Thesis (M.Sc.)--Chulalongkorn University, 2013

The ScH₃ transforms from hcp phase to intermediate phase to fcc phase under high pressure, however, the exact structure of the intermediate phase was not realized for the exact structure. The phase transition of ScH₃ had been investigated by the X-ray diffraction, Raman and infrared experiments. The theoretical calculation using density functional theory is consistent with as the experiments that the hcp phase has the enthalpy lower than the fcc phase at 0−22 GPa. The experiment showed that the fcc phase occurs at 46 GPa. The transition pressure obtained in the theoretical study that is lower than that of the experiment can be explained by the sluggishness of the intermediate phase. The structural analysis showed that the intermediate phase has the Cm symmetry. The intermediate phase is among the changing pattern of scandium planes from ABABAB to ABCABC stacking sequence. The d-spacing of the intermediate phase is in good agreement with experiment.