Electronic structure studies of bismuth compounds

Journal of Alloys and Compounds 753 (2018) 646e654

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Electronic structure studies of bismuth compounds using high energy resolution X-ray spectroscopy and ab initio calculations A.A. Mistonov a, b, *, A.P. Chumakov c, R.P. Ermakov d, L.D. Iskhakova d, A.V. Zakharova a, b, A.V. Chumakova b, K.O. Kvashnina e, f, ** a

Department of Physics, Saint-Petersburg State University, 198504 Saint Petersburg, Russia Petersburg Nuclear Physics Institute, NRC Kurchatov Institute, 188300 Gatchina, Russia European Synchrotron Radiation Facility, 38043, Grenoble Cedex 9, France d Fiber Optic Research Center of RAS, Moscow, Russia e Rossendorf Beamline at ESRF The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France f Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 December 2017 Received in revised form 13 April 2018 Accepted 17 April 2018 Available online 19 April 2018

Bismuth-based compounds are widely used as superconductors, catalysts and materials for optical devices. Their properties, and the possibility of controlling these properties, are determined by their electronic structure and the local environment of the Bi-centres. Although X-ray spectroscopy is a powerful method for revealing crystal and electronic structure, the results obtained so far have been limited by the energy resolution of the experimental data. Here, for the first time, we report X-ray absorption near edge structure (XANES) data, recorded in high energy resolution fluorescence detection (HERFD) mode at the Bi LIII and LI edges for a number of bismuth compounds. Experimental data are analysed using ab initio calculations based on a finite difference method (FDMNES) code, for metallic Bi, Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 compounds. It was observed that both the oxidation state and the length of the Bi-ligand bonds determine the exact position of the absorption edge, and this is in good agreement with other studies. Additionally, due to the high energy resolution, spectral features corresponding to strong Bi p-d orbital mixing were resolved. The results obtained here can be used as input for further investigations of the electronic structure of bismuth compounds in different chemical states. © 2018 Elsevier B.V. All rights reserved.

Keywords: Bismuth compounds Electronic structure XAS HERFD FDMNES

1. Introduction Determination of the oxidation state and local structure of bismuth is problematic for a variety of inorganic materials such as superconductors [1e5], catalysts [6e11], Bi-doped crystals [12e15], Bi-activated glasses [15e19], optical fibres [20e22], lasers [23e25], amplifiers [26,27] and superfluorescence sources [28]. Bismuth has a wide range of oxidation states (0, þ1, þ2, þ3, þ5) and a number of þ 3þ 2þ polycation clusters with different charges (Bi4þ 2 , Bi5 , Bi5 , Bi6 , 2þ 2 Bi8 , Bi2, Bi2 , Bi2 ) which may be present simultaneously in the same compound [12,16]. An understanding not only of the

* Corresponding author. Department of Physics, Saint-Petersburg State University, 198504 Saint Petersburg, Russia. ** Corresponding author. Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany. E-mail addresses: [email protected] (A.A. Mistonov), kristina.kvashnina@esrf. fr (K.O. Kvashnina). https://doi.org/10.1016/j.jallcom.2018.04.190 0925-8388/© 2018 Elsevier B.V. All rights reserved.

oxidation state but of the whole electronic structure and its relationship with the crystal structure parameters allows the creation of materials based on Bi compounds with controlled properties and characteristics that are optimal for the corresponding applications. Traditionally, spectroscopic techniques have been the most powerful methods for the investigation of electronic structure. Electrons in the X-ray absorption process are excited to unoccupied levels, thereby giving information about the chemical state and the interactions of the excited atom with the ligand environment. XANES can reflect the electronic structure of the atom within an energy range of about 30e50 eV near the absorption edge, and can contain features that are characteristic of the whole class of compounds. For instance, a chemical shift in the absorption edge is often assigned to a certain oxidation state. The position and shape of the first post-edge feature is frequently used as a fingerprint for a particular ligand and crystal structure environment. For this reason, the analysis of new compounds requires XANES spectra for reference samples with known chemical states and structural

