Thermophysical properties of the lanthanide sesquisulfides. I. Schottky functions and magnetic and electronic properties of γ-La2S3, γ-Ce2S3, γ-Nd2S3, and γ-Gd2S3

Abstract

Heat capacity measurements of four lanthanide sesquisulfides La₂S₃, Ce₂S₃, Nd₂S₃, and Gd₂S₃, prepared in the γ phase, have been obtained between 6 and 350 K by adiabatic calorimetry. The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed splitting of the ground‐state manifold of the rare earth ions occupying sites of S₄ symmetry in the Th₃P₄ structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce₂S₃ (²F_5/2) are 0, 185, and 358 cm⁻¹; for Nd₂S₃(⁴I_9/2), they are 0, 76, 150, 180, and 385 cm⁻¹. For La₂S₃, which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to 0 K. The entropy for La₂S₃ at 298.15 K (as S⁰/R) is 19.51. Schottky and magnetic ordering at lower temperatures in Ce₂S₃, Nd₂S₃, and Gd₂S₃ preclude such extrapolation techniques. Therefore the entropy at 298.15 K for these compounds {S⁰–S⁰(7 K)}/R, is 21.34, 22.38, and 20.05, respectively.