Structural and vibrational spectroscopic studies of new phases with sillenite type in the system Bi2O3- In2O3 –MgO

The anion and cation deficient phase Bi0.95 In0.05 O1.5 (Bi1.9 In0.1 O3) was synthesized and experimentally investigated using X-ray diffraction and vibrational spectroscopy (Infrared and Raman). The non-stoichiometric phases are similar to sillenite family type γBi2O3 and crystallize in the I23 space group. The crystal structure was determined by full profile Rietveld analysis of the powder diffractogram. It is formed by a sequence of BiO5E polyhedra (E lone pair of bismuth) and MO4 polyhedra (M = In, Mg). The set of MO4 polyhedra are localized in cavities generated by BiO5E polyhedra. The vibrational spectroscopic study revealed the existence of three regions; low, intermediate and high-frequency region. They are attributed to Bi-O stretching mode, In / MgO vibrations and cationic displacements respectively.


Introduction
Recently, materials having elements with outer electronic configuration ns 2 , particularly sillenite phases, have been extensively studied because of their remarkable properties such as; piezoelectricity, optoelectronics, photocatalysis, dielectricity ... 1-8 . Indeed the sillente phase is isolated for the first time by Silen in 1937 and admits as formula Bi12MO20 (M= Si, Ti, Ge) 9 . Several studies have been conducted, in particular on the crystalline structure, such as those of Levin and Roth 10 , and Radev et al. 11 that have determined the crystal structure of Bi12 (Bi2/3 Zn1/3) O19.33 and Bi12 ( 0.5 3+ 0.5 3+ )O19.5 by neutron diffraction.
The sillenite compounds Bi12 Mx O20 crystallize in the I23 space group. The crystalline structure is formed by a three-dimensional sequence of BiO5E polyhedra (E lone pair). These polyhedra share the edges and the vertices by M n+ O4 tetrahedra. The M n+ cation may be a tetravalent element in the case of stoichiometric sillenite or a combination of elements with an average value of four.
In the present work, On the one hand, the new phases isolated within the Bi2O3-In2O3-MgO system have been studied and identified as belonging to sillenite family. On the other hand, these phases were examined using Infrared and Raman spectroscopy techniques. The X-ray analysis was performed using the Rietveld structure refinement by full prof program 18 . The influence of dopant content; indium (single doping) or In and Mg (codoping) on the sillenite structural features has been discussed.

Preparation
The synthesized compounds have been stabilized with a solid-state method using the appropriate quantities of highly pure oxides powder α Bi2O3, MgO and In2O3 with hight purity (Aldrich brand). These oxides were heated at successively higher temperatures 700°C (24h) and 800°C (24h) with several intermediate grindings and followed by quenching.

X-ray diffraction
The final products have been monitored by X-ray powder diffraction (XRD) using a Philips X'Pert PRO diffractometer and Cu-K-alpha (λ=1.5406Å) radiation. The structural refinements were undertaken from the powder data. The patterns were scanned through steps of 0.02° (2θ), between 10° and 70° (2θ) with a fixed time counting of 60 s. The study of the structure is conducted by analyzing the profile of X-ray diffraction diagrams of powder with the program Fullprof 18 using the pseudo-Voight function.

IR spectrum
IR spectrum was recorded by using Bruker Tensor 27 with prepared pellets (1mg + 99mg KBr) in a frequency range of 400-4000 cm -1 .

Raman Spectrum
Raman spectrum was recorded by using DXR RAMAN spectrometer with exciter wavelength of 663nm and a power of 6 MW.
The results of the other compositions will be published later.
On the basis of γBi2O3 phase structure previously determined by Radev 11 , we carried out our structural study by distributing the cations on the possible sites. The chemical form of this compound can be written: Table 1 contains the experimental conditions and the structural parameters of this composition.

 Third case:
The atoms Bi and In, initially occupying the sites 24f and 2a respectively (according to the first case), were redistributed in the nearby sites admitting, therefore, the atomic positions given in Table 2. The final refinement seems to be in good agreement with the occupancy rates and the calculated distances (Tables 2 and 3).  (7) 109.4 (7) 109.4 (7) 1.952(4) 0.874

3.496
The experimental, calculated X-ray diffraction patterns and differential spectrum obtained by using Rietveld refinement are represented in Figure 1.
This lattice system contains tetrahedral cavities occupied by Bi 3 + / In 3 + . The substitution of bismuth by indium of a smaller size will cause a decrease of lattice parameter a (γ Bi2O3 = 10.25Å) to 10.10Å.
Some changes have been observed in this structure in comparison with the ideal sillenite structure Bi12MO20 (Bi12SiO20) (Fig.7). These modifications are manifested by a mutual displacement of Bi and In atoms and also oxygen atoms located in the equatorial plane of BiO5E polyhedra (Fig. 6). Three types of polyhedra surrounding Bi 3 + (24f site) are envisaged.
 Bi1O5E environment (Fig.3)  Bi2 atom is surrounded by 4 oxygens at distances varying between 1.93Å and 2.04Å (Fig.5). The longest distance Bi-O3 leads to a modification of the lone pair E orientation on the one hand and the environment of the connected coordination polyhedra on the other hand.
The Bi/In environment is a regular tetrahedra, as shown in Figures 5 and 6, the In-O3 distance is 1.95Å. The distribution of Bi2 3+ /In 3+ on the tetra 8c and 2a sites, respectively, generates the creation of anionic vacancies. If this bond is excluded, the environment Bi2O3 E will be surrounded by three oxygens and E. The lone pair E is directed to the missing vertex of the Bi2O3 E tetrahedra.
Furthermore, the existence of anionic and cationic vacancies in 8c site leads to changes in bond lengths within the polyhedra in Bi12O16 lattice. In Table 4, we have grouped the main distances obtained in certain sillenite phases (Bi12GeO20 and γBi2O3) compared to the distance of the new phase isolated in the Bi2O3-In2O3 -MgO.system

Raman spectroscopy analysis
Raman spectrum of the crystal Bi0.95In0.05O1.5 is illustrated in Figure.8. We have also reported the Raman spectrum of Bi12TiO20 as a comparison ( Table  5). The spectrum was recorded in 100-800 cm -1 spectral range at room temperature. The attribution of Raman peaks was based on previous work 22 . At first glance, the spectra are broadly comparable to the Bi12TiO20 spectrum with a slight difference in intensities. This difference seems to be due to the doping elements In and Mg on the one hand and the other hand, to the interaction of BiO5 polyhedra with their counterparts In / Mg O4. Indium oxide usually absorbs 800-300 cm -1 radiation 23 . In our compound (Fig.8), a peak at 818.59 cm -1 is actually observed and therefore attributed to the In-O4 tetrahedra.
The shoulder observed at about 368.15 cm -1 is probably assigned to Bi-O-In bonds because the mass of bismuth is higher than that of indium. As a result, the oscillations of In-O4 tetrahedra are small.

Conclusion
The results of structural refinements are in perfect agreement with those determined by 16,17 on sillenite compounds. The structure is formed by a sequence of BiO5 polyhedra linked together by edges. These polyhedra are connected by Bi/InO4 tetrahedra via O3 atoms.
The phases are non-stoichiometric on both the anion and cation sublattice. They are similar to γBi2O3 type sillenite phase. The substitution of Bi by In of a smaller size causes a lattice parameter decrease. Three regions are elucidated by the vibrational spectroscopic study. The low-frequency region is attributed to cations displacement, the second region, which corresponds to medium frequencies, is attributed to stretching vibration modes of Bi-O however, the high frequencies (observed in the third region) are assigned to the deformation bonds of Bi / In-O-Bi.