Bulk modulus anomaly inRCoO3(R=La, Pr, and Nd)

Abstract

In order to demonstrate the effect of hydrostatic pressure and chemical pressure on crystal structure and the spin-state transition in the perovskites $R\mathrm{Co}{\mathrm{O}}_3$ ($R=\mathrm{La}$, Pr, and Nd), x-ray diffraction has been carried out under pressure up to $80\mathrm{kbar}$. A sharp difference of the bulk modulus found between the higher-spin $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ and $\mathrm{Pr}\mathrm{Co}{\mathrm{O}}_3$ and the low-spin $\mathrm{Nd}\mathrm{Co}{\mathrm{O}}_3$ has been interpreted to reflect a pressure-induced spin-state transition in $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ and $\mathrm{Pr}\mathrm{Co}{\mathrm{O}}_3$. A change in the bandwidth of the $\sigma$ 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 $\mathrm{Co}\text{–}\mathrm{O}$ bond length in order to account for the spin-state transition under hydrostatic pressure.