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

The RCoO$_{3}$ family undergoes a transition from the low-spin state to a higher spin state with increasing temperature. The onset temperature for this thermally driven, progressive transition is 35 K for LaCoO$_{3}$; the onset temperature increases to 200K for PrCoO$_{3}$ and 300K for NdCoO$_{3}$. At room temperature the population of the low-spin state increases as the ionic size of rare earth reduces from La to Nd. High pressure stabilizes the low-spin state by enlarging the crystal field splitting. The structural study with more pressure sampling points in this work has given a more accurate V-P relationship for the RCoO$_{3}$ family. A linear fitting to the V-P relationship instead of the Birch-Murnaghan (B-M) equation with a parameter B'=4 as default has been obtained for LaCoO$_{3}$. The bulk modulus B is even lower than that reported previously. In contrast, NdCoO$_{3}$ shows a regular B by fitting the V-P curve with the B-M equation. The V-P curve of PrCoO$_{3} $ is irregular and has been explained in terms of both a pressure- induced spin-state transition and a structural transition from an orthorhombic to a rhombohedral phase.