I. INTRODUCTION
Among inorganic materials, the ordered double perovskite of the A2BB′O6 family is one of the most studies by the scientific community due to its wide variety of interesting physical properties, including magnetic, magnetocaloric, ferroelectric, superconducting, photonic, catalytic, and microwave dielectric (Cava and Batlogg, Reference Cava and Batlogg1989; Fukushima et al., Reference Fukushima, Stroppa, Picozzia and Perez-Matob2011; El Hachmi et al., Reference El Hachmi, El Ouahbi, Manoun and Lassri2022, Reference El Hachmi, Sen, Mondal, Paul, Saha, Im and Biswas2023; Guo et al., Reference Guo, An, Shu, Peng, Han, Hu, Chen, Zhao, Li and Li2022; El Hachmi and Manoun, Reference El Hachmi and Manoun2023; Algahtani et al., Reference Algahtani, Ali, Hussain, Ali, Quraishi, Tirth, Abdullaeva, Kamran, Aslam and Zaman2024).
The double perovskite structures Sr2MMoO6–δ (M = Fe, Ni, Mg, Mn, Co, Zn) have been reported using X-ray powder diffraction (XRPD) to adopt a lower symmetry of triclinic space group I–1 (Vasala et al., Reference Vasala, Lehtimäki, Huang, Yamauchi, Goodenough and Karppinen2010). The calculated Goldschmidt tolerance factor (t) has been found between 0.952 (M = Mn) and 0.984 (M = Ni). The triclinic space group I–1 has also been described for the triple perovskite LaSr2Cr2SbO9 at room temperature (Hunter et al., Reference Hunter, Battle, Sena and Hadermann2017); with unit-cell parameters a = 5.5344 Å, b = 5.5562 Å, c = 7.8292 Å, α = 89.986°, β = 90.350°, and γ = 89.926°. This material has been reported to exhibit a ferrimagnetic state below 150 K. It has been found that the B-site cations partially occupy the 2f (0,1/2,0) and 2g (1/2,0,0) Wyckoff sites.
The ordered double perovskite SrLaCuTaO6 has been reported by West and Davies (Reference West and Davies2011), who used neutron powder diffraction (NPD) at 323 K as crystallizing in a triclinic space group P–1 (No. 2) with unit-cell parameters a = 7.801 Å, b = 7.813 Å, c = 8.399 Å, α = 89.67°, β = 90.32°, and γ = 90.09°. Moreover, the tilt system assignment has been given for the P–1 triclinic structure by the notation a +b −c −. It has been noted that all atoms occupy 2i (x,y,z) Wyckoff sites.
Similar to our tellurium-based compound, it has been reported that the two materials Ba2Bi2/3TeO6 and Ba3Bi2TeO9 (with the same chemical elements and an order of 0.6667:1 and 2:1 on the B-site cations, respectively) have been prepared using the solid-state method at an annealing temperature of 950 °C and investigated by means of NPD at room temperature (Park and Woodward, Reference Park and Woodward2000). The double perovskite Ba2Bi2/3TeO6 has been described as crystallizing in cubic symmetry with a Fm–3m space group. It has also been described that the triple perovskite Ba3Bi2TeO9 adopts a trigonal structure with a P–3c1 space group.
On the other hand, the simple perovskites LaNi0.5Ti0.5O3 and LaNi0.45Co0.05Ti0.5O3 have been studied using the Rietveld method (Souza et al., Reference Souza, Maza and Tuza2021). These two compounds have been described in the orthorhombic structure, with space group Pbnm (No. 62) and Glazer notation a −a −c +. The two monoclinic structures SrLa2Ni2TeO9 and Sr3NiNb2O9 have been characterized by NPD at room temperature and 20 K, respectively; the first compound crystallizes in the P21/n space group with lattice parameters a = 5.6008 Å, b = 5.5872 Å, c = 7.9018 Å, and β = 90.021° (Sena et al., Reference Sena, Hadermann, Chin, Hunter and Battle2016), and the second structure adopts a P121/c1 space group with lattice constants a = 9.7268 Å, b = 5.6175 Å, c = 16.8704 Å, and β = 125.024° (Lee et al., Reference Lee, Choi, Ma, Sinclair, Cruz and Zhou2016). Furthermore, the Sr3FeNb2O9 compound has been presented as crystallizing in the P4/mmm tetragonal space group (Cha et al., Reference Cha, Bae, Hong, Yoon, Chung, Jeong, Kim, Borse and Lim2014).
