Crystal Fields as a Probe of the Valence Transition in Pr-based Cobaltites

Upon cooling, a unique first-order metal-insulator transition (MIT) has been observed in cobaltite perovskites at certain compositions with Pr on the A-site; this is strongly suspected as a consequence of electron transfer from Pr$^{3+}$ ions to hybridized Co-O orbitals. Here we discuss the crystal field levels of Pr$^{3+}$ and Pr$^{4+}$ ions which we have investigated using inelastic neutron spectroscopy as a function of temperature for three samples: one insulating, one metallic, and one that undergoes the MIT. We show that the insulating compound, PrCoO$_{3-\delta}$, contains Pr$^{3+}$ ions (J=4) whose ground state multiplet is split into nine singlets, and we extract its crystal field parameters. In the hole-doped metal, Pr$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$, which also contains Pr$^{3+}$ ions, the crystal field levels maintain the same energy-level structure but are noticeably broadened. The scattering from the Pr$^{3+}$ crystal fields, though present, is diminished in the sample that undergoes the MIT (Pr$_{0.63}$Y$_{0.07}$Ca$_{0.3}$CoO$_{3-\delta}$), consistent with a decrease in the concentration of Pr$^{3+}$; moreover, the levels are severely broadened. The experiments illustrate the inhomogeneous charge distribution that occurs around the Pr sites upon substitution.