Emergence of a Cubic Phase Stabilized by Intermetallic Charge Transfer in (1 – x)PbVO₃–xBiCoO₃ Solid Solutions

Abstract

In this study, the crystal structure evolution of solid solutions between giant-tetragonal (GT) perovskite-type PbVO3 and BiCoO3, namely, (1 – x)­PbVO3–xBiCoO3, is reported. The all-proportional solid solutions are successfully synthesized using a high-pressure and high-temperature method. A region of cubic phase is observed in the 0.4 ≤ x ≤ 0.75 range. The tetragonal distortion (c/a ratio) systematically decreases toward the cubic phase region, and these phases coexist in boundary regions. Magnetic susceptibility and soft X-ray absorption spectroscopy measurements reveal that 0.5PbVO3–0.5BiCoO3 possesses a valence state of V5+ and Co2+, which is different from the end members, V4+ and Co3+. The crystal structure changes stem from a self-doping effect induced by intermetallic charge transfer between the V and Co ions on the B-site (V4+ + Co3+ → V5+ + Co2+). This finding shows that tuning an electronic structure by making a solid solution is an effective way to control the stability of the GT structure.

Solid Solutions between PbVO3 and BiCoO3

“…The tetragonality monotonically decreases from each end member. The same tendencies were observed in ref . Slightly different results, compared with ref were obtained on the boundary compositions.…”

“…They were intensively studied as end members in solid solutions in research on negative thermal expansion materials due to large volume contraction during the tetragonal-to-cubic phase transitions and in research on piezoelectric materials. − The tetragonality and ferroelectric transition temperature could be successfully controlled in solid solutions. PbVO 3 and BiCoO 3 perovskites also have a spin degree of freedom and show long-range magnetic orderings at the Néel temperatures of 44 and 470 K, respectively. , A recent study of (1 – x )­PbVO 3 – x BiCoO 3 solid solutions found an intermetallic charge transfer between cobalt and vanadium cations (V 4+ + Co 3+ > V 5+ + Co 2+ ) for intermediate compositions and the stabilization of the cubic phase (instead of the P 4 mm phase) as a result . In this work, we extended the study of the (1 – x )­PbVO 3 – x BiCoO 3 system.…”

Crystal Structures and Electronic States of High-Pressure-Synthesized (1-x)PbVO3-xBiCrO3 Solid Solutions

“…This broadening may result from composition fluctuations owing to phase separation. While an additional phase was observed in the (1 − x)PbVO 3 − xBiCoO 3 solid solutions, no additional changes were observed in the present system [1].…”

“…This tetragonal structure is stabilized by the strong covalency of the Pb-O bonds and the stereochemical activity of the 6s 2 lone pairs [4]. Furthermore, the Jahn-Teller effect of V 4+ (3d 1 ) contributes to strengthening the distortion [5,6]. The t 2g orbital degeneracy is lifted in the square pyramidal geometry, splitting into stabilized 3d xy and destabilized 3d yz and 3d zx orbitals, as shown in Figure 1.…”

“…High-pressure synthesis is a powerful tool for preparing solid solutions because the applied pressure prevents ion desorption due to Le Chatelier's principle. Recently, we reported a high-pressure synthesis and elucidated the valence states of perovskite-type PbVO 3 − BiCoO 3 solid solutions [1]. An intermetallic charge transfer between vanadium and cobalt ions, that is, V 4+ + Co 3+ → V 5+ + Co 2+ , was observed, which resulted in a polar-to-nonpolar structural transition and a large volume shrinkage.…”