In this work, we describe the synthesis procedure and crystal structure determination of the double perovskite oxides Ba2Bi0.572TeO6±δ and SrLa2NiFeNbO9 from room temperature XRPD using the Rietveld method.
II. EXPERIMENTAL DETAILS
A. Synthesis
Polycrystalline powders of the Ba2Bi0.572TeO6±δ and SrLa2NiFeNbO9 ceramics were prepared using solid-state reaction route in air, and the raw materials BaCO3 (99.98%), Bi2O3 (99.9%), TeO2 (≥99%), SrCO3 (≥99.9%), La2O3 (99.999%), NiO (99%), Nb2O5 (99.99%), and Fe2O3 (≥96%) were used as starting reagents (all received from Sigma-Aldrich). The quantity of each reagent was weighed according to its stoichiometric coefficient to obtain the appropriate metal ratios of the final products, then mixed and ground in an agate mortar to form a homogeneous powder. The resulting mixtures were placed in alumina crucibles, and then increasingly heated in air at different temperature stages with intermittent grinding; these two samples were calcined at 950 °C for 12 h and 1200 °C for 24 h, respectively. During the heat treatment process, the samples were cooled to room temperature, reground and sintered several times to improve homogeneity. The chemical reactions are:
$$2\cdot {\rm BaC}{\rm O}_3{\rm} + 1{\rm /}3\cdot {\rm B}{\rm i}_2{\rm O}_ 3{\rm} + {\rm Te}{\rm O}_2{\rm} + 1{\rm /}2\cdot {\rm O}_2\displaystyle{{\Delta ( 950\,{\rm ^\circ C}) } \over {{\rm AIR}}}\to {\rm B}{\rm a}_2{\rm B}{\rm i}_{2/3}{\rm Te}{\rm O}_6{\rm} + 2\cdot {\rm C}{\rm O}_2$$
$${\rm SrC}{\rm O}_3{\rm} + {\rm L}{\rm a}_2{\rm O}_3{\rm} + {\rm NiO} + 1{\rm /}2\cdot {\rm F}{\rm e}_2{\rm O}_3{\rm} + 1{\rm /}2 \cdot {\rm N}{\rm b}_2{\rm O}_5\displaystyle{{\Delta ( {{\rm 1200 \,^\circ C}} ) } \over {{\rm AIR}}} \to {\rm SrL}{\rm a}_2{\rm NiFeNb}{\rm O}_9{\rm} + {\rm C}{\rm O}_2 $$B. X-ray powder diffraction
Diffraction data of the Ba2Bi0.572TeO6±δ and SrLa2NiFeNbO9 samples were collected at room temperature on a D2 PHASER diffractometer, with the Bragg-Brentano geometry, using a copper anti-cathode tube as the radiation source (Kα 1, Kα 2) of the wavelengths λ(Kα 1) = 1.54056 Å and λ(Kα 2) = 1.54439 Å with 30 kV and 10 mA, Soller slits of 0.02 rad on incident and diffracted beams; divergence slit of 0.5°; anti-scatter slit of 1°; receiving slit of 0.1 mm; with sample spinner, and a Lynxeye detector type with a maximum global count rate >1 000 000 000 cps. The Ba2Bi0.572TeO6±δ pattern was scanned in steps of 0.010142° (2θ), between 15 and 105° (2θ) with a fixed-time counting of 2 s per step; while the SrLa2NiFeNbO9 pattern was scanned through steps of 0.020283° (2θ) between 10 and 70° (2θ). The WinPlotr software (Roisnel and Rodríguez-Carvajal, Reference Roisnel and Rodríguez-Carvajal2001) integrated in the FullProf program (Rodríguez-Carvajal, Reference Rodríguez-Carvajal1990) was used to determine the crystal structures by means of the Rietveld method (Rietveld, Reference Rietveld1969). The peak-shape was described by a pseudo-Voigt function using linear interpolation between set background points with refinable heights.
