PRESSURE-INDUCED PHASE TRANSITION OF AgGaS₂. H. Kitahara, N. Ishizawa, F. Marumo* and Y. Noda** Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, *Department of Earth Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya-ku, Tokyo 156, Japan **Department of Materials Science, Faculty of Engineering, Tohoku University, Aramaki, Aoba-ku, Sendai 980-77, Japan

Single-crystal X-ray diffraction study under high-pressures has been carried out on AgGaS₂ to obtain crystallographic information of the high pressure phase in the range 4.2-11.6 GPa employing a diamond-anvil cell. Single-crystals of AgGaS₂ with chalcopyrite structure were grown with the Bridgman method. The crystal is tetragonal, I42d, a=5.7626(5) and c=10.3128(9) Å at ambient pressure. Above the transition point 4.2 GPa, the systematic condition of 2h+1=4n for hhl reflections was broken, showing that the diamond glide planes parallel to {110} have been disappeared in the high-pressure phase. In addition, most diffraction spots except 001 split into two or more in the high-pressure phase. They turned into single ones again when the pressure was released. These results indicate that (1) above 4.2 GPa the diamond glide planes are lost, and (2) a twin or a quadruplet seems to be formed in a way which keeps the c direction constant, and (3) the transition is essentially reversible, though the glide twins were left in the pressure-released crystals. The phase transition looks like a displacive type and the high-pressure phase apparently possesses a symmetry of C2, one of the non-isomorphic subgroups for I42d. The compressibility along the c axis is larger than; that along the a axis under pressures up to transition point. Since the intensities of 001 reflections were scarcely affected by the transition, the structural change may involve the atomic displacements on (001).