Bulk modulus anomaly in RCoO3 (R=La, Pr, and Nd)

In order to demonstrate the effect of hydrostatic pressure and chemical pressure on crystal structure and the spin-state transition in the perovskites RCoO3 (R=La, Pr, and Nd), x-ray diffraction has been carried out under pressure up to 80kbar. A sharp difference of the bulk modulus found between the higher-spin LaCoO3 and PrCoO3 and the low-spin NdCoO3 has been interpreted to reflect a pressure-induced spin-state transition in LaCoO3 and PrCoO3. A change in the bandwidth of the σ bonding electrons due to the structural distortion has been shown to be the driving force for the spin-state transition caused by chemical pressure. On the other hand, the changes in this bandwidth must be overcompensated by the cubic-field splitting resulting from a shorter Co-O bond length in order to account for the spin-state transition under hydrostatic pressure.