Ab Initio and Experimental Insights on Structural, Electronic, Optical, and Magnetic Properties of Cr-Doped Bi₂Ti₂O₇

Combined ab initio and experimental study of Cr doping into bismuth titanate pyrochlore was carried out for the first time. Accurate first-principles 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_1.5Cr_0.5Ti₂O₇ (Bi site doping) in a fine powder state was shown via coprecipitation method, while the single-phase Bi₂Ti_1.5Cr_0.5O₇ (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_1.5Cr_0.5Ti₂O₇ compound is an A-site deficient pyrochlore (Bi_1.38Cr_0.30)(Ti_1.84Cr_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_1.6Cr_0.1Ti₂O_7-δ) and 3.01 μB (Bi_1.5Cr_0.5Ti₂O₇) was experimentally observed. All chromium in magnetically diluted pyrochlore Bi_1.6Cr_0.1Ti₂O_7-δ exists in the form of Cr³⁺ monomers, whereas in the more concentrated magnetic Bi_1.5Cr_0.5Ti₂O₇ composition Cr³⁺-O-Cr³⁺ 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.