1 Introduction

Alkaline-earth aluminium borates are known to be potential phosphors in designing efficient luminescent materials [1, 2]. Emergence of this material is associated with its low synthesis temperature, high UV transparency, high luminescence brightness, good thermal stability and also nonlinear optical properties that make the material useful for application in plasma display panels (PDPs), UV devices and nonlinear optical materials [3,4,5]. This matrix is well known as an ideal host to several rare earths and contributes towards developing WLED (white light emitting diodes) materials [6,7,8,9]. Among the alkaline-earth aluminium borates, BaAl2B2O7 has evolved as a potential phosphor material. The most fascinating about Alkaline-earth aluminium borate (BaAl2B2O7) being a host is its crystal structure that was first described by Hubner and latter it was discussed by Ye et al. [10, 11]. The BaAl2B2O7 lattice follows rhombohedral structure, here the prominent units are AlO4 tetrahedra and BO3 triangles and in between these two units the Ba ion gets sandwiched. The Ba ion gets stabilized in a large cavity surrounded by six oxygen atoms at its vertices forming an octahedron like structure and similarly another six more distant oxygen atoms form an octahedral arrangement. The well-occupied Ba ion sites seem to be ideal host sites for many guest ions. Pekgo¨zlü et al. [12] have reported blue emission from Pb2+ doped BaAl2B2O7. Camardello et al. [13] have studied optical spectroscopy, thermal quenching and electron- vibrational interaction of BaAl2B2O7: Eu2+. Recently, an abnormal reduction in Eu3+ to Eu2+ in oxidizing environment under UV excitation is also observed in BaAl2B2O7 matrix [5]. However, incorporation of actinides in alkaline-earth aluminium borate lacks literature support. At present, solid host matrices doped with uranium are being investigated with an approach of storage matrices for radioactive waste [14,15,16,17,18,19]. Although many works have been carried out on uranium doped borates but uranium doped alkaline-earth aluminium borate is still an untouched one [20,21,22]. In the present investigation, luminescence of uranium in BaAl2B2O7 host matrix has been carried out. As it is known that solubility and site occupancy of uranium in solid matrices, being a heavy metal ion is always challenging so it is interesting to study BaAl2B2O7: U system. Uranium shows various oxidation states i.e. from trivalent to hexavalent and interestingly each of these state shows their characteristic optical spectroscopic properties [23,24,25,26,27,28,29]. The uranium in hexavalent state appears in different forms such as UO42−, UO66− or UO22+. So, the form of stabilization or speciation is also an important factor for investigation and towards this end luminescence technique is very useful. The present study is intended to probe BaAl2B2O7: U system and extract information regarding speciation of uranium, its photophysical properties as well as its site occupancy in BaAl2B2O7 host matrix.

2 Experimental

The uranium doped and undoped samples were prepared via solution combustion technique. Analytical reagent (AR) grade chemicals were used. In order to synthesize, stoichiometric weight proportions of Ba (NO3)2, Al (NO3)3 9H2O, H3BO3 (5 mol % excess), and UO2(NO3)2 6H2O were taken. These salts were ground together in an Agate Mortar with little mixing of distilled water and an aqueous homogeneous solution was obtained. Excess water was made to evaporate by heating at low temperature, and precursor solution was obtained. Then the precursor solution was heated at 600 °C inside a muffle furnace. The solution attended boiling and foams were appeared and suddenly it got ignited and flame was observed. After the flame disappeared a voluminous, foamy precursor powder was obtained. The entire combustion process was for 5 min only. The well-mixed precursor powders were made into pellets. The pellets were annealed in a muffle furnace at a temperature of 900 °C for 3 h and quenched to room temperature.

The physical characterization was carried out by powder X-ray diffraction technique. The used instrument is a Philips diffractometer (model PW 1071) operated with monochromatic CuKα (λ = 1.5418 Å) radiation. The Shimadzu RF 5301pc spectrofluorometer with 150W CW (continuous wave) xenon lamp as the excitation source was used for Photo luminescence (PL) studies. In this instrument, the emission monochromator is protected by a long-wavelength-pass filter (UV-35, Shimadzu) that reduces incident beam scattering. Similarly, the Xe lamp output is also get auto corrected by a monitoring photomultiplier that dynamically correct the gain of the photomultiplier tube. The room temperature PL decay time studies were done by Edinburgh FLS-900 time resolved fluorescence spectrometer.

