title: Jumps in entropy and magnetic susceptibility at the valence and spin-state transition in a cobalt oxide.
abstract: A wide family of cobalt oxides of formulation (Pr(1-y)Ln(y))(1-x)Ca(x)CoO3 (Ln being a lanthanide) exhibits a coupled valence and spin-state transition (VSST) at a temperature T*, which involves two concomitant modifications: (i) a change in the spin state of Co(3+) from low-spin (T < T*) to a higher spin state (T > T*) and (ii) a change in the valence state of Pr, from a mixed Pr(4+)/Pr(3+) state (T < T*) to a purely trivalent state (T > T*), accompanied by an ~ 90 K is investigated by magnetization and heat capacity measurements.First, we quantitatively characterized the jumps in magnetic susceptibility (χ) and entropy (S) around T*. Then, these values were compared to those calculated as a function of the variations in the population of the different cationic species involved in the VSST. X-ray absorption spectroscopy experiments recently showed that the higher spin state above T* should be regarded as an inhomogeneous mixture between low-spin (LS) and high-spin (HS) states. In the frame of this description, we demonstrate that the jumps in both χ and S can be associated with the same change in the Co(3+) HS content around T*. This result lends further support to the relevance of the LS/HS picture for the VSST, challenging the currently dominant interpretation based on the occurrence of an intermediate-spin (IS) state of Co(3+) above T*.
authors.affiliation: Laboratoire CRISMAT, ENSICAEN, UMR 6508 CNRS, Caen, France. vincent.hardy@ensicaen.fr
authors.forename: V
authors.initials: V
authors.lastname: Hardy
authors.forename: F
authors.initials: F
authors.lastname: Guillou
authors.forename: Y
authors.initials: Y
authors.lastname: Bréard