR technologies must be clearly identified in the study; (c). Learning effectiveness: The learning effectiveness of the study must be evaluated, and the variables used must be clearly defined. After excluding the articles that were not specifically addressed the required information above, merely 19 articles were selected and analysed ( Table 1).
As shown in Table 1, in popular science, XR technologies are primarily used in museum education, helping users interact with cultural heritage and acquire scientific knowledge. AR is mostly used (15 studies), followed by VR (six studies), yet only one used MR. Evidently, MR is still in its infancy that needs more empirical studies to identify and verify its pedagogical values. Almost all the studies indicated that XR provides an immersive, interactive, and authentic learning environment and hands-on activities/experiments, which can entertain users and effectively contribute to positive attitudes, affection, and meaningful learning (Damala et al. 2016;Neuburger and Egger 2018;Errichiello et al. 2019;Kennedy et al. 2021;Gong et al. 2022). Also, XR allows users to interact with abstract or unseeable objects, helping improve their cognitive ability and spatial understanding and consolidating knowledge retention (Sugiura et al. 2019;Yoon et al. 2012). The affordance of XR seamlessly echoes constructivism learning, providing students with a simulated environment where students construct their knowledge through manipulation and interaction with objects or people. Specifically, in the cases above, the participants' scientific knowledge was improved (Sugiura et al. 2019;Yoon et al. 2018), and art/painting and cultural knowledge were enhanced (Clini et al. 2014;Ghouaiel et al. 2017;Hammady et al. 2020a).  However, it is found that most studies were conducted in museum settings for the public but not for a specific group of learners. For instance, the research about pupils' experience in XR-based museums is scant and worth further exploration. Furthermore, according to the different degrees of "individual"-group" and "passive consumptionactive creation", in most XR learning settings, students were "consumers" instead of "creators", "problem-solvers", or "collaborators", which are essential dimensions in DL (please refer to Figs. 2 and 3 for further details). From the perspective of DL, it is believed that the potential of XR is still untapped and using XR to cultivate other higher-order thinking skills is promising and feasible in future popular science education.

The dimensions of digital literacy in XR learning
Literacy refers to individuals' ability to read and write (Reddy et al. 2022); however, with the development of technology, the definition of literacy has kept evolving. Gilster (1997)'s book "Digital Literacy" popularised the use of the term and invoked people's awareness of the importance of DL, an ability to understand and use information from various digital sources. With the proliferation of digital devices and social media, the definition of DL keeps expanding. Thus, there is a practical and academic need to review the scholarly works on DL through the past years, which helps explore the changing DL and grasp essential dimensions for establishing the DL-XR framework. The strengths and constraints of applying AR, VR, and MR in education should be discussed. It is fair to say that different technologies can provide different learning experiences and contribute to different DL dimensions. By using the keywords of "digital literacy" to search literature through online databases in October 2022, including EBSCO, Science Direct, Google Scholar, and ERIC, and select the articles through the following criteria: (a). The articles should be journal papers, book chapters, or government/professional organisation reports; (b). explaining the definition of digital literacy and its framework; and (c). using a self-developed framework instead of an existing one, a total of 13 papers met the criteria and were included and analysed (Table 2). By reviewing literature regarding XR popular science education (Table 1) and digital literacy frameworks (Table 2), eight DL dimensions in XR learning settings are categorised, including "access and understanding", "evaluation", "ethics and well-being", "interaction", "collaboration", "creation", "problem-solving", and "civic engagement and responsibility":

Access and understanding
Access and understanding refer to the ability to find helpful information, understand the meanings, and construct understanding from virtual content. For other technologies, determining the appropriate website and searching for useful information from digital sources is a critical ability in the era of information explosion (Alt and Raichel 2020). Users also need to know the capabilities of each platform, so they can choose the appropriate one to get relevant information (Park and Burford 2013). XR learning environments are different from computer-assisted learning settings, as students are immersed in a virtual space (MR/VR), or virtual objects appear in the real world (MR/AR). Students have to interpret virtual representations and understand virtual learning materials. At the same time, they must concentrate on learning materials and avoid being distracted from the presented content (Southgate et al. 2019). This dimension is closely related to individuals' photo-visual skills (understanding text-and graphic-based information), branching skills (constructing knowledge from non-linear and non-orderly information), and real-time thinking skills (processing various kinds of stimuli) (Eshet-Alkalai 2012).