A.A. Mistonov et al. / Journal of Alloys and Compounds 753 (2018) 646e654

parameters. It has previously been shown [29] that the XANES spectra at the Bi LIII absorption edge depend strongly not only on the oxidation state but also on the local environment of the Bi centre. However, the energy resolution of the experimental data reported earlier [3,16,17,29,30] was not sufficient to resolve all spectral features, preventing a detailed analysis of the electronic structure. Better energy resolution of the XANES spectra can be obtained using state-of-the-art experimental techniques involving the high energy resolution fluorescence detection (HERFD) mode [31]. This approach allows the resolution of fine features that reflect the complexity of the electronic structure such as pre-edge transitions, which are often not noticeable in conventional XANES experiments. It is possible to achieve a clear understanding of the electronic structure if theoretical approaches are exploited [32]. Ab initio calculations of the electronic density of states (DOS) for the absorbing atom and ligands allow the XANES spectrum to be modelled; this can then be compared with the experimental results. The correspondence between the experimental and theoretical spectral features reflects the correctness of the assumptions used, since the XANES spectrum is very sensitive to the crystal structure parameters. In this work, experimental HERFD data are recorded at the Bi LI and LIII absorption edges for metallic Bi, a-Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 and are compared to the theoretical calculations using a FDMNES code [36]. The paper is organised as follows. Section 2 gives details of the sample preparation, theoretical calculations and experimental setup. In Section 3, the results and a discussion are presented. Section 4 presents concluding remarks. 2. Samples and methods Experimental data were obtained for Bi, a-Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 compounds. Metallic bismuth and a-Bi2O3 were bought from Sigma Aldrich. The BiPO4 sample was synthesised at room temperature using a precipitation method. White homogeneous BiPO4 precipitate was produced as a result of mixing 0.5 M bismuth nitrate, water and phosphoric acid, using a stoichiometric relationship. This was then calcinated at 800+C. Monocrystalline Bi4(GeO4)3 was grown using the Czochralski process at the Research Centre for Laser Materials and Technologies at the A. M. Prokhorov General Physics Institute, Russian Academy of Sciences. Sodium bismuthate (NaBiO3) was synthesised by the Aldrich Chemical Company. The measurements were performed at beamline ID26 [33] of the European Synchrotron (ESRF) in Grenoble. The incident energy was selected using the 〈311〉 reflection from a double Si crystal monochromator. Rejection of higher harmonics was achieved using three Cr/Pd mirrors at an angle of 2.5 mrad relative to the incident beam. The incident X-ray beam had a flux of approximately 2 1013 photons/s at the sample position. XANES spectra were measured simultaneously in total fluorescence yield (TFY) mode using a photodiode and in HERFD mode using an X-ray emission spectrometer [34,35]. The sample, five analyser crystals and a photon detector (silicon drift diode) were arranged in a vertical Rowland geometry. The Bi HERFD spectra at the LIII edge were obtained by recording the maximum intensity of the Bi La1 emission line (10.839 keV) as a function of the incident energy, while Bi HERFD data at the LI edge were obtained at the maximum of the Bi Lb3 emission line (13.211 keV). The La1 emission energy was selected using the 〈844〉 reflection of four spherically bent Ge crystal analysers (with 1 m bending radius) aligned at a Bragg angle of 82+, and the Lb3 emission energy was selected using the 〈880〉 reflection of five spherically bent Si crystal analysers (with 1 m bending radius) aligned at a Bragg angle of 78+. A combined (incident convoluted

