Introduction

The recent development of such industries as chemical, electronic, energy, textile or automotive industries requires the search for new, previously unknown phases that can be components of modern, advanced materials with attractive and competitive in terms of application, physicochemical properties.

Research in the field of materials science often concerns mainly on the modification of already used materials in order to improve their functional properties, a reduced consumption of raw materials and energy by industrial production technologies, as well as limited their negative impact on the environment. Less frequently, such research includes the design and production of new materials, especially advanced ones, with the desired chemical, mechanical, thermal and other properties, in particular, due to their time-consuming and complex nature. Application research is always preceded by comprehensive, continuous basic research, from which we know not only the synthesizing methods of new phases, but also their physicochemical and structural characteristics. Such knowledge is the basis for searching for their functional properties and potential applications.

The literature data show that an interesting group of such new phases are compounds and solid solutions formed, inter alia, in multicomponent oxide systems, especially those containing d and f-electron metal oxides [1,2,3,4,5,6,7,8,9,10,11].

The choice of the research object analyzed in this study was made on the basis of very limited literature data on the three-component Fe2O3–Ta2O5–VO2 system [12] and the research experience of employees of the Department of Inorganic and Analytical Chemistry of the Faculty of Chemical Technology and Engineering, ZUT [9,10,11, 13,14,15,16].

The most thermodynamically stable forms of oxides, i.e., the hexagonal form α-Fe2O3, low-temperature β-Ta2O5 crystallizing in the orthorhombic structure and the monoclinic vanadium(IV) oxide polymorph [17,18,19], were used for the syntheses in this work.

The physicochemical properties of these types of oxides and the range of their applications are well known [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33].

It is known that hexagonal iron(III) oxide, due to its electrical, optical and magnetic properties [20,21,22], has many applications, including the production of inorganic pigments, gas sensors and electronic devices such as devices for data storage and magnetocaloric cooling, bioprocessing or in the production of ferrofluids and adsorbents for wastewater treatment [23, 24].

Tantalum(V) oxide with an orthorhombic structure due to its high dielectric and refractive index as well as excellent photoelectric parameters [25, 26] is used in the production of photovoltaic, electronic devices, and also as a material for the anti-reflective coatings. Moreover, Ta2O5 has several advantages over the other materials like low cost, non-toxicity, availability and structural stability [27,28,29].

Vanadium dioxide, the third component of the reaction mixtures, is a typical metal–insulator transition material (MIT) that passes from the monoclinic insulating phase (M) at room temperature to the high-temperature rutile metallic phase (R) [30, 31]. The phase transition of VO2 is accompanied by sudden changes in electrical conductivity and optical transmittance. Due to its specific phase transition characteristics, VO2 is widely studied for applications in electrical and optical devices, smart windows, sensors, actuators, etc. [32, 33].

In the available literature, no studies of the phases formed in two-component oxide systems, i.e., Fe2O3–VO2 and Ta2O5–VO2, were found, which are side limitations of the Fe2O3–Ta2O5–VO2 system selected for this study. It is known that in the third of these lateral systems, namely Fe2O3–Ta2O5, only one compound with the rutile structure is formed, i.e., FeTaO4 [34,35,36]. The methods of synthesis of this compound and its magnetic properties are known [34, 35]. From the few literature data [9] it is also known that in the Fe2O3–Ta2O5–VO2 system only one phase is formed, the limited solid solution type with the formula FexTaxV2−2xO4 and the homogeneity range for 0.01 ≤ x ≤ 0.35. The FexTaxV2−2xO4 phase was obtained by heating FeTaO4 mixtures with VO2 in a vacuum, sealed quartz ampoules at a temperature of 1100 °C for 40–50 h [9].

The main goal of this study was to determine the synthesis conditions of the previously unknown FeVTaO6 compound, formed with the participation of three oxides Fe2O3, Ta2O5 and VO2, and to study its properties and structure. In addition to the classic high-temperature solid-phase synthesis method, an attempt was made to obtain this new compound by a mechanochemical method that guarantees smaller crystallite sizes, which is important due to its potential use, inter alia as a catalyst.