Evaluation
Evaluation refers to critically verifying the credibility of information and discerning facts from various digital sources, including XR (Alt and Raichel 2020). Students should not only know how to use, receive, and understand digital content from devices but also need to evaluate and use information critically (Buckingham 2006). The ability of information assessment is essential for individuals to discern object information from personal opinions. Each information provider has its own purpose and ideological biases. Digital literate users can trace the source origins and compare media content with different sources (Polizzi 2020). Students should be taught to critically read every piece of information and understand the purpose of information providers. After critical and reflective consideration, they make their decisions about whether they should believe the information or find other credible information. Also, the rapid development of XR games has raised much concern, as some games include violent and sexual plots, which may negatively influence children's values and behaviours (Southgate et al. 2017). Students must cultivate their critical thinking to reflect on how XR games form their values and thoughts, identify the commercial purpose, implicit meanings, and values behind the games, and further construct their values and knowledge.

Ethics and well-being
Ethics and well-being refer to ethically and legally using digital devices and taking care of self-physical and psychological health (Feerrar 2019;Hsu et al. 2019;JISC 2022). Due to the infancy of XR technology, the ethics of this technology has not gained much attention. However, some potential issues should be thoroughly thoughts about before happening. Peer-to-peer misuse among students may be a potential issue (Heimo et al. 2014). AR allows users to modify objects' appearances, including looking and clothing, and even zombify others. If someone modifies others ridiculously and uploads pictures or videos on the Internet without the subjects' permission, this may cause cyberbullying (Heimo et al. 2014). Also, students should be informed that when they wear XR headsets, everything they see may be recorded, and their biometrics data (e.g. eye-tracking data and emotional and physical reactions) can be collected by systems (Bye et al. 2019). Some XR games include pornographic and violent content, which may implicitly encourage children's anti-social or violent behaviours and convey inappropriate values (Southgate et al. 2017). Furthermore, children are more easily drawn into immersive virtual worlds than adults, hardly discerning reality and virtuality and controlling their emotions and behaviours (Baumgartner et al. 2008). The children's physical and psychological development issues merit experts to work on them. Other issues may emerge when XR is more commonly embedded in daily life, such as equality, data ownership, and surveillance issues (Heimo et al. 2014). Before using XR, students should be informed of the ethics and potential risk.

Interaction
Interaction refers to the ability to interact with people or virtual content when using XR. This dimension is seldom emphasised by other digital literacy frameworks, as other technologies' functions are unlike XR technology, of which tangible interaction is a nascent and prominent characteristic (Azuma 1997;Eisenlauer 2020). Before students can collaborate with other people or virtual characters, they must know how to interact with XR systems and people first. The first facet is the interaction between students (human-human interaction). In most cases, students learn in a group with one XR device and receive knowledge from the content; however, they do not collaborate to complete a task or a mission (Bekele et al. 2021). They are just "consumers" and "co-users" instead of "collaborators" of technology. The other facet is the interaction with virtual content (human-system interaction). The essential characteristic of XR is that it allows users to manipulate simulated objects and provide a hands-on experience (Eisenlauer 2020). This kind of learning method is totally different from other technologies and sometimes is not easy for students (Pellas et al. 2020), requiring competency far beyond reading, writing, and understanding online content. Students should know how to interact with virtual items and construct knowledge from the interaction.

Collaboration
Collaboration refers to collaborating with others in virtual/ physical environments. Technology-supported collaboration is a specific form of interaction (not all interactions are collaboration) in which students have a particular task and work together to accomplish it (Borokhovski et al. 2016). Due to the different affordances and constraints of XR and different learning tasks, collaboration can be divided into several types. Regarding collaborative environments, AR can only allow students to collaborate in a physical environment, while MR can create a virtual-physical-combined environment for students to work together. However, it is difficult for users to collaborate in VR settings, as VR immerses users in a totally virtual environment and separates them from other users (Bekele et al. 2021). Compared to AR and VR, collaboratively manipulating virtual objects in an MR setting (a physical-virtual hybrid environment) is more feasible (Bekele and Champion 2019). Two or more people can cocreate and co-construct a virtual artefact or collaborate to solve virtual problems presented by MR (Wang and Dunston 2013). In these new types of collaboration, students should know how to effectively express themselves, understand others' meanings, and then construct knowledge in hybrid or virtual environments. Regarding collaborative partners, most frameworks focus on peer-to-peer collaboration, but peer-to-system collaboration is seldom mentioned. With the rapid development of technology and the emergence of the metaverse, it is possible to collaborate with systems or non-player characters to co-create or co-solve problems in the future.