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with emitted) energy resolution of 1.8 eV (2.2 eV) at the LIII (LI) edge was obtained by measuring the full width at half maximum (FWHM) of the elastic peak. The intensity was normalised to the incident flux. Since Bi is easily oxidised, which changes the valence, we studied the effects of radiation damage induced by incident Xrays. We performed several fast scans (1 s per scan) for each sample, but found no damage caused by X-ray irradiation. The XANES spectra at the Bi LI and LIII edges were simulated using an FDMNES approach [36]. This code allows the calculation of the unoccupied projected density of states (DOS) in relation to the €dinger equation in the X-ray absorption process by solving the Schro monoelectronic approximation. Final states were calculated inside a spherical cluster. Only the atoms inside this sphere were considered. Simulations were performed for an atomic cluster with a radius of 6 Å, using the Cartesian coordinates listed in the Inorganic Crystal Structure Database (ICSD) [37]. Quadrupole transitions were taken into account and the multiplet effect was omitted. The local density approximation was used to calculate the potential. Relativistic self-consistent field calculations using the Dirac-Slater approach were performed for each atom in the cluster. In order to introduce core-hole lifetime broadening, which is drastically reduced in the case of HERFD, the absorption spectra were convoluted using a Lorentzian of the corresponding width (2 eV at the LI edge and 0.7 eV at the LIII edge). An example of an FDMNES input file can be found in the Supplementary Information. 3. Results and discussion The Bismuth ion has a ground state electron configuration of [Xe] 4f145d106s26p3, and depending on the oxidation state, the valence electrons are situated at the 6s and 6p orbitals. The 6p valence electrons in Bi can be probed using XANES at the Bi LI edge, since the dipole allows 2s / 6p transitions, while the 6s and 5d electrons can be probed using XANES at the LIII edge, due to the excitations from the 2p3/2 orbital to the empty s or d orbitals. A XANES experiment in the HERFD mode has a specific advantage in that the core-hole lifetime broadening, which is of the order of 14.9 eV at the Bi LI edge and 5.9 eV at the Bi LIII edge (observed in TFY mode), can be greatly reduced, providing high energy resolution data for the recorded XAS transitions. For heavy ions, this enhancement allows featured spectra to be obtained rather than broad, almost featureless, conventional XANES spectra, which are hard to analyse and interpret. Both HERFD and TFY measurements were performed for several Bi systems: Bi, Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 with different oxidation states at the Bi LI and LIII edges. The experimental data are shown in Fig. 1. Compared to the TFY spectra, HERFD data are much more informative, since the peaks are sharper and more intense. Several features cannot be observed in the TFY plots, especially at the LIII absorption edge. In particular, the pre-edge peak can be only slightly resolved in the TFY spectrum of the NaBiO3 compound, while for others it is not seen at all. The LI edge HERFD spectrum also noticeably differs from the TFY spectrum; the white-line intensity is higher, and the feature at 16.400 keV is significantly pronounced for all compounds studied, in addition to Bi4(GeO4)3. The main difference between the two absorption edges is the shape of the LI and LIII edge transitions. The LI HERFD spectra for several systems look very similar: each spectrum shows the main edge transitions (white-line transitions) and one or two additional features in the post-edge region. In contrast to the LI spectra, the profiles of the LIII edge spectra [Fig. 1b] differ significantly between the various Bi systems. Additionally, the Bi LIII HERFD spectra are shifted to a higher energy following changes in oxidation state when the ligand environment remains the same. An energy shift is also observed when only the ligand environment or crystal structure is altered.

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Fig. 1. Experimental TFY (a,b) and HERFD (c,d) spectra obtained at LI (a,c) and LIII (b,d) edges of bismuth.

This difference is due to the fact that different molecular orbitals are probed at the LI and LIII absorption edges. The form and occupation of the final orbital and the involvement of that particular orbital in the bonding strongly determine the shape of the spectrum. The LI edge profile is primarily formed due to transitions to the p orbital, while the LIII profile is due to transitions to the s and d orbitals. This gives rise to the significant difference between the shapes of the LI and LIII spectra, including their shifts. We first analyse the electron structure by discussing the oxidation state, which usually corresponds to the shift in the absorption edge, often determined as a first derivative maximum of the white line. The values of the main edge transitions at the Bi LI and LIII edges are summarised in Table 1, together with the structural parameters of all the Bi compounds. It is clear that the position of the LI edge increases with the oxidation state, and most significantly for the 5 þ systems; for LIII,

