Low-Temperature Formation of Cubic Th3P4-type Gadolinium and Holmium Sesquisulfides

Abstract

The formation behavior of gadolinium and holmium sesquisulfides was examined by studying the sulfurization of their inorganic and organic salts such as nitrate, carbonates, oxalates, acetates and octanoate by carbon disulfide (CS2) gas. In previous studies, α-Gd2S3, which is low temperature phase, was produced by the sulfurization of the oxide at 1023 K. In the present study, single phase γ-Gd2S3, which is stable at relatively high temperature, was formed by the sulfurization of octanoate at 1073 K and oxalate at 873 K. It has also been found that gadolinium salts are thermally decomposed at temperatures higher than 500 K. The thermal decomposition leads to the formation of a gadolinium oxide via an oxycarbonate. In the case of holmium sesquisulfides, the sulfurization of holmium oxide and nitrate provide the mixture of δ-Ho2S3 and Ho2O2S impurity, while pure γ-Ho2S3 is exclusively formed from oxalates, acetates, or carbonates. These results reveal that the formation of oxycarbonate such as Gd2O2CO3 and Ho2O2CO3 play an important role in the formation of γ-phase. After sulfurization, the synthetic powder of γ-Ho2S3 was consolidated by pulse electric current sintering to investigate its high-temperature stability. Holmium sesquisulfide transformed from the γ-phase to the δ-phase at sintering temperatures above 1073 K while δ-phase was stable even at a high sintering temperature of 1773 K.