Creation
Creation refers to the "production" and "creation" of digital artefacts or content by using XR, including activities to create new and original content and improve others' ideas (Redecker and Punie 2017). This dimension has been mentioned in many frameworks, but the difference between "production" and "creation" is usually unclear. "Producing" refers to generating digital content and editing content in different formats, while "creating" refers to developing original and diverse content and innovatively improving others' ideas (Organisation for Economic Co-operation and Development 2022). Some frameworks focus on the former one (Lazonder et al. 2020;Park and Burford 2013), whereas some focus on the latter one ( (Liu et al. 2007), students input vocabulary into the MR system. Then, the system searched a database according to the vocabulary and created a poem based on the given words. Students could explore and input a much more appropriate vocabulary in every attempt, or they could further refine the poem produced by the system. In this case, students co-created with the MR system in the real world.

Problem-solving
Problem-solving refers to the ability to solve problems in authentic or simulated environments by involving XR technology. Most frameworks focus on using technology to solve a real-world problem, or users can solve technical problems. For example, in Hsu et al. (2019)'s study, students were required to make simple AR content, such as campus tours and animal life circles. Students found their AR video quality was not good, so they used the Google search engine to scrutinise the problem and creatively solve it. XR can also offer other types of problem-solving, allowing students to enter a hybrid or virtual environment and solve a simulated problem presented by XR systems, although the cases are few. One reason may be that its pedagogical use is still in its initial stage; on the other hand, most teachers do not have programming savvy, so they cannot develop XR content that fits their educational goals (Pellas et al. 2019). With the rapid development of XR and the metaverse, it is anticipated that in the future, students can solve a simulated problem with avatars or non-player characters in a totally virtual setting.

Civic engagement and responsibility
Civic engagement and responsibility refer to knowing own responsibility in a shared and virtual society and bringing a positive contribution to establishing a harmonious virtual environment. DL is no longer an individual ability; instead, it entails being aware of the social well-being and environmental impact of technology (Redecker and Punie 2017). Social justice and equity have also become important values for DL (Feerrar 2019). The emergence of the metaverse, a cutting-edge technology based on the XR technique, has drawn people's attention to the intergrowth and co-prosperity of virtual environments. The metaverse is a perpetual and persistent multiuser environment where users can communicate in real time (Mystakidis 2022), opening a new form of pedagogical style and may tremendously impact daily life. Although the development of the metaverse is yet full-fledged, it is a prospective field in education. Like other technologies, some essential issues should be considered beforehand, such as data management and intellectual property protection (Hirsch 2022). For instance, in Wiederhold (2022)'s paper, when an unfamiliar avatar suddenly appears and touches you without your consent, what we should do and where we can seek help in this kind of harassment is worth further discussion. Establishing global guidelines and regulations for the metaverse is important for ensuring the well-being of avatars in virtual society (Hirsch 2022). Students should know they are citizens in the virtual world and must cultivate their civic consciousness to establish a harmonious virtual society. In most scholarly works, XR is used to demonstrate information or knowledge for users instead of targeting nurturing the users' higher-order thinking skills. Zhou et al. (2022) proposed that future scholarly works can explore how to use XR to develop users' critical thinking, collaboration, and other higher-order thinking skills. Bekele et al. (2021) indicated that users usually hope to have social interaction, not only interaction with systems when using XR. Furthermore, collaboration, creation, and problem-solving have been extensively emphasised in education in recent decades. Future XR studies may consider designing and bettering a learning environment that allows users to co-create and cosolve problems. Based on the different levels of "collaborative environments" and "partners", the user's collaborative styles can be further categorised into four quadrants (Fig. 2). In terms of collaboration environments, collaborating in real-world environments means that students can see the real world in the collaboration process. This type can be found in AR and MR learning settings (Bekele and Champion 2019). Another side of the continuum is collaboration in virtual environments, where users can see a virtual world. This type of working environment can be created by MR and VR (Bekele et al. 2018). MR entails the characteristics of AR and VR, creating real, virtual, or hybrid world working places and a more immersive and interactive environment than AR and VR (Hammady et al. 2020a). Regarding partners, users can collaborate with real people (whether they turn into avatars) or XR systems.