Determining the basic physicochemical properties of FeVTaO6 will help identify other potential areas of its applications, including in electrical and optical devices or in the process of photocatalytic hydrogen production [23,24,25,26, 32, 33, 37].

The latter potential application [37] may be of particular interest, as photocatalytic hydrogen evolution represents a transformative pathway in addressing fossil fuel challenges, heralding a renewable and pristine alternative to conventional fossil fuel-based energy paradigms. A formidable challenge, therefore, is to develop a stable photocatalyst with high efficiency that optimizes solar energy transduction and charge sharing even under adverse conditions.

Experimental procedure

Sample preparation

To synthesize monophase samples contained FeVTaO6, the following reagents were used: Fe2O3—a.p. (POCh, Poland), Ta2O5—a.p. (Alfa Aesar, Germany), VO2—a.p. (Alfa Aesar, Germany) and separately obtained compound—FeTaO4 (PDF card—01-071-0932) by method described, for example, in the paper [35].

The high-temperature solid-state reaction

Reagents weighed in suitable proportions were homogenized and calcined in argon atmosphere in the temperature range 700–920 °C in a tube furnace PRC 50/170/M (Czylok, Poland). After each heating stage, the samples were gradually cooled in the furnace to room temperature, weighed (changes in their mass and their color were recorded) and analyzed.

Mechanochemical synthesis

Mechanochemical synthesis (MChS) using laboratory planetary ball mill Pulverisette-6 (Fritsch GmbH, Germany) with vessel and balls made of zirconium was carried out under the following conditions: rpm = 500, BPR = 1:20, time = 3.0 and 6.0 h under argon atmosphere, the diameter of the milling balls used is 10 mm. The conditions for mechanochemical syntheses were chosen based on the authors’ experience in this field and information available in the scientific literature [38,39,40].

Methods

The obtained samples were tested using the following methods:

  • XRD (the diffractometerEMPYREAN II, PANalytical, Netherlands) analysis using CuKα radiation (λ = 1.5418 Å) with graphite monochromator. The phases were identified on the basis of XRD characteristics contained in the PDF cards [41]. The powder diffraction pattern of FeVTaO6 was indexed by means of FullProf [42] and EXPO2014 programs [43].

  • The crystallite size determination was done using the PANalytical Software HighScore Plus 4.0 based on the Rietveld Method [44, 45] and the Williamson–Hall theory [46]. The procedure and its assumptions have been published in detail in previous works [47, 48].

  • SEM (FE-SEM Hitachi SU-70 microscope, EDS X-ray Microanalysis using NORAN™ System 7 of Thermo Fisher Scientific (UltraDry X-ray detector).

  • EDXRF (Epsilon3 energy-dispersive X-ray fluorescence spectrometer, Malvern Panalytical)

  • IR spectroscopy (SPECORD M-80 Carl Zeiss, Jena, Germany). The measurements were taken within the wave number range of 1100–300 cm−1. The infrared spectra were made by pelleting a sample with KBr in the weight ratio of 1:300.

  • DTA–TGA (SDT 2960, TA Instruments, USA) measurements in the temperature range 20–1500 °C in argon atmosphere, mass of samples ~ 20 mg, heating rate 10 deg/min.

  • DTA–TGA (SDT 650, TA Instruments, USA) measurements in the temperature range 20–1250 °C in air atmosphere, mass of samples ~ 20 mg, heating rate 10 deg/min.

  • UV–Vis-DR spectroscopy (V-670 JASCO, Japan) equipped with a reflecting attachment for the solid-state investigation (integrating sphere attachment with horizontal sample platform PIV-756/(PIN-757). The spectra were recorded in the wavelength region of 200–750 nm at room temperature.

  • The density of FeVTaO6 were determined in argon (5N purity) with the help of an Ultrapyc 1200e ultra pycnometer (Quantachrome Instruments, USA).

