Formation of unusual Cr~(5+) charge state in CaCr_(0.5)Fe_(0.5)O_3 perovskite

A new oxide CaCr_(0.5)Fe_(0.5)O_3 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 Cr~(5+)/Fe~(3+) with the presence of unusual Cr~(5+) state in octahedral coordination, although Cr~(4+) and Fe~(4+) occur in the related perovskites CaCrO_3 and CaFeO_3.The randomly distributed Cr~(5+) and Fe~(3+) spins lead to short-range ferromagnetic coupling, whereas an antiferromagnetic phase transition takes place near 50 K due to the Fe~(3+)–O–Fe~(3+) interaction. In spite of the B-site Cr~(5+)/Fe~(3+) disorder,the compound exhibits electrical insulating behavior. First-principles calculations further demonstrate the formation of CaCr_(0.5)~(5+)Fe_(0.5)~(3+)O_3 charge combination, and the electron correlation effect of Fe~(3+) plays an important role for the insulting ground state. CaCr_(0.5)Fe_(0.5)O_3 provides the first Cr~(5+) perovskite system with octahedral coordination, opening a new avenue to explore novel transition-metal oxides with exotic charge states.     

多阶有序钙钛矿多铁性材料的高压制备与物性

钙钛矿是研究磁电多铁性最重要的材料体系之一.由于高的结构对称性,在以往的立方钙钛矿晶格中尚未发现多铁现象.另外,现有的单相多铁性材料很难兼容大电极化和强磁电耦合,严重制约多铁性材料的潜在应用.本文简单综述了利用高压高温条件制备的两个多阶有序钙钛矿氧化物的磁电多铁性质.在具有立方晶格的多阶钙钛矿LaMn_3Cr_4O_(12)中,观察到自旋诱导的铁电极化,表明该材料是第一个被发现的具有多铁性的立方钙钛矿体系.在另一个多阶有序钙钛矿BiMn_3Cr_4O_(12)中,随温度降低该材料依次经历了I类多铁相和II类多铁相.正因为这两类不同多铁相的同时出现,BiMn_3Cr_4O_(12)同时展示了大的电极化强度和强的磁电耦合效应,并且通过不同的电场调控可实现四重铁电极化态,为开发多功能自旋电子学器件与多态存储提供了先进的材料基础.