The following structural and instrumental parameters: scale factor, unit-cell parameters, pseudo-Voigt corrected for asymmetry parameters, FWHM parameters (U, V, and W), preferred orientation, atomic positions, and isotropic thermal factors are included in the refinement of the investigated XRD patterns.
III. RESULTS AND DISCUSSION
A. Crystallite size and microstrain
The crystallite size of these two studied compounds was determined by means of X-ray line broadening using the Scherrer equation and the Williamson-Hall (W–H) method. X-ray peak broadening may be due to crystallite size, microstrain, instrumental profile, temperature factors, and solid solution inhomogeneity. The Scherrer equation can be described by the following expression (Scherrer, Reference Scherrer1918):
$$D = \displaystyle{{K\lambda } \over {\beta _{\rm Sch}\cos \theta }}$$where D is the average crystallite size in (nm), K is the Scherrer constant equal to 0.9, λ is the X-ray wavelength (Cu Kα average = 1.54178 Å), θ is the Bragg angle of the most intense X-ray diffraction peak and β Sch is the grain size broadening (i.e. full width at half maximum) of the highest peak. Herein, the crystallite size was deduced from the XRD pattern in the strongest peak at 2θ ≈ 29.60° for Ba2Bi0.572TeO6±δ and 2θ ≈ 31.93° for SrLa2NiFeNbO9.
According to the Williamson-Hall technique, the strain-induced broadening that results from imperfections and distortions was described by the following equation (Williamson and Hall, Reference Williamson and Hall1953):
$$\varepsilon {\rm} = \displaystyle{{\beta _{\rm s}} \over {4\tan \theta \;}}$$where ε ≈ Δd/d is the upper limit on the lattice distortion, and β s is the strain broadening in the material. Assuming that the contributions of particle size and strain to the diffraction line broadening are unrelated to each other and that they both have a Cauchy-type profile, the observed peak breath βhkl can be considered as the sum of the β s and β Sch widths (El Hachmi and Manoun, Reference El Hachmi and Manoun2023):
$$\beta _{hkl}{\rm} = \beta _{\rm s} + \beta _{\rm Sch}$$and from Eqs (1) and (2), we get
$$\beta _{hkl}\cos \theta {\rm} = \varepsilon ( 4\sin \theta ) {\rm} + \displaystyle{{K\lambda } \over D}$$By plotting the values of βhkl cosθ on the y-axis against 4sinθ on the x-axis, we can determine crystallite size directly from the y-intercept (Kλ/D) and microstrain from the slope (ɛ) of the linear regression line. The values for the crystallite size and microstrain of these two compounds are summarized in Table I. It can be noted that the crystallite size derived by the Williamson-Hall method is larger than that obtained by the Scherrer formula.
TABLE I. Details of Rietveld refinement, crystallite size, and microstrain.
B. Crystal structure determination and structure description
At room temperature, the XRD patterns of the two compounds Ba2Bi0.572TeO6±δ and SrLa2NiFeNbO9 are successfully identified by the PDF2 database (Gates-Rector and Blanton, Reference Gates-Rector and Blanton2019) integrated into the HighScore plus software (Degen et al., Reference Degen, Sadki, Bron, König and Nénert2014) and analyzed using the DICVOL program (Louër and Boultif, Reference Louër and Boultif2014) to adopt a single phase related to perovskite structure. Previously, the Ba2Bi2/3TeO6 structure was reported to crystallize in cubic symmetry with a Fm–3m space group (Park and Woodward, Reference Park and Woodward2000). The crystal structures of the two studied compounds were determined using the Rietveld analysis method (Rietveld, Reference Rietveld1969).