  • The transmission electron microscopy (TEM) observations were performed using a JEOL high-resolution (HRTEM) JEM 3010 microscope operating at a 300 kV accelerating voltage, equipped with a Gatan 2 k × 2 k Orius™ 833SC200D CCD camera. The powder samples were suspended in isopropanol and the resulting material after dispersion in ultrasonic bath was deposited on a Cu grid with an amorphous carbon film standardized for TEM observations. Selected area electron diffraction (SEAD) patterns were indexed using dedicated ElDyf software.

Results and discussion

Synthesis and properties of the FeVTaO 6 compound

The synthesis of FeVTaO6 was carried out using two different methods: high-temperature solid-phase reactions and mechanochemical synthesis.

The synthesis of FeVTaO6 by the “high-temperature solid-phase” method was carried out in an argon atmosphere using the following starting reagents: a mixture of oxides composed of 25.00 mol% Fe2O3, 50.00 mol% VO2 and 25.00 mol% Ta2O5 and a mixture of 50.00 mol% VO2 and 50.00 mol% FeTaO4.

After homogenization and pelletization, mixtures of appropriate reagents were heated, under argon, in the following stages: 700 °C (12 h) → 800 °C (12 h) → 900 °C (12 h) → 925 °C (12 h).

The results of the X-ray phase analysis of the samples after the last stage of their heating showed that, regardless of the type of reactants contained in the reaction mixtures, as a result of reactions taking place in the solid phase, a new compound with the formula FeVTaO6 is formed, according to the following reaction equations:

$${\text{Fe}}_{{2}} {\text{O}}_{{{3}({\text{s}})}} + {\text{2VO}}_{{{2}({\text{s}})}} + {\text{Ta}}_{{2}} {\text{O}}_{{{5}({\text{s}})}} = {\text{2FeVTaO}}_{{{6}({\text{s}})}}$$
(1)
$${\text{FeTaO}}_{{{4}({\text{s}})}} + {\text{VO}}_{{{2}({\text{s}})}} = {\text{FeVTaO}}_{{{6}({\text{s}})}}$$
(2)

The mechanochemical method was also used to obtain FeVTaO6 compound after two 3-h steps of grinding the mixture of substrates with the composition of 50.00 mol% VO2 and 50.00 mol% FeTaO4 (reaction 2), under argon atmosphere.

FeVTaO6 has a graphite color. Figure 1 shows, next to fragments of XRD patterns of starting mixtures (Fig. 1a, b), a fragment of a diffractogram of FeVTaO6 compound obtained by high-temperature synthesis (Fig. 1c) and the mechanochemical method (Fig. 1d).

Figure 1
figure 1

Fragments of XRD patterns a a starting mixture with the composition of 25.00 mol% Fe2O3, 50.00 mol% VO2 and 25.00 mol% Ta2O5, b the initial mixture composed of 50.00 mol% VO2 and 50.00 mol% FeTaO4, c FeVTaO6 obtained by the method of high-temperature reactions, d FeVTaO6 obtained by a mechanochemical method

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In Fig. 1, the diffraction patterns of initial mixtures (Fig. 1a and b) and diffractograms of final products obtained by the high-temperature reaction method (WT) (Fig. 1c) and mechanical synthesis (MCS) (Fig. 1d) are presented. The diffraction lines on the diffraction pattern of FeVTaO6 obtained by the mechanochemical method (Fig. 1d) are very broad compared to the analogous ones recorded for this compound obtained by high-temperature solid-state synthesis (Fig. 1c). This result indicates that the size of the crystallites of the FeVTaO6 compound obtained by the mechanochemical method leads to formation of nanocrystalline phases. In order to confirm this fact, an analysis of the diffraction profiles using the Williamson–Hall method was performed. The obtained results showed that values of crystallite size and lattice strain of FeVTaO6 obtained by high-temperature reaction method are: ~ 1000 Å and 0.10%, respectively, and obtained by the mechanochemical method are 151 Å and 1.08%, respectively. It is worth pointing out that in the case of both methods, the same phase is formed, which is clearly visible in Fig. 2.