this trend persists, but the difference in position is observed for the non-oxidised sample. This also demonstrates the difference between the origins of the LI and LIII edges. It is well known that the energy shift in the absorption feature can be used to determine the oxidation state. For some elements and their substances, this link is so univocal that it can easily be used to reveal an unknown oxidation state or even a mixture of states, and arises from initial core-hole shielding. The potential energy of electron in an atom is determined by its attraction to a positive nucleus and repulsion from the other electrons. A reduction in the number of electrons (oxidation) results in a lowering of the nucleus screening; thus, the energy needed for its escape from the atom increases and absorption edge shifts towards a higher energy. When the number of electrons increases, the edge shifts to lower energies [43]. However, this shift can also be due to the difference in bond

Table 1 Positions of the LI and LIII absorption edges and structural parameters for the studied compounds. Compound

m0max [LI](eV)

m0max [LIII](eV)

Oxidation state

Cell parameters (Å) and angle (+) a

b

c

b

Bi [38] Bi2O3 [39] BiPO4 [40] Bi4(GeO4)3 [41] NaBiO3 [42]

16376.5 16377.5 16377.5 16377.1 16378.6

13417.3 13420.9 13421.2 13421.7 13421.8

0 3þ 3þ 3þ 5þ

4.546 5.849 4.882 10.524 5.567

4.546 8.166 7.068 10.524 5.567

11.862 7.510 4.703 10.524 15.989

90 113 96.290 90 90


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Fig. 2. Unit cells of Bi (a), Bi2O3 (b), BiPO4 (c), Bi4(GeO4)3 (d) and NaBiO3 (e) structures. Bismuth atoms are shown by green balls, oxygen d red, phosphorus d yellow, germanium d blue, sodium d orange. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

length, i.e. the local structure [44]. In view of this, theoretical calculations of the HERFD spectra were performed that took into account the crystal structure parameters of all Bi samples. For example, Bi metal atoms are packed in an R3m structure [38]; the three nearest neighbours are located at a distance of 3.07 Å and next three neighbours at 3.53 Å [Fig. 2a]. These values significantly differ from the bonding lengths of the other compounds (see below). Thus, one can clearly observe from Fig. 1 the difference in the edge position for the LIII. However, there is no difference for the LI edge. The formal oxidation state of a-Bi2O3, BiPO4, Bi4(GeO4)3

compounds is 3þ, but the structures differ considerably [Table 1]. Bismuth oxide has two positions that are not equivalent to each other. Each Bi atom with its five neighbouring oxygens forms a strongly distorted octahedron or pyramid with a slightly distorted basal plane (one atom is located almost at the centre of the pyramid base, but it is out of the plane) [Fig. 2b] [39]. In bismuth phosphate, the Bi atom is surrounded by oxygen atoms forming a pentahedron, while the phosphorus atom is in a tetrahedral oxygen environment [Fig. 2c]. A single crystal of Bi4(GeO4)3 is an array of BiO6 octahedra and GeO4 tetrahedra [41] [Fig. 2d]. On the other hand, the average Bi-O bond distances are 2.329 Å

Fig. 3. Experimental and calculated HERFD spectra obtained at LI (a) and LIII (b) edges for metallic Bi sample as well as corresponding DOS. The spectra are shifted in vertical direction for clarity.

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[29], 2.397 Å [40] and 2.386 Å [41] for the oxide, phosphate and germanate, respectively. The bond distance will always affect the electron density distribution. We therefore have a set of reference systems which are almost the same from this point of view. Although the difference in the crystal structure is expected to cause changes in the electronic structure of the Bi atom, it does not significantly affect the edge position in Bi2O3,BiPO4, Bi4(GeO4)3 due to the similarity of the Bi-O bond lengths. Interestingly, the LIII edge in NaBiO3 is also close to those of the 3 þ compounds. Its crystal structure consists of BiO6 and NaO6 octahedra [Fig. 2e], with a Bi-O distance of about 2:12 A [29,42], which is smaller than in the oxide, phosphate and germanate. Clearly, both the oxidation state and the bond length affect the shift compensation. Nevertheless, this shift is only a preliminary indicator of the oxidation state or local environment of the absorbing atom. The most informative aspect is the analysis of the shape and the specific absorption features above and below the main edge transitions. Particular attention should be paid to the pre-edge features. While an absorption spectrum is primarily due to the dipole transitions, the pre-edge is mostly formed by the quadrupole transitions. The corresponding selection rules are Dl ¼ ±1 for the dipole transition and Dl ¼ 0,±2 for the quadrupole transition, where l d is the orbital quantum number. This means that at the Bi LIII absorption edge, mainly from 2p3/2/ 7s and 2p3/2/ 6d transitions, the 6p and 5f orbitals can also be probed. Experimental observation of the pre-

