Simultaneous valence shift of Pr and Tb ions at the spin-state transition in ((Pr$_{1-y}$Tb$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$

TL;DR

This study investigates the simultaneous valence shifts of Pr and Tb ions during a spin-state transition in a cobaltite system, revealing correlated valence changes at low temperatures associated with the metal-insulator transition.

Contribution

It provides the first direct measurement of valence shifts of both Pr and Tb ions during the transition in this specific cobaltite system, highlighting the role of RE ion substitution.

Findings

Pr and Tb ions exhibit increased valence below the transition temperature.

Valence shifts are correlated with the metal-insulator and spin-state transition.

No valence change detected for other rare earth ions like Sm and Eu.

Abstract

Temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at the Pr $L_{3}$- and Tb $L_{3}$-edges was measured for the (Pr$_{1-y}$Tb$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$ system, in which a metal-insulator (MI) and spin-state (SS) transition took place simultaneously at a critical temperature $T_{\rm MI}$. A small increase in the valence of the terbium ion was found below $T_{\rm MI}$, besides the enhancement of the praseodymium valence; the trivalent states, which are stable at room temperature, change to a 3+/4+ ionic mixture at low temperatures. In particular for the $y$=0.2 sample, the average valence determined at 8 K amounts to 3.03+ and 3.25+ for the Tb and Pr ion, respectively. In analogous (Pr$_{1-y}$RE$_{y})_{0.7}$Ca$_{0.3}$CoO$_{3}$ samples (RE=Sm and Eu), in which the MI-SS transition also took place, no valence shift of the RE ion was detected in the XANES spectra at the RE ion $L_{3}$-edge. The role of the substituted RE ion for the Pr-site on the MI-SS transition is discussed.