Reentrant Structural Transitions and Collapse of Charge and Orbital Orders in Quadruple Perovskites.

Charge and orbital degrees of freedom determine properties of many materials, and are central to many important phenomena. At high temperatures, thermal fluctuations overcome them, and high-symmetry structures are realized. On decreasing temperature, different charge- and orbital-order transitions take place accompanied by symmetry lowering. Remarkable exceptions to this general tendency, realized in Cu-doped BiMn O quadruple perovskites, are presented. Introduction of Cu produces mixtures of Mn and Mn and charge degree of freedom. BiCuMn O (and compositions in the vicinity) exhibits well-defined 1:3 charge order of Mn and Mn and orbital order of Mn near room temperature, but both charge and orbital orders collapse below about 115 K with the reentrance of the high-temperature cubic Im3‾ phase. What is interesting the collapse can be controlled by a magnetic field even without long-range magnetic order, and the collapsed phase shows nearly zero thermal expansion.