Figure 2
figure 2

Comparison of diffraction patterns of FeVTaO6 obtained by the high-temperature reaction method (WT—red line) and by the mechanochemical method (MCS—blue line). Miller indices obtained from EXPO analysis (Table 1)

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Indexing of the powder diffraction pattern of FeVTaO6 was performed using the FullProf and EXPO2014 programs [42, 43]. 25 successive diffraction lines registered in the angular range 10–130° 2θ (CuKα) were subjected to indexing. When choosing solution, high values of the “figure of merit” F (25) = 24 (0.021507) were taken into account.

Based on the collected powder diffraction patterns for the FeVTaO6 compound after mechanochemical synthesis the full Rietveld refinement was performed using FullProf program suite. The fitting results can be seen in Fig. 3.

Figure 3
figure 3

Result of the Rietveld refinement of the FeVTaO6 sample obtained by the mechanochemical method. Red circles represent experimental points, black solid line represents calculated profile, blue solid line shows the difference curve whereas the green vertical lines indicate the Bragg positions. Insets shows image of the resulting crystal structure—metal atoms are shown as the grayish ball inside green octahedra with oxygen atoms on the corners shown as red balls

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The Rietveld refinement results for the studied compound are given in Table 1.

Table 1 Results of the Rietveld refinement for the studied compound
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The studied structure crystalizes in tetragonal crystal system in the P42/mnm space group (group number 136). Metal atoms are located on 2a special Wyckoff position, whereas oxygen atom is located on 4f special Wyckoff position what indicate that only the oxygen atom x = y coordinates can be refined (obtained value from the refinement is 0.2978(5)). The theoretical density of the compound FeVTaO6 has been calculated and is 652 g/cm3.

Scanning electron microscope (SEM)

In the next stage of the research, the FeVTaO6 compound was examined by scanning electron microscopy. Figure 4a shows the image of the crystals of the compound obtained by the solid-state method, and in Fig. 4b, the mechanochemical method.

Figure 4
figure 4

SEM image of FeVTaO6 crystals obtained by the method a high-temperature solid-state reactions, b mechanochemical method

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Figure 4a and b shows significant differences in the crystal morphology. The crystals of the FeVTaO6 compound obtained by solid-state synthesis have the shape of irregular polyhedra and the size of ~ 2 µm, while the crystals of the compound obtained mechanochemically have the shape of fine plates with sizes ~ 20 nm.

The X-ray microanalysis of the content of metallic elements in the tested samples was performed at several points, selecting crystals for testing, one of the faces of which was perpendicular to the electron beam. Analysis showed the presence of iron, vanadium and tantalum in the following averaged amounts:

  • For FeVTaO6 obtained in solid-state method: Fe—22.15 mass%, V—14.6 mass% and Ta—63.53 mass%,

  • For FeVTaO6 obtained mechanically: Fe—19.70 mass %, V—17.63 mass% and

  • Ta—62.45 mass%, in relation to the calculated from the formula Fe—19.41 mass%,

  • V—17.70 mass% and Ta—62.89 mass%.

  • Chemical composition of the obtained compound was determined also with XRF method and was as follows:

  • For FeVTaO6 obtained in solid-state method: Fe—19.46 mass%, V—14.99 mass% and Ta—65.55 mass%,

  • For FeVTaO6 obtained mechanically: Fe—20.20 mass%, V—15.85 mass% and Ta—63.95 mass%.

The correctness of the proposed formula is indicated by Rietveld analysis, as well as by the composition of the substrate mixtures used in the reaction, which corresponds to the stoichiometry of the FeVTaO6 compound, and the course of the reaction without a significant change in mass.

As part of the work, the density of FeVTaO6 compound was determined using an ultrapiknometer, which is: ~ 6.18 g/cm3 (mechanochemical synthesis) and ~ 6.51 g/cm3 (synthesis by high-temperature reactions).

