Preparations of γ-Pr2S3 and γ-Nd2S3 powders by sulfurization of Pr6O11 and Nd2O3 powders using CS2 gas, and their sintering

In the synthesis of alpha-Pr2S3 via the sulfurization of Pr6O11 powder using CS2 gas, PrS1.7, Pr4O7 and Pr2O2S were formed as intermediate products. At sulfurization temperatures above 1123 K, Pr2O2S was formed in the initial stage of reaction, and beta-Pr2S3 and alpha-Pr2S3 were formed for a shorter period of time at higher temperature instead of Pr2O2S. In the synthesis of alpha-Nd2S3 via the sulfurization of Nd2O3 powder using CS2 gas, only Nd2O2S formed as an intermediate product was formed in the initial stage of reaction, and beta-Nd2S3 and alpha-Nd2S3 were also formed for a shorter period of time at temperatures of 1073 K or higher. beta-Pr2S3 and beta-Nd2S3 were also thought to be oxysulfides having a limiting composition of Pr10S14O and Nd10S14O, respectively. Next, the synthetic powders were sintered by the pressureless sintering technique. When the synthetic powder of alpha-Pr2S3 having a trace of residual beta-Pr2S3 containing a low amount of oxygen was sintered, the compact of gamma-Pr2S3 having a trace of residual beta-Pr2S3 was formed at the sintering temperature of 1973 K. On the other hand, when the synthetic powder of the same alpha- beta-Pr2S3 containing a high amount of oxygen was sintered, the single-phase beta-Pr2S3 compact was formed at the same sintering temperature. Phase transformation of beta-Pr2S3 to gamma-Pr2S3 progressed preferentially with a decrease in the oxygen content of synthetic powder. In the sintering of synthetic powder of alpha-Nd2S3 having a trace of residual beta-Nd2S3, a synthetic powder containing a low amount of oxygen was used. As a result, a single phase of gamma-Nd2S3 could be prepared by sintering at 1673 K for 3.6 ks. In hot-pressed sintering, the densification behavior of gamma-Pr2S3 and gamma-Nd2S3 powder prepared by the pressureless sintering and subsequent milling were also examined. The sintering kinetics could be explained by a grain boundary-diffusion mechanism that had an apparent activation energy of 247 kJ . mol(-1) in gamma-Pr2S3 and 347 kJ . mol(-1) in gamma-Nd2S3.