The thermal decomposition of ammonium thiocyanate (NH 4 SCN) was studied by thermogravimetry, differential thermal analysis, and mass spectrometry. It occurred in the temperature range from 400 to 530 K. The decomposition products contained NH 3 , CS 2 , H 2 S, and HNCS gases. Rare-earth oxides were reacted with these gases at 1273 K for 8 h in order to prepare rare-earth sulfides. The single tetragonal -La 2 S 3 phase was formed after the sulfurization of La 2 O 3 . In contrast, the single orthorhombic -Gd 2 S 3 phase was formed after the sulfurization of Gd 2 O 3 . These powders were consolidated by pressure-assisted sintering to fabricate the thermoelectric elements.
The electrical resistivity, thermopower, and thermal conductivity have been measured for the lanthanum sesquisulfide ͑La 2 S 3 ͒ of which the crystal phase is controlled by the Ti additive. In all the samples, the thermopower is negative between 300 and 1000 K. The sample with 8 wt % Ti, which consists almost of the cubic ␥ phase, behaves as a degenerate semiconductor. The thermoelectric figure of merit ZT increases with increasing temperature, reaching a value of 0.21 at 1000 K. In contrast, the sample with 2 wt % Ti consists almost of the tetragonal  phase. The transport mechanism can be well explained by the model of the Anderson localization. The ZT value increases abruptly with increasing temperature. At 1000 K, this ZT value is comparable with that of the sample with 8 wt % Ti.
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