Human-to-human collaboration in a real-world environment can be embodied by AR and MR, which can create a real-virtual-hybrid environment where users can see each other in the real world and manipulate virtual objects simultaneously (Bekele et al. 2021). However, VR immerses users into a simulated world which separates the users from the real world. It is difficult for them to work with other people face-to-face. In most cases, users can only interact (not collaborate) with virtual objects independently (Bekele and Champion 2019). Thus, although MR and VR can provide a fully immersive environment, MR has great potential for allowing human-to-human collaboration in a virtual or hybrid setting, whereas VR can only allow remote collaboration using avatars. However, this kind of collaboration is seldom mentioned in VR-embedded popular science education (Bekele and Champion 2019). It is predicted that when the metaverse becomes fully fledged, more and more people can work with each other in a fully simulated environment.
Cases of human-to-system collaboration in real-or virtual-world environments are scant, at least in popular science education (Zhou et al. 2022). It is highly applicable for AR and MR to fulfil human-to-system collaboration in the real world, in which users can still see the real world when co-creating or co-solving problems with systems. As mentioned, the BlogWall MR game may allow users to cocreate a poem with the MR system in the real world (Liu et al. 2007). Human-to-system collaboration in the virtual world may be fulfilled by MR and VR, where users can enter a totally simulated world and work with systems, even with virtual characters (non-player characters). The emergence of the metaverse makes this collaboration style likely to happen in the future. When the time comes, people can not only work with people (or avatars) in a virtual world, but they also can collaborate with virtual characters to perform complicated tasks. It should be noted that the four quadrants only show the current affordance of XR and possible limitations of contemporary XR education. With the development of XR technology, its usage may be expanded, and categories may be different. DL, from the individual ability to the group dynamic. The first level is "access and understanding", which is an essential thinking skill and an individual ability for all XR users in all the existing digital literacy frameworks. Once XR users know how to interpret digital content and understand its meanings individually, they start "human-to-system" or "human-to-human" interaction, a prominent dimension specialised for XR technologies. The significant difference between XR and other technologies is that XR allows users to manipulate virtual content, requiring their whole-body gestures. Students should know how to interact with peers and systems; then, they can work with a team to complete complicated problem-solving or content-creation missions in which their teammates may be real-world or virtual users, even non-playable characters. Collaboration, creation, and problem-solving have been emphasised in education. However, they are still scantly explored in the contemporary XR studies.
Differently but interconnectedly, the Y-axis is the development of cognitive skills, from basic to higher-order thinking skills. Once users can decode and understand XR content, their "critical evaluation" skills can be nurtured, which requires users to reflect on what they receive, identify objective messages, and establish their values. This critical and deep-thinking ability is the basis for creativity and problemsolving. Students internalise what they learn and then use the knowledge to create something new or solve problems creatively. The "ethics and norms" and "civic engagement and responsibility" are two affectional dimensions of DL. When students interact with others, they should behave ethically to protect each other from potential risks. Behaving ethically among peers is the foundation for entering the virtual society, the metaverse, a shared virtual society where users can have real-time interactions. As a citizen in the virtual world, creating a harmonious society is everyone's responsibility.
The framework can help researchers categorise contemporary XR-embedded studies, locate the current situation of their studies, and offer guidance for future research and educational practices. For example, Agostini (2019) and his team conducted XR-embedded cultural heritage education in the Italian city of Verona, a UNESCO World Heritage Site. They applied XR technologies to outdoor heritage education to help primary school fifth graders learn more about Verona and its historic buildings. In terms of AR, students used smartphones and tablets to see 3-D images superimposed on real scenery. They could see the Roman monument, which existed in the Roman time, through the tablets. Besides, students wore VR headsets to see the arena. With VR, students enter an entirely virtual environment to see the ancient arena and its surroundings. Based on the existing AR, MR was also developed to create a time-travel experience in which students could see the existing monument and the surrounding landscape two thousand years ago. After the Verona 1-day tour, Agostini found that students using XR retained more information than the control group, who only had booklets during the trip. Also, the experimental group students could draw 3-D buildings resembling the original ones in their painting homework. Agostini (2019)'s case indicated that XR can be effectively used in cultural heritage education. However, this kind of technology use is primarily categorised in the "access and understanding" dimension ( Fig. 3), yet other higher-order thinking skills (e.g. creation and problem-solving) are not particularly emphasised, echoing the current use of most XR devices-knowledge demonstration (Zhou et al. 2022).
