Magnetic and structural transitions in La$_{1-x}A_{1-x}$CoO$_{3}$ ($A$ = Ca, Sr, and Ba)

TL;DR

This study investigates how doping LaCoO_3 with Ca, Sr, and Ba affects its thermal, structural, and magnetic properties, revealing phase transitions and spin-state changes that depend on doping level and temperature.

Contribution

It provides new insights into the doping-dependent phase diagram and spin-state transitions in LaCoO_3-based compounds, supported by comprehensive thermal, structural, and calorimetric data.

Findings

Suppression of the maximum in thermal-expansion coefficient with doping

Identification of multiple phase transitions as a function of doping and temperature

Proposal of an extended phase diagram for La_1-xCa_xCoO_3

Abstract

We report thermal-expansion, lattice-constant, and specific-heat data of the series La_1-xA_xCoO_3 for 0<= x <= 0.30 with A = Ca, Sr, and Ba. For the undoped compound LaCoO_3 the thermal-expansion coefficient alpha(T) exhibits a pronounced maximum around T=50K caused by a temperature-driven spin-state transition from a low-spin state of the Co^{3+$ ions at low towards a higher spin state at higher temperatures. The partial substitution of the La^{3+} ions by divalent Ca^{2+}, Sr^{2+}, or Ba^{2+} ions causes drastic changes in the macroscopic properties of LaCoO3. The large maximum in alpha(T) is suppressed and completely vanishes for x> 0.12. For A = Ca three different anomalies develop in alpha(T) with further increasing x, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data we identify several phase transitions as a function of the doping concentration x and temperature. From these data we propose an extended phase diagram for La_1-xCa_xCoO_3.