Transport and Magnetic Studies on the Spin State Transition of Pr_1−xCa_xCoO₃ up to High Pressure

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Transport and magnetic measurements and structural and NMR studies have been carried out on (Pr_1−yR′_y)_1−xA_xCoO₃ {R′ = (rare earth elements and Y); A = (Ca, Ba and Sr)} at ambient pressure and 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. 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 ⁵⁹Co-NMR spectra and magnetic susceptibility of Pr_1−xCa_xCoO₃, the electronic-energy-separations among the different spin states of Co³⁺ and Co⁴⁺ are roughly estimated.