edge feature is difficult due to the ratio of probabilities of dipole and quadrupole transitions. However, if there is a strong hybridisation (or mixing) of the main orbitals (s and d in this case) and additional orbitals (p and f), the pre-edge peak becomes noticeable. The HERFD detection method also helps in observing this, due to the possibility of distinguishing the fine structure of the absorption features. We performed electronic structure calculations using the FDMNES code, with the aim of assigning the fine-structure components to particular electronic transitions. For all the compounds studied, we obtained the density of unoccupied electronic states (DOS) and spectra convoluted by the core-hole lifetime broadening; these are presented below, together with the experimental spectra. It should be understood that the DOS does not depend on the probed absorption edge, but on the different molecular orbitals contributing to the spectral profiles at the LI and LIII absorption edges. In order to show these more clearly, the intensity was either reduced or increased, as shown in Figs. 3e7. Valence band is composed of following states: 5d, 6s and 6p states. Density of states plotted in all figures is a sum over all principle quantum numbers of the particular state projected on angular momentum. It is worth noting that although we measured X-ray absorption at Bi edges, strongly hybridized molecular orbitals allows to indirectly probe the orbital contributions of the ligands, which in this case are O, P and Ge. However, a direct probe of the electronic

Fig. 4. Experimental and calculated HERFD spectra obtained at LI (a) and LIII (b) edges for a-Bi2O3 sample as well as corresponding DOS. The spectra are shifted in vertical direction for clarity.


structure of ligands can be achieved only by measurements at the O, P or Ge K-edge. Metallic Bi: The experimental and theoretical results for metallic bismuth are shown in Fig. 3. Calculations were performed for the central Bi atom in a cluster of 26 atoms (cluster radius R ¼ 6 Å). The main edge absorption features of the LI spectrum are due to the contribution of the p states, while the LIII edge shows the main impact of the d states. Since a metal always combines with oxygen from the air to form an additional product (the oxide), the spectra of pure bismuth and a-Bi2O3 were mixed in a 3:1 ratio. The experimental data are better reproduced by this type of calculation for comparison with pure metallic calculations, and showed the presence of 25% of the oxide phase (SI.Fig. 1), in good agreement with the X-ray diffraction data. This addition smooths the feature just after the white line in the LI spectrum, and also slightly increases the intensity of the LIII spectrum at 13.433 keV. According to our calculations, weak mixing of Bi p-d and s orbitals takes place, giving rise to the pre-edge feature observed in the Bi LIII spectrum; however, this was not observed in the experiment. We believe that this is due to experimental broadening and the weakness of the pre-edge feature. a-Bi2O3: As mentioned above, metallic Bi is easily oxidised in air. The most stable oxide (under ambient conditions) is a-Bi2O3. The valence 6p shell of Bi becomes completely empty in the 3 þ oxidation state. The crystal structure of a-Bi2O3 contains two