Another example is in Hammady et al. (2020b)'s study, describing how an MR touring guide allowed visitors to interact with others. Hammady et al. developed the MR tour guide, the MuseumEye, in an Egyptian Museum in Cairo. With the MR headset, visitors could see the co-existence of genuine antiques and virtual information (historical scenery, animated characters, virtual guide avatar) and listen to the audio narration simultaneously. Visitors could interact with spatial visuals (tapping or rotating visual items). Also, the MuseumEye system allowed visitors to see the same visual information, facilitating communication between visitors in the physical-virtual hybrid environment. According to the visitors' responses, their interest and engagement increased, and they were satisfied with the communication function of the device. A similar human-to-human interaction MR function can also be found in Bekele et al. (2021)'s study. In the past, the use of XR is mostly located in "access and understanding" and "human-to-system interaction" dimensions. Nowadays, researchers have started exploring the possibility of "human-to-human interaction". Also, social interactions satisfy users' desire to communicate with others when they use XR devices. Future studies can go beyond "interaction" and explore the possibility of "collaboration". Considering the different affordances and characteristics of XR, MR has the greatest potential to empower or facilitate peer-to-peer collaboration for co-creation or problem-solving in a physical, virtual, or hybrid setting (Bekele et al. 2021). As shown in the DL-XR framework (Fig. 3), the future direction is to develop an XR system in which users can not only interact with others but also collaborate to create and solve problems, training users' other dimensions of digital literacy.

Discussion
Higher-order thinking development and interrelationships building are paramount goals in education. It is essential to seek new opportunities for integrating XR into education, vitalising and optimising the use of this cutting-edge technology to support students' learning. Based on the literature review of XR popular science education and DL, the study developed the DL-XR framework, which articulates the status quo of XR popular science education, and offers a direction for maximising and vitalising the use of XR in the future. It is fair to say that the mainstream of XR education lies on "access and understanding" and "interaction (humansystem interaction)" dimensions, whereas other higherorder thinking and social dimensions include "creation", "problem-solving", "collaboration", and "ethics and wellbeing", are hardly discussed (Bekele et al. 2021;Zhou et al. 2022). To cope with this dilemma, the researchers propose the use of project-based learning (PBL) to shift the users' work from an individual to a group basis and transform the focus of learning from passive consumption/reception to active creation. PBL refers to a learning approach in which students collaboratively explore a real-world and complicated problem, collect and synthesise information, and then produce a product (e.g. artefacts or reports) to address the issue (Kokotsaki et al. 2016;Morales et al. 2013). It is PBL that allows learners to apply popular science knowledge in real life, featuring technology-enhanced self-directed/regulated learning, experiential learning, and immersive learning (Fidan and Tuncel 2019;Halabi 2020), which are all the strengths of XR, allowing learners to actively interact with virtual content, immerse themselves in a simulated world, and construct knowledge by themselves. This concept reflects constructivist and constructionist learning. The former refers to the active construction of ideas and concepts individually, while the latter refers to collaborative construction and creation through group conversation and sharing (Rob and Rob 2018), benefitting the DL dimensions beyond "access and understanding", which is mainly focused in current use of XR. Echoing this, PBL is one of the feasible ways to cultivate students' DL, including higher-order thinking and communication skills (Hsu et al. 2019;Kimbell-Lopez et al. 2016;Loizzo et al. 2018), changing learners' roles from "passive consumers" to "active creators". As shown in Morales et al.'s (2013) and Southgate's (2022) study, by creating VR content, students started peer mentoring and collaboration and formed a knowledge-building community of practice. They had a deeper understanding of knowledge, especially abstract concepts, and expressed their knowledge creatively through VR, showing their digital creativity. The pedagogical concepts of these examples of VR-enhanced PBL happen can somehow be used in popular science education and is worthwhile for further exploration.
