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 CoO 3 (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 equivalent charge transfer within the Co 3+ /Co 4+ subsystem. In the present paper, the VSST taking place in (Pr 0.7 Sm 0.3 ) 0.7 Ca 0.3 CoO 3 at T*  90 K is investigated by magnetization and heat capacity measurements. First , we quantitatively characterized the jumps in magnetic susceptibility () 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 lowspin (LS) and high-spin (HS) states. In the frame of this description, we demonstrate that the jumps in both  and S can be associated to 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 intermediatespin (IS) state of Co 3+ above T*.

Electrical resistivity anomaly, valence shift of Pr ion, and magnetic behavior in epitaxial (Pr1-yYy)1-xCaxCoO3 thin films under compressive strain

“…In the bulk (Pr 1-x RE y ) 1-x Ca x CoO 3 systems under ambient pressure, which show the simultaneous MI-SS transition, a volume contraction of $1% took place below T MI $ 100 K. 44 The entropy change makes DS $ 5 JÁK À1 Ámol À1 , 45,46 which means that the internal energy of the low-temperature phase is about DSÁT MI $ 500 JÁmol À1 lower, compared with the high-temperature phase. The present PYCCO films are fixed on the substrate and the lattice parameters and the unit cell volume are difficult to change.…”

Excitonic condensation of strongly correlated electrons: The case ofPr0.5Ca0.5CoO3

“…A decade ago, Tsubouchi et al [1, 2] observed a metal-insulator transition in Pr 0.5 Ca 0.5 CoO 3 associated with a drop of magnetic susceptibility and a sharp peak in the specific heat indicating the collective nature of the transition. Subsequently, the transition was observed in other PCCO materials with x and y in the ranges 0.2-0.5 and 0-0.3, respectively [3][4][5]. Despite the evidence for a continuous, or very weakly first order, phase transition and the experimental effort [6] no long-range order could be identified.…”

Thermodynamic model of the coupled valence and spin state transition in cobaltates

“…A quite unique type of transition takes place in a class of mixed valent cobalt oxides, involving both a change in the mean valence of cobalt (Co 3+ /Co 4+ ) and in the spin state of Co 3+ (low-spin/high-spin) (for an overview, see for instance [1,2] and references therein). This «valence and spin state transition» (VSST) strongly impacts all the physical properties, e.g., the magnetic susceptibility, the heat capacity, as well as the electrical and thermal conductivities [3][4][5][6][7][8][9][10][11][12]. There is no change of space group associated to this transition, but one observes a large reduction in the unit cell volume when crossing the VSST upon cooling [3,10,[13][14][15].…”