Table I gives the crystal data, data collection, and details of the Rietveld refinements, including lattice parameters, cell volumes, crystal system, space group, and various statistical agreement factors. The values of the reliability factors, R B, R F, R p, R wp, and R exp, and the goodness-of-fit (χ 2), are small and indicate that we have obtained a good quality of the refinements. The quality of the fits is also confirmed by Figures 1 and 2, which provide excellent agreement between the experimental (dots) and theoretical (solid line) patterns. For the Ba2Bi0.572TeO6±δ compound, the impurity Ba0.89Bi8.11O13.05 (PDF# 00-045-0289) was included in the refinement as a second phase crystallizing in a rhombohedral symmetry R–3m (No. 166) with lattice constants a = 3.9942 and c = 28.4667 Å. Based on the most intense peaks of both phases, the impurity content is estimated at around 3.5%. The crystal structure presented in Figures 3 and 4 for these two studied compounds were drawn by means of the Vesta software (Momma and Izumi, Reference Momma and Izumi2011).
Figure 1. Final Rietveld plot of the triclinic compound Ba2Bi0.572TeO6±δ. The experimental pattern is represented by dots (red) and the calculated pattern is represented by a solid line (black). The vertical (green) marks indicate the Bragg positions of the main phase and the impurity. The lower curve (blue) is the difference diagram.
Figure 2. Final Rietveld plot of the orthorhombic compound SrLa2NiFeNbO9. The experimental pattern is symbolized by dots (red) and the calculated pattern is represented by a solid line (black). The vertical (green) marks designate the Bragg positions. The lower curve (blue) is the difference diagram.
Figure 3. Structural views of the I–1 triclinic phase Ba2Bi0.572TeO6±δ. It illustrates octahedra tilt effects in accordance with Glazer's notation a –b –c –.
Figure 4. Structural representation of the Pbnm orthorhombic phase SrLa2NiFeNbO9. It shows BO6 octahedra sharing corners in 3D, as well as octahedral tilting effects in accordance with Glazer notation a –a –c +. Ni2+/Fe3+/Nb5+ cations are located inside octahedra. Sr2+/La3+ cations are represented by green spheres.
1. Ba2Bi0.572TeO6±δ
Rietveld refinement analysis reveals that this structure adopts the triclinic symmetry of space group I–1 (No. 2) with unit-cell parameters a = 6.0272(2) Å, b = 6.0367(1) Å, c = 8.5273(3) Å, α = 90.007(7)°, β = 90.061(2)°, and γ = 90.015(4)°. The cell volume is calculated to be 310.26(2) Å3. The number of unit formulas within a cell is Z = 2. Reflections from the triclinic structure with space group I–1 (No. 2) are correctly indexed on the studied XRD pattern; where the reflection conditions can be described as follows: hkl → h + k + l = 2n; 0kl → k + l = 2n; h0l → h + l = 2n; hk0 → h + k = 2n; h00 → h = 2n; 0k0 → k = 2n; 00l → l = 2n (Cockcroft, Reference Cockcroft1999). The double perovskite Ba2LaRuO6 (PDF# 01-085-1103) was used as a starting model for refinement (Battle et al., Reference Battle, Goodenough and Price1983). It is worth mentioning that the triclinic space group I–1 is a non-standard setting of the triclinic space group P–1 (No. 2). Fractional atomic coordinates and isotropic thermal parameters are provided in Table II. The structural information indicates that the formal oxidation state of bismuth was oxidized to +3.5 and that of tellurium was oxidized to +6, under synthesis conditions. The site occupancy fraction for bismuth has been refined alone to 0.572(8), and with the constrained occupancies (1 for each Ba, Te, and oxygen), the Ba2Bi0.572TeO6±δ structure can be considered almost electroneutral. The isotropic displacement parameters B iso (Å2) for Ba, Bi, and Te atoms were refined separately, while for oxygen atoms; they were considered equivalent. It can be seen that the isotropic displacement factor B iso of the Bi atom (1.21(1) Å2) is greater than that of the Ba, Te, and oxygen atoms. These anomalous isotropic displacement factors can be explained by assuming the presence of a possible positional disorder (either static or dynamic) on the bismuth atoms. The two Bi and Te cations were found to occupy octahedral sites of Ci symmetry at 2f (0,1/2,0) and 2g (1/2,0,0) positions, respectively. The Ba2+ and O2– ions were located at 4i (x,y,z) positions of C 1 symmetry.