The integration of PBL with XR education has pedagogical significance and practical appropriateness. A range of educational programmes have been launched, utilising PBLintegrated XR to develop students' self-regulated learning, which aims to enable them to become life-long learners (Ministry of Education 2017, 2020). As an example, our research team recently collaborated with a primary school to develop a series of PBL courses that incorporate XR technology. In this particular project, primary school students were given the opportunity to collaborate with students from a school in the UK, with the objective of enhancing their respective local markets. Together, students from both schools designed immersive 360-degree VR experiences showcasing their local markets. Through the utilisation of VR technology, students were able to immerse themselves in different cultural contexts and gain insights into the strengths and weaknesses of each location. This facilitated an exchange of ideas between children from two distinct regions, focusing on various aspects of market improvement such as layout and decorations. During this PBL, children need to know how to make VR, collaborate with people from different cultural and language backgrounds, and then innovate their market environments. Besides VR, various technologies are involved, such as tablets, online meeting software, and video editors. The activity provides ample opportunities for students to apply knowledge in real life and train their higher-order DL, including collaboration, creative creation, and even civic engagement. Alt and Raichel (2021) indicated that constructivist activities are helpful in developing students' DL, in which students can use technology to create digital content (e.g. digital storytelling, digital games, or VR content in this example) involving authentic issues or ill-structured problems (p. 27). They do not just passively receive knowledge but actively create and innovate. From this example, considering the current XR affordances and technical limitations, we can see that XR-enhanced PBL should be complemented by other technologies. Different technologies have their strengths and constraints, which should be identified and complemented by each other, helping learners complete a project.
However, it is challenging to find empirical studies that aimed to combine XR and PBL to enhance students' learning. How to use XR and other technology in popular science PBL merits further development and research, which allows students to construct and apply knowledge in various contexts, including cultural heritage, museums, and daily life. Thus, it has theoretical and practical significance to explore the feasible approaches to integrate XR into PBL systematically and synthetically to support students' inquiry and collaboration process. An XR-embedded PBL approach seems to be an important breakthrough to extend and augment the use of XR in educational settings. Future researchers can also improve and refine the DL-XR framework by integrating the core elements of PBL.
Striking a balance between the pros and cons of XR can shed light on its indispensable and irreplaceable power. The frequently reported benefits of XR from popular science education are its immersion, authenticity, concept visualisation, and hands-on activities (Besoain et al. 2022;Huang et al. 2019;Yoon et al. 2018). The critical and central questions are "can the strengths of XR make a more powerful and positive impact on students' learning than other technologies? And "how to design a course that can maximise XR strengths to support learning activities?" If the current use of XR merely focused on its "knowledge retention and reception", it may easily be substituted by other low-cost and common technologies, such as mobile phones or videos, even 3D physical objects/models, which can also allow people to see something unseeable and make abstract concepts concrete (Hung et al. 2016;Wang 2020).
In terms of the impact of XR on learning outcomes, some studies indicated that learning performance was not significantly different between XR-embedded and conventional learning methods (Kaplan et al. 2021;Meireles et al. 2022). In museums, if XR is mostly used to superimpose information on physical items, this function is very similar to paper-based labels used for exhibits (Zhou et al. 2022). XR-enhanced learning is not merely using XR to receive scientific, abstract, and unseeable information, but to use its strengths to support students' entertaining learning experiences, inquiry processes, creativity, and other digital literacy dimensions (Ibáñez and Delgado-Kloos 2018).
It is critical for researchers and educators to identify and justify the specific benefits of XR that cannot be replaced by other technologies and thus embed them into course design (Kaplan et al. 2021). It is unreasonable to embed XR in education only because of its novelty and entertaining features while not carefully connecting to learning goals (Fu et al. 2022). Innovative XR-embedded teaching approaches, design guidelines, and frameworks should be developed to augment the use of XR and maximise its strengths (Akçayır and Akçayır, 2017). For example, XR-game-based learning has been studied recently, which can help strengthen its benefits and better its pedagogical and educational practices (Fu et al. 2022;Pellas et al. 2019). Also, identifying different technologies' strengths and constraints can help educators combine these technologies, complement each technology's limitations, and augment learners' learning. Future studies can explore how to integrate XR's special features like playfulness, immersion, and transferability with PBL or other learning theories and approaches.