The investigation on site occupancy of dopant was carried out by doing X-ray absorption (XAS) measurements at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. Here the facility of EXAFS beamline i.e. BL-9 at the Indus-2 Synchrotron Source (2.5 GeV, 200 mA) was used for carrying out both X-ray near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) measurements. The details of the beam line are given elsewhere [30, 31].

EXAFS measurement at U L3 edge has been carried out in the fluorescence mode. The sample was placed at 45° to the incident X-ray beam, and the Si drift detector was placed at 90° to the incident beam to collect the fluorescence signal (\(I_{{\text{f}}}\)). An ionization chamber placed prior to the sample measures the incident beam (\(I_{0}\)). The X-ray absorption co-efficient of any sample is determined by \(\mu = I_{{\text{f}}} /I_{0}\). EXAFS measurements at Ba L3 edge have been done in transmission mode by placing the sample in between two ionization chamber detectors. The first ionization chamber measures the flux (\(I_{0}\)) of incident radiation and the second ionization chamber measures the transmittance (\(I_{{\text{t}}}\)) thus the absorbance of the sample is calculated as (\(\mu = \exp ( - \frac{{I_{{\text{t}}} }}{{I_{0} }})\)).

3 Results and Discussion

3.1 Crystal Structure and Morphology

Figure 1 represents the powder X-ray diffraction (XRD) patterns of uranium doped and undoped BaAl2B2O7. The XRD patterns shown here are in line with the standard reference data of JCPDS file no. 01–086-2168. BaAl2B2O7 matrix shows rhombohedral crystal structure and it has been observed that, incorporation of uranium does not disturb the original structure. The BaAl2B2O7 system is composed of BO3 triangles, BaO6 octahedra and AlO4 tetrahedra units (Fig. 2). The BO3 units are connected alternatively by Al-O–Al bridges and Ba atoms thus a a rigid framework of corner-shared AlO4 tetrahedra and BO3 triangles are evolved. In present case, the uranium doped host lattice has two possible regular lattice sites i.e. Ba2+ and Al3+ for incorporation of uranium. The ionic radius of hexavalent uranium (0.87 Å) is greater than the ionic radius of Al3+ (0.51 Å) but less than that of Ba2+ (1.34 Å). Therefore, it is interesting to probe the site occupancy of uranium in BaAl2B2O7.

Fig. 1
figure 1

XRD patterns of un-doped and uranium doped BaAl2B2O7 with a reference of standard ICDD 86–2168 pattern

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Fig. 2
figure 2

Schematic presentation of crystal structure of BaAl2B2O7 system

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3.2 PL Investigations

Uranium (1 mol %) doped BaAl2B2O7 shows a broad emission spectrum (Fig. 3). The emission spectrum ranges from 475 to 650 nm with a maximum at 550 nm upon excitation at wavelength of 285 nm. Here, the range of emission wavelength indicates the absence of U (III), U (IV) and U (V) form of uranium as they emit at higher wavelength regions [24,25,26, 28], whereas the emission lines from U (VI) falls under this region. In case of stabilization as U (VI), there are possibilities that uranium may get stabilized as uranyl ion (UO22+), tetrahedral uranate ion (UO42−) or in the form of octahedral uranium ion. Under octahedral form there are three possible structure such as UO66−, U6+ and UO912−. The observed emission profile corresponds to green range emission thus presence of uranium in the form of tetrahedral (UO42−) group, that emits in red region is not valid [32, 33]. Considering the spectra profile i.e. absence of vibronic progression lines, the probability of stabilization of uranium as uranyl form is not getting satisfied. In uranyl, there exist a strong coupling between the ground state Raman active symmetric vibrational (ν1) mode and the excited 3πu electronic state and thus vibronic progressions are originated. In addition to this a prominent feature is the zero-point line which involves a transition of electron from oxygen to one of the higher non-bonding uranium orbitals i.e. (πg → \(\sum\)g +) resulting a spectrum in the range from 471 to 520 nm [34]. In present case the emission profile lacks of all these uranyl emission characteristics. Thus the presence of uranium in the form of uranyl is ruled out. So, now the broadened emission spectrum is an indicative of uranium ions influenced by multiple coordination sites afforded by its surrounding. Thus, it may stabilize in the form of UOn (n = number of oxygen present at equatorial as well as axial position). Here, the coordination number at equatorial position may vary from 4 (as in UO6 6−) to lower than 7 (as in UO912−). However, the observed spectra closely resemble to UO66− as reported in various literature [35,36,37]. Although uranium faces f-f parity forbidden charge transfer transition issues but the involvement of vibronic coupling of 5f–t1u with ungerade vibrational modes of UO66− octahedron facilitates relaxation.

