Transport and Magnetic Studies on the Spin State Transition of Pr_1-<i>x</i>Ca_<i>x</i>CoO₃up to High Pressure

Abstract

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