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 $R{\mathrm{MnO}}_3$ $(R=\mathrm{La},$ Pr, Nd) and ${\mathrm{LaMn}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_3$ $(0\mathrm{}<~x<~0.5).$ The data reveal two well-defined transition temperatures $T^{*}$ and $T_{\mathrm{JT}};$ at $T<\mathrm{}{T}^{*},$ a cooperative Jahn-Teller ordering of the occupied $e_g$ orbitals of the ${\mathrm{MnO}}_{6/2}$ octahedra stabilizes an antiferromagnetic-insulator phase, and at $T>\mathrm{}{T}_{\mathrm{JT}},$ short-range orbital fluctuations stabilize a conductive, ferromagnetic phase. $T^{*}$ and $T_{\mathrm{JT}}$ vary linearly with $〈{\cos }^2\phi 〉,$ as does the Néel temperature $T_{\mathrm{N}},$ in the $R{\mathrm{MnO}}_3$ 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 ${\mathrm{LaMn}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_3$ systems for $0<~x<\mathrm{}\mathrm{0.4.}\mathrm{}$ The phase diagrams for the $R{\mathrm{MnO}}_3$ and $R{\mathrm{NiO}}_3$ families show common features; differences in their properties are due to a Hund intra-atomic exchange in $R{\mathrm{MnO}}_3$ that is not present in $R{\mathrm{NiO}}_3$ perovskites and to a larger O $2p$ component in the $e_g$ orbitals of the $R{\mathrm{NiO}}_3$ family.