Transport and Magnetic Studies on the Spin State Transition of Pr1-xCaxCoO3 up to High Pressure

TL;DR

This study investigates the spin state transition in Pr1-xCaxCoO3 under high pressure, revealing a pressure-dependent phase diagram and the relationship between structure, doping, and spin states through transport, magnetic, structural, and NMR analyses.

Contribution

It provides a detailed phase diagram of Pr1-xCaxCoO3 under pressure and links structural parameters to spin state transitions, highlighting the roles of doping and local structure.

Findings

The low temperature phase expands with pressure.

The transition is not due to order-disorder mechanisms.

Structural factors influence the transition temperature.

Abstract

Transport and magnetic measurements and structural and NMR studies have been carried out on (Pr1-yR'y)1-xAxCoO3 {R'=(rare earth elements and Y); A=(Ca, Ba and Sr)} at ambient pressure or under high pressure. The system exhibits a phase transition from a nearly metallic to an insulating state with decreasing temperature T, where the low spin (LS) state of Co3+ is suddenly stabilized. For y=0, we have constructed a T-x phase diagram at various values of the external pressure p. It shows that the (T, x) region of the low temperature phase, which is confined to a very narrow region around x=0.5 at ambient pressure, expands as p increases, suggesting that the transition is not due to an order-disorder type one. For the occurrence of the transition, both the Pr and Ca atoms seem to be necessary. The intimate relationship between the local structure around the Co ions and the electronic (or spin) state of Co3+ ions is discussed: For the smaller unit cell volume or the smaller volume of the CoO6 octahedra and for the larger tilting angle of the octahedra, the temperature of the transition becomes higher. The role of the carriers introduced by the doping of the A atoms, is also discussed. By analyzing the data of 59Co-NMR spectra and magnetic susceptibilities of Pr1-xCaxCoO3 the energy separations among the different spin states of Co3+ and Co4+ are roughly estimated.