Formation of unusual Cr⁵⁺ charge state in CaCr_0.5Fe_0.5O₃ perovskite

A new oxide CaCr_0.5Fe_0.5O₃ was prepared under high pressure and temperature conditions. It crystallizes in a B-site disordered Pbnm perovskite structure. The charge combination is determined to be Cr⁵⁺/Fe³⁺ with the presence of unusual Cr⁵⁺ state in octahedral coordination, although Cr⁴⁺ and Fe⁴⁺ occur in the related perovskites CaCrO₃ and CaFeO₃. The randomly distributed Cr⁵⁺ and Fe³⁺ spins lead to short-range ferromagnetic coupling, whereas an antiferromagnetic phase transition takes place near 50 K due to the Fe³⁺-O-Fe³⁺ interaction. In spite of the B-site Cr⁵⁺/Fe³⁺ disorder, the compound exhibits electrical insulating behavior. First-principles calculations further demonstrate the formation of CaCr_0.5⁵⁺Fe_0.5³⁺O₃ charge combination, and the electron correlation effect of Fe³⁺ plays an important role for the insulting ground state. CaCr_0.5Fe_0.5O₃ provides the first Cr⁵⁺ perovskite system with octahedral coordination, opening a new avenue to explore novel transition-metal oxides with exotic charge states.