TABLE II. Atomic coordinates and isotropic temperature factors for triclinic Ba2Bi0.572TeO6±δ and orthorhombic SrLa2NiFeNbO9 phases.
The selected interatomic distances (Å) and angles (deg.) are given in Table III. The average distances 〈Ba–O〉, 〈Bi–O〉, and 〈Te–O〉 obtained at room temperature after Rietveld refinement of the Ba2Bi0.572TeO6±δ structure are about the same as those found in other perovskite compounds containing Ba2+, Bi3+, and Te6+ cations: Bi3+MTe6+O6 (M = Cr3+, Mn3+, Fe3+) (Kim et al., Reference Kim, Deng, Fischer, Lapidus, Stephens, Li and Greenblatt2016); Ba2Bi3+MO6 (M = Bi5+, Sb5+) (Kennedy et al., Reference Kennedy, Howard, Knight, Zhang and Zhou2006); and are very close to those calculated from Shannon's effective ionic radii: Ba2+ (1.61 Å), Bi3+ (1.03 Å), Te6+ (0.56 Å), and O2− (1.40 Å) for coordination number 12 and 6 (Shannon, Reference Shannon1976). The Bi and Te cations are surrounded by 6 oxygen ions, and the Ba2+ cations are surrounded by 12 oxygen anions.
TABLE III. Selected interatomic distances (Å) and angles (°).
The structural view of the triclinic compound Ba2Bi0.572TeO6±δ with space group I–1 (No. 2) is made up of alternating octahedra sharing corner by means of oxygen atoms, as displayed in Figure 3. Along the c-axis, the BiO6 and TeO6 octahedra are linked by oxygen atoms O(3) of 4i (x,y,z) positions with x = 0.494(13), y = 0.018(18), and z = 0.225(11). Due to the positions occupied by the Bi and Te atoms in sites 2f (0,1/2,0) and 2g (1/2,0,0), respectively, and by the O(3) oxygen atoms, looking through the c-axis, the bond angle Bi–O3–Te is found to be 174°. In the ab-plane, the octahedra are connected by the oxygen atoms O(1) and O(2) at sites 4i (x,y,z), where the bond angles Bi–O1–Te and Bi–O2–Te are calculated to be 163° and 151°, respectively. The tilting system conforms to Glazer's notation (a –b –c –); it indicates that the octahedra tilting out-of-phase along the three pseudo-cubic directions [100]p, [010]p, and [001]p with an unequal amplitude, as derived for the ordered double perovskites with an I–1 (No. 2) space group of triclinic crystal system (Battle et al., Reference Battle, Goodenough and Price1983). Each BiO6 octahedron consists of six long distances between Bi and O with an average distance of 2.37 Å. While the TeO6 octahedron consists of six short distances between Te and O, with an average distance of 1.96 Å. The volume of the BiO6 octahedron is calculated to be 17.204 Å3; and is larger than that observed in the TeO6 octahedron which is 9.673 Å3.
Since the cell angles determined in the triclinic symmetry are very close to 90°, we attempted to resolve this structure in the Pbnm orthorhombic space group, but found that the coordination of the cations on the A and B-sites increased to 16 instead of 12 for Ba and to 8 instead of 6 for the Bi/Te atoms. For this reason, we concluded that the orthorhombic symmetry is not appropriate for the Ba2Bi0.572TeO6±δ structure.
2. SrLa2NiFeNbO9
This compound was analyzed using the Rietveld refinement method and revealed that its crystal structure adopts an orthorhombic system of the Pbnm (No. 62) space group with lattice parameters a = 5.6038(5) Å, b = 5.5988(4) Å, and c = 7.9124(6) Å. The cell volume was calculated to be 248.25(4) Å3. The number of unit formulas contained in a cell is Z = 4. It is important to note that the orthorhombic structure with the Pbnm (No. 62) space group is properly identified on the XRD pattern as the single perovskite Sr0.2La0.8Ni0.8Te0.2O3 (PDF# 01-087-0276); where the reflection conditions can be expressed as follows: 0kl → k = 2n; h0l → h + l = 2n; h00 → h = 2n; 0k0 → k = 2n; 00l → l = 2n (Cockcroft, Reference Cockcroft1999). Impurity phases were not detected in XRD pattern of the as-synthesized sample.
