Pressure effects on charge-ordering transitions in Perovskite manganites

Effects of chemical and external pressures have been investigated on the two types of charge-ordering (CO) systems of perovskite manganites with the use of single-crystal specimens: One is ${\mathrm{Nd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{MnO}}_3$ with moderate CO instability occurring only near x=1/2 and the other is ${\mathrm{Pr}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{MnO}}_3$ with stronger CO instability extending over a wide x region 0.3⩽x⩽0.7. We have partially substituted the Nd ions of ${\mathrm{Nd}}_{1\mathrm{/}2}$${\mathrm{Sr}}_{1\mathrm{/}2}$${\mathrm{MnO}}_3$ with larger La ions or applied external pressure on them with the aim of destabilizing the CO state via an increase of the 3d-electron hopping interaction. An electronic phase diagram relevant to the CO transition was derived for (${\mathrm{Nd}}_{1\mathrm{-}\mathrm{z}}$${\mathrm{La}}_{\mathrm{z}}$${)}_{1\mathrm{/}2}$${\mathrm{Sr}}_{1\mathrm{/}2}$${\mathrm{MnO}}_3$ by such a control of the one-electron bandwidth (W). With an increase of W, the enhanced ferromagnetic double-exchange interaction increases the Curie temperature (${\mathrm{T}}_{\mathrm{C}}$) and suppresses the charge-ordered state with a concomitant antiferromagnetic charge-exchange-type spin ordering (AF-CE). In a narrow window of z (0.4⩽z⩽0.6) or in the pressurized state for z=0.4, another type of antiferromagnetic (perhaps the A type) phase replaces the AF-CE state. Application of external pressure and resultant enhanced carrier itineracy suppresses the CO transitions also for ${\mathrm{Pr}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{MnO}}_3$. For the x=0.30 crystal, application of pressure induces a metallic phase from the low-temperature side in the charge-ordered insulating phase. The pressure-temperature phase diagrams relating to the CO transition or the concurrent insulator-to-metal transition were shown to scale well with the magnetic-field–temperature phase diagrams.