Thermal change in hot-pressed ZrS2 was investigated in an argon atmosphere up to 1500°C. During hot-pressing, ZrS2 released some sulfur atoms and the S/Zr ratio decreased. The crystal phase changed from ZrS2 to Zr3S4 at 1300°C-1500°C. In order to investigate this crystal phase change due to the variation in sulfur content, several Zr-S compounds in the range of S/Zr ratio 1.0-2.2 were synthesized. The lattice parameter measurements by the X-ray powder diffraction method revealed that the non-stoichiometric range of ZrS2 is narrow, while that of Zr3S4 is wide.
The bulk density and compressive strength of the sintered body increased with increasing hot-pressing temperature, but the crystal phase change from ZrS2 to Zr3S4 during hot-pressing gave a bad effect to densification and strength of the sintered body.
SEM photographs of the fractured surface of the sintered body showed grain growth along the direction parallel to the basal plane of ZrS2. It is supposed that the diffusion of Zr and S atoms to the direction parallel to van der Waals' layers is easier than that to the perpendicular direction in ZrS2. In Zr3S4, the partially occupied Zr layers extend in many directions and so the directional difference in the mobility of atoms is small compared with the case in ZrS2.