Transmission electron microscopy (TEM)

Performed TEM observations confirmed the results obtained by the X-ray diffraction analysis. Vast differences in the samples morphology can by observed (Fig. 5). Sample after high-temperature solid-state reactions is composed mostly of the large crystallites whereas sample obtained by a mechanochemical method is composed of nanosized crystallites. Performed manual image analysis indicated that after measuring 198 particles the average size is 36 nm with standard deviation of 25 nm. Such high standard deviation indicates the broad distribution of the particles size, what can be seen of Fig. 6.

Figure 5
figure 5

Bright field TEM images of the FeVTaO6 after a high-temperature solid-state reactions and b obtained by a mechanochemical method. Right part of the figure shows selected area electron diffraction patterns collected form the observed regions. Red ring indicates theoretical rings corresponding to the FeVTaO6

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Figure 6
figure 6

Particles size distribution of the FeVTaO6 sample obtained by a mechanochemical method

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The particles size distribution follows closely the LogNormal distribution as can be expected. The recorded selected area electron diffraction (SAED) patterns are in good agreement to the FeVTaO6 phase for both studied samples: after high-temperature solid-state reactions and obtained by a mechanochemical method.

Thermal analysis (DTA-TG)

In the next stage of work, the FeVTaO6 compound was tested by differential thermal analysis combined with thermogravimetry up to 1500 °C in an argon atmosphere. In the DTA curve of the FeVTaO6 compound, presented in Fig. 7, one endothermic effect with the onset temperature equal to 1225 °C was recorded. It was also found that the sample after this test was molten, which means that the compound FeVTaO6 melts under an argon atmosphere at a temperature of 1225 °C.

Figure 7
figure 7

DTA curve of the FeVTaO6 compound

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Phase analysis (XRD) of the sample after DTA-TG examination revealed mainly the presence of tetragonal FeTaO4 (PDF no 01-071-0932) as a solid melting product of FeVTaO6 in argon.

In order to determine the thermal stability of FeVTaO6 in air, this compound was subjected to DTA-TG tests in such an atmosphere, but due to technical limitations only up to the temperature of 1250 °C. No significant thermal effect was recorded on the DTA curve, but on the TG curve, the weight loss of the sample started as early as 925 °C.

To determine the melting or decomposition products of FeVTaO6 in air, a sample containing only this compound was heated in a muffle furnace at 925 °C for 3 h and then rapidly cooled to ambient temperature. X-ray phase analysis of the sample melted under these conditions showed the presence of FeTaO4 compound and V2O5 oxide. Vanadium(V) oxide no longer exists as a solid phase at a temperature of 925 °C, so it seems to be one of the phases crystallizing from a liquid. An additional confirmation of this fact is that the diffraction lines characterizing this oxide were shifted toward lower angles, and the mutual relations of their intensity significantly differed from the intensities reported in the literature [49].

At this stage of the research, it cannot be ruled out that in the air atmosphere the compound initially decomposes into FeTaO4 and VO2 oxide, and then vanadium(IV) oxide is oxidized with oxygen from the air to V2O5, which is crystallizes from the liquid after melting.

Regardless of the melting /decomposition mechanism, the DTA-TG studies undoubtedly allowed to state that FeVTaO6 is stable in the air atmosphere up to the temperature of 925 °C, and in the argon atmosphere up to the temperature of 1225 °C.

This temperature difference (~ 300 °C) is due to the reaction with oxygen in the air, one of the decomposition products, i.e., VO2 to V2O5.

Uv–vis-DR spectroscopy

The Uv–Vis-DR spectroscopy method was used to determine the energy gap of the obtained compound, FeVTaO6. For this purpose, the UV–vis-DR spectra of the compound obtained both by solid-state synthesis and mechanochemically were recorded and then transformed using the Kubelka–Munk function according to the following relationship [50]:

$${\text{F}}\left( R \right) = \left( {{1} - R} \right)^{{2}} /{2 }R$$
(3)

where R is reflectance. The value of the energy gap Eg and the absorption coefficient α are related to the following relationship [50, 51]:

$$F\left( R \right) \times hn = a \times hn = A(hn - {\text{Eg}})^{2}$$
(4)

where: A—constant characterizing a given material, h—Planck's constant, ν—light frequency.

