Electronic Structure of Rare-Earth Sesquisulfide Crystals

Abstract

Single crystals of R₂S₃, R = Dy, Gd, Nd, and Y, have been grown by flame fusion in an argon plasma torch and by the Bridgman technique. The purification of R₂O₃ starting materials, synthesis and analysis of compound powders R₂S₃, crystal growth, analysis to determine if single crystals are produced, and measurement of electrical resistivity and thermoelectric power coefficient from 300° to 1300°K and discussed. The electrical measurements indicate highly degenerate semiconductor behavior with ρ and α increasing with temperature in contrast with previous reports for rare-earth tellurides and selenides. The presence of the rare-earth ions in their trivalent state and the absence of divalent ions have been confirmed independently by reflectance spectra from powders, by absorption spectra of single crystals, and by measurement of magnetic susceptibility of single crystals. The materials are unusual because they are semiconductors with the (4f)ⁿ levels of the rare-earth ions spanning the bandgap in ladderlike fashion. The free ion energy levels of Dy³⁺ were obtained by diagonalization of the electrostatic and spin—orbit matrices arising from a perturbation calculation, and least-squares fitting of the calculated levels to experimental values. The Dy₂S₃ spectra are correlated with DyF₃ and Dy₂O₃ results.