To justify the orthorhombic Pbnm model, we also tried to refine the structure in the monoclinic space group P21/n as was proposed earlier; by introducing the parameters a = 5.6132 Å, b = 5.5973 Å, c = 7.9036 Å, and β = 90.01° (PDF# 01-088-0137), and the atomic coordinates of Sr/La at 4i (x,y,z) site, Ni/Fe/Nb(1) at 2c (0,1/2,0) site, Ni/Fe/Nb(2) at 2d (1/2,0,0) site, and oxygen atoms at 4i (x,y,z) site. This model did not refine well at all stages. For example, the full divergence occurs when refining the FWHM (U, V, and W) and isotropic displacement parameters. It seems that the P21/n structural model is not very satisfactory.
Atomic positions, isotropic displacement parameters, occupancy, and site symmetry are provided in Table II. The isotropic displacement parameters (B iso) for the oxygen atoms were taken as equivalent, the same for the Ni/Fe/Nb atoms at 4b-sites, and Sr/La atoms at 4c-sites. It can be noted that the isotropic displacement factor (B iso) of Ni/Fe/Nb atoms (0.18(7) Å2) is quite low compared to that of Sr/La atoms 1.05(9) Å2, assuming that B-site cations are more ordered than A-site cations. The fraction of site occupancy for all elements was not refined. Despite some differences in ionic size and valence charge, the three elements Ni2+, Fe3+, and Nb5+ have only one crystallographic B-site. As well, the two elements Sr2+ and La3+ have only one crystallographic A-site in the AA′2B2B′O9 triple perovskite structure. The Sr2+/La3+ cations and O2–(1) anions were located at the 4c (x,y,1/4) positions of the Cs symmetry. The Ni2+, Fe3+, and Nb5+ cations were arranged at the 4b (1/2,0,0) positions with Ci symmetry, and the O2–(2) anions were distributed at the 8d (x,y,z) sites with C 1 symmetry. The Sr2+/La3+ cations are surrounded by 12 oxygen ions, and the Ni2+/Fe3+/Nb5+ cations are surrounded by 6 oxygen anions. The selected bond lengths (Å) and angles (deg.) obtained from the refined XRD data of the SrLa2NiFeNbO9 structure are listed in Table III.
The average distances 〈Sr/La–O〉 and 〈Ni/Fe/Nb–O〉 obtained at room temperature are comparatively very close to those found in other perovskite compounds containing Sr2+, La3+, Ni2+, Fe3+, and Nb5+ cations; such as SrLa2Ni2TeO9 (Sena et al., Reference Sena, Hadermann, Chin, Hunter and Battle2016), Ba3NiNb2O9 (Lufaso, Reference Lufaso2004), and Sr3Fe2+xMo1–xO9–3x/2 (El Hachmi et al., Reference El Hachmi, Manoun, Sajieddine, Tamraoui and El Ouahbi2021), and are roughly identical to the distances calculated from Shannon's effective ionic radii; where Sr2+ (1.44 Å), La3+ (1.36 Å), Ni2+ (0.69 Å), Fe3+ (0.55 Å), Nb5+ (0.64 Å), and O2− (1.40 Å) those are for 12- and 6-fold coordination (Shannon, Reference Shannon1976). The structural lattice of the orthorhombic phase (space group Pbnm, Z = 4) of the triple perovskite SrLa2NiFeNbO9 is composed of octahedra with shared corners in three dimensions (3D), as depicted in Figure 4. Along the c-axis, the BO6 octahedra (B = Ni/Fe/Nb) are linked by O(1) oxygen atoms of positions (x,y,1/4); where the bond angle B–O1–B is calculated to be 168.7°. However, in the ab-plane, the BO6 octahedra are connected by O(2) atoms of positions (x,y,z) and the bond angle B–O2–B is found to be 156.3°.
