Orbital order-disorder transition in single-valent manganites

The temperature dependence of the resistance and the thermoelectric power have been measured to high temperatures on single-crystal RMnO₃ (R=La, Pr, Nd) and LaMn_1-xGaₓO₃ $(0<~x<~0.5).$ The data reveal two well-defined transition temperatures T* and TJT; at T<T*, a cooperative Jahn-Teller ordering of the occupied eg orbitals of the MnO6/2 octahedra stabilizes an antiferromagnetic-insulator phase, and at T>TJT, short-range orbital fluctuations stabilize a conductive, ferromagnetic phase. T* and TJT vary linearly with 〈cos²φ〉, as does the N\'eel temperature TN, in the RMnO₃ family where {φ} is the bending of the (180^∘{}-{φ}) Mn-O-Mn bond angle. All three critical temperatures vary linearly with x in the LaMn_1-xGaₓO₃ systems for $0<~x<0.4.$ The phase diagrams for the RMnO₃ and RNiO₃ families show common features; differences in their properties are due to a Hund intra-atomic exchange in RMnO₃ that is not present in RNiO₃ perovskites and to a larger O $2p$ component in the eg orbitals of the RNiO₃ family.