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 RMnO 3 (R=La, Pr, Nd) and LaMn 1 - x Ga x O 3 (0≤x≤0.5). The data reveal two well-defined transition temperatures T* and T J T ; at T T J T , short-range orbital fluctuations stabilize a conductive, ferromagnetic phase. T* and T J T vary linearly with , as does the Neel temperature T N , in the RMnO 3 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 - x Ga x O 3 systems for 0≤x<0.4. The phase diagrams for the RMnO 3 and RNiO 3 families show common features; differences in their properties are due to a Hund intra-atomic exchange in RMnO 3 that is not present in RNiO 3 perovskites and to a larger O 2p component in the e g orbitals of the RNiO 3 family.