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inequivalent positions of Bi atoms. The calculations shown in Fig. 4 were performed for both Bi atoms, and then averaged with respect to the number of particular atoms in the corresponding structure. The LI spectrum was calculated for the atomic cluster with a radius of 5 Å (30 atoms), and the LIII spectrum for the cluster with a radius of 6 Å radius (57 atoms). This was due to the fact that the 6 Å calculations consumed excessive computational resources. In general, the difference in cluster size may have an effect on the DOS shape and consequently on the theoretical spectral features. However, this is mainly observed for smaller cluster sizes (less than 3e4 Å). The LI edge experimental spectrum is very well reproduced by theoretical simulations for a cluster size of 5 Å. The existence of a strong pre-edge feature is due to the mixing of the p and d orbitals of Bi and the p states of O. BiPO4: Fig. 5 shows the experimental and theoretical data for the BiPO4 system. Calculations were performed for the 6 Å cluster (66 atoms). Taking into account the low symmetry of the Bi environment, a strong pre-edge feature can be expected, and this has been observed in both calculations and experimental data The phosphorus atoms are situated at 3.19 Å from the Bi atoms, and another bismuth atom is at 4.06 Å [32,40]. It is clear that the LImain edge features are due to the p states of Bi, with a small admixture of the s and d states of Bi and the s and p states of O and P. The view of the LIII edge spectrum is strongly determined by the distribution of d states. A clearly resolved peak of the d DOS of Bi at

Fig. 5. Experimental and calculated HERFD spectra obtained at LI (a) and LIII (b) edges for BiPO4 sample as well as corresponding DOS. The spectra are shifted in vertical direction for clarity.

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13.446 keV (between two vertical dotted lines in Fig. 5) is not shown in the theoretical and experimental spectra. The existence of a well-resolved pre-edge structure, as mentioned above, confirms the p-d mixing of Bi orbitals. It is also worth noting that the p and s orbitals of both P and O mix with those of Bi, giving rise to the preedge peak and the feature near 13,425 keV. Bi4(GeO4)3: The cluster of Bi4(GeO4)3 modelled here was of 6 Å radius and included 54 atoms. The BiO6 octahedra forming the structure are distorted, and the inversion symmetry is therefore broken; p-d mixing is then also expected for the compound. Fig. 6 shows that all calculated and measured features coincide well enough for both absorption edges, although the intensity distribution in the region between 13.427 keV and 13.440 keV is different. Here, the form of the d DOS of Bi looks very similar to results of the experiment. The calculated spectrum reflects the existence of d-f orbital mixing. In a similar way to the compounds considered above, it can be seen that ligands contribute significantly to the beginning of the spectral range. In addition, Ge d states also contribute at energies near 13.455 keV at the LIII edge spectrum and 16.425 keV at the LI edge spectrum. A further feature appearing at an energy close to 13.440 keV is observed in the spectra of all compounds with Bi in the 3 þ oxidation state; in the spectrum of BiPO4, this feature is shifted to lower energies. NaBiO3: In the NaBiO3 compound, Bi is in its highest oxidation state (5þ), meaning that both the 6p level and the 6s level are

empty, and the dipole-allowed transitions occurring at the LIII edge differ from those in Bi3þ compounds. Fig. 7 shows the experimental and calculated spectra for the LI and LIII absorption edges for Bi. The cluster size used for the calculations was 6 Å (57 atoms). The spectra presented in Fig. 7a agree fairly closely, while the spectra obtained at the LIII edge are similar only in terms of certain features. The main difference is observed in the pre-edge region, where a strong peak is clearly seen in the experimental data, while the calculated spectrum shows two small features. This difference may be due to an inappropriate crystal structure being used for the calculations. According to the X-ray diffraction data (SI.Fig. 2), there is evidence for the formation of not only the NaBiO3 phase but also the NaBiO,32H2O phase during synthesis. Unfortunately, there is no information about this structure in the crystal structure database that would allow refinement of the theoretical calculations. Nevertheless, the pre-edge structure in the HERFD LIII spectra is probably due to the lowering of the symmetry of the Bi environment, caused by water embedded into the structure of the main phase. Thus, studies of the NaBiO,32H2O crystal structure are important for further investigations of the electronic structure of Bi compounds in its highest oxidation state (5þ). Comparison between samples. The main absorption peak at the LI edge can be ascribed to the 2s / 6p transition. The 6p state has vacancies in all the samples, but in metallic Bi it is half-filled, and hence the intensity of the main peak is the lowest. The

Fig. 6. Experimental and calculated HERFD spectra of Bi4(GeO4)3 sample, obtained at LI (a) and LIII (b) edges as well as corresponding DOS. The spectra are shifted in vertical direction for clarity.


