Temperature-induced intersite charge transfer involving Cr ions in A-site-ordered perovskites ACu₃Cr₄O₁₂ (A=La and Y)

Changes in the valence state of transition-metal ions in oxides drastically modify the chemical and physical properties of the compounds. Intersite charge transfer (ISCT), which involves simultaneous changes in the valence states of two valence-variable transition-metal cations at different crystallographic sites, further expands opportunities to show multifunctional properties. To explore new ISCT materials, we focus on A-site-ordered perovskite-structure oxides with the chemical formula AA'₃B₄O₁₂, which contain different transition-metal cations at the square-planar A' and octahedral B sites. We have obtained new A-site-ordered perovskites LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ by synthesis under high-pressure and high-temperature conditions and found that they showed temperature-induced ISCT between A'-site Cu and B-site Cr ions. The compounds are the first examples of those, in which Cr ions are involved in temperature-induced ISCT. In contrast to the previously reported ISCT compounds, LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ showed positive-thermal-expansion-like volume changes at the ISCT transition. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)