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Combined ab initio and experimental study of Cr doping into bismuth titanate pyrochlore was carried out for the first time. Accurate firstprinciples density functional theory calculations were performed considering a Hubbard U correction (DFT+U) to account for on-site Coulomb interactions of the Cr 3d states. The possibility to synthesize a novel pyrochlore-type compound with a high dopant content Bi 1.5 Cr 0.5 Ti 2 O 7 (Bi site doping) in a fine powder state was shown via coprecipitation method, while the single-phase Bi 2 Ti 1.5 Cr 0.5 O 7 (Ti site doping) could not be obtained. Detailed descriptions of thermostability, structural, optoelectronic, and magnetic properties of Cr-doped pyrochlores in the fine (particles size 100− 300 nm) and "bulk" (1−50 μm) powder states are presented based on the well-matched results of theoretical and experimental investigations. According to the Rietveld refinement of the X-ray diffraction data, Bi 1.5 Cr 0.5 Ti 2 O 7 compound is an A-site deficient pyrochlore (Bi 1.38 Cr 0.30 )-(Ti 1.84 Cr 0.16 )O 6.44 with chromium distribution between both cationic sites. Metastability of fine powder Cr-containing pyrochlore phase was revealed during long-thermal annealing, while the bulk powder sample was stable up to its melting point 1230 °C. According to the study of electronic structure and optical properties, Cr-doped pyrochlores are wide-band semiconductors with light absorption in the range of 300−500 nm and perspective as photocatalytic active materials under visible light irradiation. Paramagnetic behavior with effective magnetic moment 3.92 μB (Bi 1.6 Cr 0.1 Ti 2 O 7−δ ) and 3.01 μB (Bi 1.5 Cr 0.5 Ti 2 O 7 ) was experimentally observed. All chromium in magnetically diluted pyrochlore Bi 1.6 Cr 0.1 Ti 2 O 7−δ exists in the form of Cr 3+ monomers, whereas in the more concentrated magnetic Bi 1.5 Cr 0.5 Ti 2 O 7 composition Cr 3+ -O-Cr 3+ dimers may also be present, with a fraction equal to 0.39. This investigation constitutes the first approach to the electronic, structural, optical, and magnetic properties of d-elements doped bismuth titanate pyrochlores from experimental and theoretical viewpoints, emphasizing the power of DFT+U to provide insights and to complement the experimental characterization of these new compounds.

The photocatalytic properties of Bi2–x Ti2O7–1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi2Ti2O7) and 550 °C (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 phase in comparison with the Bi2Ti2O7 phase. The estimated crystallite size is 30–40 nm with noticeable agglomerates of about 100–300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510–580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of −33 mV (Bi2Ti2O7) and −27 mV (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). The use of the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical E g values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/– eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi2Ti2O7 shows the highest activity in comparison with Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

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Solid electrolytes with high values of lithium-ion conductivity are required for the creation of high-energy lithium and lithium-ion power sources, and compounds with a garnet structure based on Li7La3Zr2O12 (LLZO) are one of the candidate materials for this purpose. In the present work, solid electrolytes of the Li7–x La3Zr2–x Ta x O12 system with x = 0.0–2.0 were synthesized using the sol–gel method. According to X-ray diffraction analysis, all of the compounds with x ≥ 0.1 have the same cubic modification with the space group Ia3̅d. However, an increase in Ta concentration affects the short-range order crystal structure of these materials, resulting in higher local distortions, which was shown by pair distribution function (PDF) analysis. Particularly, the PDF data indicate an increase in the probability of Li ions to locally occupy not only two typical positions, Li196 h and Li224 d, but also a third one, Li348 g. The maximum value of lithium-ion conductivity in the studied system was observed for the Li6.4La3Zr1.4Ta0.6O12 compound (i.e., x = 0.6) and had the value of 1.4 × 10–4 S cm–1 at 25 °C. This is consistent with the results of density functional theory (DFT) modeling, which confirmed that a moderate Ta-doping (up to x < 1.0) is most suitable for enhancing Li diffusion in LLZO materials. A combination of DFT modeling, structural characterization of the short and average structures, and conductivity measurements in this work allowed getting insight into this important class of Li-conducting oxides and ideas on improving their properties.

The obtained results explain features of LaScO3 optical properties and the effect of acceptor doping and H2 and H2O uptake on them.

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