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Fig. 7. Experimental and calculated HERFD spectra of NaBiO3 sample at LI (a) and LIII (b) edges as well as corresponding DOS. The spectra are shifted in vertical direction for clarity.

difference in intensities between the 3 þ samples is obviously due to the slight difference in crystal structure. For NaBiO3, the peak amplitude decreases. The transitions observed in the spectra at the LIII edge for the Bi samples in the 0 and 3 þ oxidation states are due to the 2p 3/2/ 7s and 2p 3/2/ 6d excitations. According to our calculations, the d DOS are major contributors, while the s DOS are minor contributors; this is in agreement with previously reported work by Jiang et al. [29]. It was found that the p DOS of Bi also play a significant role in the shape of the spectral transitions. Moreover, the spectra of the 3 þ oxidation state compounds can be divided into two types depending on the stereochemical activity of the Bi electron lone pair, based on the intensity and position of the white line. In this case, BiPO4 belongs to one type, while a-Bi2O3 and Bi4(GeO4)3 belong to the other. Due to the high energy resolution, it can clearly be seen that the range 13.420 keVe13.440 keV contains three spectral features, positioned at 13.424 keV, 13.429 keV and 13.440 keV. The last of these has a similar intensity and position for all three compounds, and thus weakly depends on the local structure or lone pair role. The main difference is observed at 13.429 keV, meaning that the type of environment primarily determines these transitions. A strong pre-edge feature, produced by the 2p 3/2/ 6p transition, can be observed in all spectra of Bi compounds. This pre-edge structure was not observed in the experiment for metallic Bi compounds, but is expected to be there based on the calculations. The low intensity of this feature, which is detected only in the case of metallic Bi rather than the other

samples, is due to the fact that its p-orbital has fewer vacancies (i.e. a lower p DOS value) than the other samples, and that ligands (especially oxygen) also have a significant p DOS. The experimental existence of the pre-edge in case of NaBiO3 cannot be explained by the results of our calculations. 4. Concluding remarks The electronic structure of the Bi ion in different local environments and oxidation states is studied here using HERFD spectroscopy and ab initio calculations. We show that common L-edge XANES spectroscopy is unsuitable for the investigation of Bi compounds due to a large amount of core-hole lifetime broadening. By combining HERFD and FDMNES calculations, we observe and interpret spectral features that are inaccessible to conventional XANES. An analysis of the changes in the oxidation state of several Bi compounds is presented here, due to its importance in understanding the performance of Bi centres in several applications (e.g. superconductors and optical devices). We investigate metallic Bi, and a-Bi2O3, BiPO4, Bi4(GeO4)3 and NaBiO3 compounds. By resolving the pre-edge spectral peak, we observe that strong p-d mixing of Bi orbitals takes place in all compounds. We find a significant contribution of ligands such as O, P and Ge to the Bi p DOS, and that these play a crucial role in the shape of the spectrum. These results could be obtained due to the high energy resolution at the synchrotron facility. It was also shown that the spectral feature

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at an energy of about 13.440 keV is specific to the 3 þ oxidation state. We observe that the edge position is determined primarily by the Bi-O bond length, confirming results reported by other researchers. Theoretical calculations are also performed for the above crystal structures; however, errors may occur during synthesis or due to the oxidation process of materials in air (as in the case of metallic Bi). Therefore, a careful characterisation of each sample by X-ray diffraction or other independent methods is necessary prior to using HERFD experiment.

[21]

Acknowledgements

[22]

[17]

[18] [19]

[20]

Authors acknowledge L. I. Ivleva for providing Bi4(GeO4)3 sample and M. Sukhanov for synthesis of BiPO4 sample. Authors also would thank staff of the ID26 beamline at ESRF for the granting the beamtime and for the hospitality, and especially Lucia Amidani for the invaluable help during the whole experimental time.

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.04.190.

[27]

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