Rare-Earth-Containing Selenides and Oxyselenides

Abstract

In this thesis, rare-earth-containing chalcogenides and oxychalcogenides were sought with the aim to identify new crystal structures and to evaluate their potential as optical and thermoelectric materials. They were synthesized in the form of pure microcrystalline powders and single crystals through high-temperature reactions. To address frequently made claims that the bonding in oxychalcogenides can be partitioned separately within the more ionic oxide vs. the more covalent chalcogenide blocks within their structures, electron localization function maps were examined carefully to evaluate the bonding character. Many ternary chalcogenides RE-Tr-Ch (RE = rare-earth metal; Tr = Ga, In, Tl; Ch = S, Se, Te) are known, but most of them (about 75%) are sulfides, while the corresponding selenides and tellurides are still underexplored. Among the series of chalcogenides RE3TrCh6, there remain some ambiguities in the existence of several members and their structures. Several new compounds in the RE3InSe6 series were prepared. They exhibit an unusual structural evolution in which the In atoms occupy tetrahedral or octahedral sites, depending on the RE. They have relatively narrow band gaps in the attractive range of 1.2-1.4 eV, making them suitable as potential photovoltaic materials. The major portion of the thesis was motivated by an initially naive hypothesis that oxychalcogenides could be prepared by successively substituting O for Ch atoms within the existing series of chalcogenides. In this way, Sm3GaSe5O was obtained; however, its structure is new and unrelated to RE3GaSe6. Instead, it contains unprecedented GaSe5 units in trigonal pyramidal geometry. Attempts to substitute other RE components led to two series of oxychalcogenides: La4Ga2Se6O3 and RE4Ga2Se7O2 (RE = Pr, Nd). They adopt new framework structures that can be contrasted with the layered structures more commonly associated with oxychalcogenides. They have relatively small band gaps of 1.7–1.8 eV, as determined from optical diffuse reflectance spectra, which suggests that they could be viable candidates as thermoelectric materials. To evaluate this potential application, electronic and thermal properties were calculated from the first principles.