The interatomic distances between Ni/Fe/Nb and O(1) were calculated to be 1.988(2) Å, and between Ni/Fe/Nb and O(2) were found to be 1.95(2) and 2.09(2) Å. The volume of the BO6 octahedra was calculated to be 10.7127 Å3. The octahedral lattice corresponds to the Glazer tilting system (a –a –c +); it indicates that the octahedra tilt out-of-phase along the two [100]p and [010]p pseudo-cubic axes with an equal amplitude and in phase along the [001]p axis with a different amplitude, as determined for the “orthorhombic” ABO3 simple perovskites with the Pbnm (No. 62) space group (Mitchell et al., Reference Mitchell, Welch and Chakhmouradian2017). The I/I o intensities of the XRPD data presented in Tables IV and V are obtained from the calculated intensities of the Rietveld refinement and the observed intensities of all peaks. The values of the d-spacings d(Å) were calculated for Cu Kα 1 radiation of wavelength 1.54056 Å.
TABLE IV. Powder diffraction data of Ba2Bi0.572TeO6±δ (Cu Kα 1, λ = 1.54056 Å).
TABLE V. Powder diffraction data of SrLa2NiFeNbO9 (Cu Kα 1, λ = 1.54056 Å).
IV. CONCLUSION
The crystals of the Ba2Bi0.572TeO6±δ and SrLa2NiFeNbO9 perovskites were obtained by conventional solid-state reaction techniques. Their structures were determined at room temperature from XRPD data using the Rietveld method. The crystal structure of the Ba2Bi0.572TeO6±δ phase adopts an I–1 (No. 2) triclinic system with unit-cell parameters a = 6.0272(2) Å, b = 6.0367(1) Å, c = 8.5273(3) Å, α = 90.007(7)°, β = 90.061(2)°, and γ = 90.015(4)°. The Bi and Te cations fully occupy the octahedral sites at positions 2f (0,1/2,0) and 2g (1/2,0,0), respectively. Ba2+ cations and O2– anions are located at 4i (x,y,z) sites. The average distance of 〈Bi–O〉 is 2.37 Å, and that of 〈Te–O〉 is 1.96 Å. The BiO6 and TeO6 octahedra share corners via oxygen atoms. The tilt system is given by the notation (a –b –c –); this means that the octahedra tilting out-of-phase along the three pseudo-cubic axes [100]p, [010]p, and [001]p with an unequal amplitude. On the other hand, Rietveld analysis of the crystal SrLa2NiFeNbO9 reveals that its structure adopts an orthorhombic space group Pbnm (No. 62) with lattice constants a = 5.6038(5) Å, b = 5.5988(4) Å, and c = 7.9124(6) Å. The BO6 octahedra (B = Ni/Fe/Nb) sharing the corners in 3D. Along the c-axis, the octahedra are connected by O(1) atoms of (x,y,1/4) positions; while in the ab-plane, they are linked by O(2) atoms of (x,y,z) positions. The bond angle of B–O1–B is 168.7° and that of B–O2–B is 156.3°. The octahedral lattice corresponds to the tilt pattern (a –a –c +); it indicates that the octahedra tilt out-of-phase along the two [100] and [010] axes and in phase along the [001] axis. The three Ni, Fe, and Nb elements have only one crystallographic B-site; similarly, the Sr and La elements have only one crystallographic A-site in the structure of AA′2B2B′O9 triple perovskite. The Sr2+/La3+ cations and O2– (1) anions were located at the 4c (x,y,1/4) positions. The Ni2+, Fe3+, and Nb5+ cations were situated at the 4b (1/2,0,0) positions, and the O2– (2) anions were distributed at the 8d (x,y,z) sites.
V. DEPOSITED DATA
Selected powder patterns from this XRD study have been submitted to ICDD for inclusion in the Powder Diffraction File. The Crystallographic Information Framework (CIF) files containing the results of the Rietveld refinement (including the raw data) were deposited with the ICDD. The data can be requested at [email protected].
CONFLICT OF INTEREST
The authors have no conflicts of interest to declare.
Open access
