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

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 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 BiCrTiO (Bi site doping) in a fine powder state was shown via coprecipitation method, while the single-phase BiTiCrO (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, BiCrTiO compound is an A-site deficient pyrochlore (BiCr)(TiCr)O 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 (BiCrTiO) and 3.01 μB (BiCrTiO) was experimentally observed. All chromium in magnetically diluted pyrochlore BiCrTiO exists in the form of Cr monomers, whereas in the more concentrated magnetic BiCrTiO composition Cr-O-Cr 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.