Fig. 3
figure 3

PL emission spectra of the BaAl2B2O7: U system at an excitation wavelength of 285 nm

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Figure 4 represents the excitation spectrum of the BaAl2B2O7: U recorded at 550 nm emission wavelength. The observed excitation spectrum was a charge transfer spectrum i.e. a charge transfer from oxygen (2p) to uranium (6d) orbitals resulting a broad spectrum ranging from 220 to 300 nm with a maximum at 285 nm. In addition to this, excitation peak at around 434 nm is due to hexavalent uranium. Although the PL studies confirm the stabilization of uranium in the form of UO66− but there is no strong clue about the site occupancy. As, Ba exists as an octahedra BaO6 in BaAl2B2O7 moiety, uranium may prefer to reside in Ba regular sites rather than Al3+ sites which remains as AlO4 tetrahedra structure. As discussed earlier the ionic radii of host ions (Ba2+, Al3+) are also not in good agreement with ionic radii of dopant ion (U(vi)). There is a huge difference in the oxidation states of host ions and dopant ion too. In this regard luminescence decay time studies are of great importance.

Fig. 4
figure 4

PL excitation spectra of the BaAl2B2O7: U system at an emission wavelength of 550 nm

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3.3 PL Decay Time Studies

The photo luminescence decay time studies were carried out to probe the influencing environmental coordinating sites as depending upon the site symmetry there may be multiple life-time values. The sample was excited at 285 nm, and emission was monitored at 550 nm to measure luminescence decay time. The observed life-time/decay curve (Fig. 5) could be fitted as bi-exponential decay: I (t) = A1 exp (−t/τ1) + A2exp (−t/τ2) + y0 where A1 and A2 are pre-exponential factors and are scalar quantities, ‘t’ is the time of measurement and τ1 and τ2 are the decay time values. The PL decay time values were measured to be τ1 = 24 µs (65%) and τ2 = 86 µs (35%). The double decay time values infer the presence of uranate ions at two different types of sites having different surrounding coordination environment. In this case, two possibilities are there. First one is the uneven distribution of defect centres around the uranium ion i.e. the one surrounded with more defect centres may result in smaller decay time values through non radiative relaxation. Secondly the double decay time may be presenting two different lattice sites occupied by uranium i.e. regular lattice sites of Al and Ba ions. As Ba ion possesses an octahedra geometry, uranium may experience a better symmetric coordination environment compared to tetrahedral environment of Al ion and thus uranium placed at Ba site may decay slower than the uranium placed at Al site owing to symmetry forbidden rules. Information from PL and PL decay time studies are not corroborating each other and thus investigation of site occupancy of uranium in BaAl2B2O7 remains interesting for further studies.

Fig. 5
figure 5

PL decay curve of the BaAl2B2O7: U system at an λemission of 550 and λexcitation of 285 nm

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3.4 EXAFS Studies on BaAl 2 B 2 O 7 : U

3.4.1 Ba L 3 Edge

Figure 6 shows the XANES spectra of the BaAl2B2O7 and U doped BaAl2B2O7 samples at Ba L3 edge. The XANES spectra do not show any significant change except a small increase in the white line intensity for the U doped BaAl2B2O7 sample. A standard procedure was followed to extract the EXAFS oscillations from the obtained absorption spectra [38, 39]. The EXAFS oscillations at high \(k\) are amplified by taking k2 to the consideration i.e. the \(\chi (k)\) function is weighted by k2. The Fourier transformation of functions \(\chi (k)\) k2 in \(r\) space was carried out to generate the \(\chi (r)\) versus \(r\) plots. These plots are Fourier transformed-EXAFS spectra in terms of the real distances from the centre of the absorbing atom. In the Fourier Transform the k range used was 2–8.5 Å−1. The short \(k\) range is due to the presence of Ba L2 edge at 5624 eV which is at a difference of 377 eV from the L3 edge (5247 eV). EXAFS Data Analysis program Athena and Artemis in the Demeter software package has been used for data processing and data fitting, respectively [40].

