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

Abstract

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.

Photocatalytic Properties of Bi2–xTi2O7–1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

“…These compositions have been experimentally prepared using the coprecipitation method, which is the cheapest, fastest, and easiest way to obtain nanomaterials for photocatalytic applications. 41 A detailed certification of the compounds was carried out with a subsequent evaluation of the photocatalytic activity of the Bi 2−x Ti 2 O 7−1.5x (x = 0, 0.5) pyrochlores during the decomposition of rhodamine B under visible light irradiation.…”

Low‐temperature dielectric behavior of Sc‐ and In‐doped bismuth titanate pyrochlores

“…Thus, the dopants (Sc or In) can also be located at the sixfold octahedral coordination of the pyrochlore structure ( B ‐site). This phenomenon was also demonstrated for Bi 1.5 Cr 0.5 Ti 2 O 7 , which was found to be the A ‐site deficient pyrochlore (Bi 1.38 Cr 0.30 )(Ti 1.84 Cr 0.16 )O 6.44 with chromium distribution over both cationic sites 45 …”

“…The models with A ‐cations at 96 h or 96 g sites have the same refinement parameters, and we are presenting here 96 h models. This indistinguishability of 96 h or 96 g based on the XRD data was also previously defined 44,45 . Moreover, the location of the O′ at 8 a was chosen for all compositions (Tables 1–4).…”

“…This phenomenon was also demonstrated for Bi 1.5 Cr 0.5 Ti 2 O 7 , which was found to be the A-site deficient pyrochlore (Bi 1.38 Cr 0.30 )(Ti 1.84 Cr 0.16 )O 6.44 with chromium distribution over both cationic sites. 45 One of the required parameters of the dielectric materials for LTCC technology is their compatibility with metallic electrodes (Ag, Cu, etc.) during co-firing.…”

“…This indistinguishability of 96h or 96g based on the XRD data was also previously defined. 44,45 Moreover, the location of the O′ at 8a was chosen for all compositions (Tables 1-4). It should be noted that O′ moving from 8a to 32e site resulted in the better fitting parameters for (Bi 1.6 Sc 0.2 )Ti 2 O 6.7 and (Bi 1.6 M 0.4 )Ti 2 O 7 (M -Sc, In).…”