Challenges and future research suggestion
The current study reviewed the use of XR in popular science education from the view of DL, which provides an insightful revelation of future improvement and directions. Currently, the challenge of XR is that its use is primarily located on the "access and understanding" and "human-system interaction" DL dimensions, while other dimensions are still yet exploited. Learners mostly passively receive knowledge provided by XR, but they seldom have opportunities to co-create and co-solve problems. Although MR has the highest potential for developing higher-order DL dimensions, as it allows human-to-human/system collaboration in a real and virtual world (Bekele and Champion 2019), we can only find one popular science article using MR, showing that the application of MR in education is still in its infant stage, meriting future exploration to develop its usefulness and evaluate its pedagogical value (Pellas et al. 2020). MR is the newest among the three technologies and requires the highest computing power and high-level technological devices (Bekele and Champion 2019). The challenge may be finding an economical way of extensively using MR in popular science education, even formal education. This requires constant improvement of the XR techniques and devices. We also found that in most studies, learners can only use previously made XR content for learning. They do not have the freedom to adjust the content or interface of XR. The difficulty in adjusting the intended content may limit the use and extension of XR, as educators cannot make their own content for their own needs. Also, users cannot adjust the content if the interface is not user-friendly. As shown in Bekele et al.'s (2021) study, they found that although the MR devices have rich functions and allow peer-to-peer interaction in a hybrid environment, some museum visitors might find interacting with the system difficult. The collaboration between educators, researchers, and technology industries is essential, so the XR content can be practical and meet the needs of educators and users. Researchers can evaluate the effectiveness of XR content and provide further improvement suggestions. Besides, creating no coding and easily made XR is worthwhile, allowing educators in museums and schools to tailor XR content for their specific needs.
To augment the use of technology and cultivate DL, several researchers proposed using PBL, which creates a productive environment for improving knowledge and giving students opportunities to practice and display their DL (Alt and Raichel 2020;Loizzo et al. 2018). Although PBL is seldom used in popular science education, it is a promising approach in this field involving using XR. The European Commission (2019) proposed that technologies used in PBL can support students' DL, as PBL give students opportunities to actively participate in innovation and collaboration activities across disciplines through various technologies. Constructivist activities (e.g. digital content development) and real-life relevance PBL have been regarded as the most feasible way to cultivate students' DL because students have opportunities to put DL into practice and perceive learning as meaningful and life-relevant. Learners do not just passively receive information presented by digital content (consumers); instead, they actively create, collaborate, and solve problems (creators). We proposed that only using XR in PBL is not practical due to current XR's technical and content limitations (e.g. students may need computers or tablets to search for information or use online meeting software for online discussion); instead, educators and researchers need to identify the strengths and constraints of XR and other technologies and complement each other, so its learning effectiveness can be augmented.
In terms of the research methodology employed in this study, the authors initially utilised keywords to identify relevant articles. Subsequently, we engaged in discussions with three scholars who possess expertise in the fields of educational technology, XR, AR, VR, and MR to establish the criteria for article selection. Following this, the authors meticulously examined and analysed all the selected papers, and further discussions were conducted with the aforementioned experts to enhance the robustness of the findings. Future studies can consider incorporating advanced techniques to analyse articles. Additionally, considering articles written in languages other than English and from diverse countries and cultures would be beneficial in contextualising and expanding upon the findings.

Conclusion
The proliferation of technology contributes to learning and teaching tremendously, urging educators and researchers to prioritise learners' DL, which is critical for students to use technology effectively and develop their high-order thinking. Among all technologies, XR has gained momentum and has been extensively used in education to provide students with a fresh and immersive learning experience. To better the use of XR in education and explore its future applications, the researchers proposed the DL-XR framework, including eight essential digital literacy dimensions for XR and the metaverse: access and understanding, evaluation, ethics and well-being, interaction, collaboration, creation, problem-solving, and civic engagement and responsibility. To our knowledge, this is the first framework entailing DL and XR and provides a foundation for further discussion and refinement, which entitles the study to its significance and appropriateness. The framework can serve as guidance for future researchers and educators, which articulates the optimistic future of XR and clearly highlights the dimensions of DL that are still untapped. Future researchers, educators, and practitioners can refer to the DL-XR framework and consider whether the embedment of XR can facilitate users' DL development, especially those dimensions labelled as high-order thinking and group dynamic. It should be noted that although most findings and the DL-XR framework development were mainly based on popular science education literature review, it is believed that the facets of DL and the specific features of AR, VR, and MR can be transferable to other disciplines, which also helps illuminate the strengths, constraints, and future directions of XR learning.
The framework introduced in this study can be perceived as a comprehensive blueprint for the prospective integration of XR in educational contexts to enhance digital literacy.