Fig. 6
figure 6

XANES spectra of the BaAl2B2O7 and U doped BaAl2B2O7 samples at Ba L3 edge

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Figure 7 shows the \(\chi (r)\) versus \(r\) plots of the BaAl2B2O7 and U doped BaAl2B2O7 samples at Ba L3 edge. Taking the crystal structure of BaAl2B2O7 in to account, the \(\chi (r)\) versus \(r\) data have been fitted from 1.5 to 2.8 Å assuming two Ba–O paths at 2.75 Å(× 6) and 3.25 Å(× 6), respectively [41]. For a comparison, a theoretical fit following the same procedure has been plotted in Fig. 7 and the fitting result is shown in Table 1. From the results, it can be observed that the Ba–O bond length decreases on U doping. From Fig. 7, it is also evident that the intensity of the peak near 3.2 Å (phase uncorrected) which corresponds to Ba-Al coordination shell at 3.75 Å according to BaAl2B2O7 crystal structure, increases on U doping. The increase in the intensity of this peak may be attributed to the replacement of Al with U. Since the atomic number of U(92) is much higher than that of Al(13), therefore the scattering amplitude of U is much higher compared to Al which may result in an increase of peak intensity. Also, the ionic radius of U (0.87 Å) is higher than that of Al (0.675 Å), therefore the replacement of Al with a larger ion like U may result in shortening of nearby bonds as has been observed for Ba–O bonds. It should be noted that due to the short k range (2–8.5 Å−1) of the data, the \(\chi (r)\) versus \(r\) data have been fitted only up to the first shell.

Fig. 7
figure 7

\(\chi (r)\) versus \(r\) plots of BaAl2B2O7 and U doped BaAl2B2O7 samples at Ba L3 edge

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Table 1 EXAFS Results of Ba L3 edge
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Figure 8 shows The XANES spectra of the U doped BaAl2B2O7 sample at U L3 edge are shown here in Fig. 8. along with a standard reference of UO2CO3, where U cations exist in + 6 oxidation state. Comparing the XANES spectra of the U doped BaAl2B2O7 with UO2CO3 standard, it is evident that U cations exist in + 6 oxidation state in the U doped BaAl2B2O7 sample. Figure 9 shows the \(\chi (r)\) versus \(r\) plots (or FT-EXAFS spectra) of the U doped BaAl2B2O7 sample at U L3 edge. The FT-EXAFS spectrum is extracted from the EXAFS spectra using the Fourier transform k range of 2–9 Å−1 and is fitted in the r range of 1–2.5 Å using an axial U–O shell with coordination 2 and an equatorial U–O shell with coordination of 4 following the Uranate structure as predicted from Photoluminescence measurements done on the samples.

Fig. 8
figure 8

XANES spectra of the U doped BaAl2B2O7 sample at U L3 edge along with UO2CO3 standard

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Fig. 9
figure 9

versus r plots (or FT-EXAFS spectra) of the U doped BaAl2B2O7 sample at U L3 edge

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The coordination number of the axial U–O path is kept fixed, while all other parameters of the axial and equatorial U–O paths are varied during the fitting. In Fig. 9, the experimental data as well as the best theoretical fit data are plotted and the EXAFS fitting results are mentioned in Table 2. Analysis of the EXAFS fitting results (Table 2) clearly revealed that the two axial oxygen atoms exist at a bond distance of 1.89 Å and four equatorial oxygens exist at a distance of 2.27 Å around U cations which is consistent with Uranate structure [42, 43]. So, it is evident From the EXAFS results that there is a possibility of U replacing Al in the BaAl2B2O7 lattice. However, the U L3 edge EXAFS results suggest that U cations exists in distorted octahedral geometry as Uranate in the lattice. Therefore, from the EXAFS analysis at both Ba and U L3 edge, it appears that though U replace the tetrahedral coordinated Al, but it rearranges its local environment as distorted octahedral geometry of the Uranate structure.

Table 2 EXAFS Results of U L3 edge
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4 Conclusion

Uranium-doped BaAl2B2O7 is successfully synthesized by solution combustion synthesis method and it is validated by PXRD studies. The PL studies have shown that BaAl2B2O7 host matrix is an ideal matrix for uranium luminescence studies as uranium gets stabilized in this host as uranate (UO6 6−). This host matrix offers two different environments (with respect to defect centres) for uranium stabilization which is understood from PL life-time measurement indicating two life-time values of 24 µs (65%) and 86 µs (35%). Several studies in EXAFS have evolved the information regarding the site occupancy of uranium in BaAl2B2O7. Although both Ba and Al are the existing sites and moreover the Ba ion has a greater ionic radius but the study reveals that the regular Al3+ site is the preferred site of stabilization for uranium and subsequently uranium undergoes a rearrangement and get stabilized in a distorted octahedral geometrical environment.