The value of the energy gap of the analyzed solutions was determined from the intersection with the axis of the tangential energy led to the linear segment of the relationship (α⋅hν)2 (Fig. 8).

Figure 8
figure 8

The relationship (αhν)2 as a function of the photon energy hν with the determined value of the energy gap Eg for a FeVTaO6 compound obtained in the solid-state reaction, b compound FeVTaO6 obtained mechanochemically

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FeVTaO6 has been found to be a semiconductor. The value of the energy gap determined for the compound obtained by solid-state synthesis is Eg ~ 1.75 eV, and obtained by the mechanochemical method Eg ~ 2.10 eV. The obtained results are consistent with the literature data, which show that with the increase in the size of the crystallites, the value of the energy gap decreases [52].

IR spectroscopy

Figure 8 presents an IR spectrum of a mixture of oxides comprising 25.00 mol% Fe2O3, 25.00 mol% Ta2O5 and 50.00 mol% VO2 (a) next to an IR spectrum of the compound obtained from this mixture, i.e., FeVTaO6 by the mechanochemical method (b) and by solid-state reaction (c).

Figure 9 indicates that the IR spectrum of the compound differs from the spectrum of the oxides mixture both as to the number of the registered bands, their location and as to their intensities. In accordance with the literature data, absorption bands comprising the IR spectrum of the oxides mixture (Fig. 9a) correspond to the stretching or deformation vibrations of the bonds in the coordinating polyhedra in the structure of these oxides [53,54,55,56,57].

Figure 9
figure 9

IR spectrum of a a mixture of oxides comprising 25.00 mol% Fe2O3, 25.00 mol% Ta2O5 and 50.00 mol% VO2 b FeVTaO6 obtained by the mechanochemical method c FeVTaO6 obtained by solid-state reaction

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Absorption bands characteristic for the FeVTaO6 phase were recorded within the wave-number range 1100–3000 cm−1. Unlike the IR spectrum of the oxides mixture, the spectrum of FeVTaO6 possesses two absorption band. As follows from peruse of the literature, these bands are characteristic of solid tantalanes(V), antimoniates(V) and vanadates(V) with a rutile and trirutile type structure [58,59,60]. The first broad absorption band is recorded within the range of wave numbers 850–400 cm−1, with the maximum at 650 cm−1. This band can be ascribed to stretching vibrations of the bonds Fe–O in FeO6 octahedra [61,62,63] as well as to stretching vibrations of the bonds Ta–O in TaO6 octahedra [64, 65]. The position of the absorption band of maximum at 330 cm−1 in the vibration spectrum of FeVTaO6 implies that it can be assigned to the deformation vibrations of the O–M–O bond (O–Fe–O, O–V–O, O–Ta–O) and V–O bonds in the distorted MO6 octahedra or it has a mixed character [66,67,68].

The IR spectrum of FeVTaO6, a phase whose structure has not previously been known, implies that it is built from MO6 (where M = V, Fe, Ta) octahedra and adopt rutile type structure.

Conclusions

As part of the research, a new, previously unknown compound FeVTaO6 were obtained as a result of the reaction of the substrate mixture containing 50.00 mol% VO2 and 50.00 mol% FeTaO4, in an argon atmosphere by two different methods, i.e., the high-temperature solid-state method and the mechanochemical synthesis. The compound was also obtained by the high-temperature method by reacting a mixture of oxides; 25.00 mol% Fe2O3, 50.00 mol% VO2 and 25.00 mol% Ta2O5, under argon. The new compound crystallizes in the tetragonal system and in the P4/mmm space group. The IR spectrum of FeVTaO6 suggests that the compound is composed of MO6 octahedrons (where M = V, Fe, Ta) and has a rutile-type structure.

FeVTaO6 is stable in the air atmosphere up to the temperature of 925 °C, and in the argon atmosphere up to the temperature of 1225 °C. FeVTaO6 is a semiconductor. The energy gap value of the compound obtained by the solid-state method is Eg ~ 1.75 eV, and for the compound obtained by mechanochemical method Eg ~ 2.10 eV.