MONOCLINIC HIGH-PRESSURE PHASE OF AGGAS2

Single-crystal x-ray diffraction study under high pressure has been ca rried out on AgGaS2 to obtain crystallographic information on the stab le phase in the pressure range between 4.2 and 11.6 GPa. The high-pres sure phase has monoclinic symmetry with space group Cc, a=8.065(4) Ang strom, b=8.039(1) Angstrom, c=6.226(3) Angstrom, and beta=128.50(3)deg rees at 5.6 GPa. The monoclinic cell can be approximated as a pseudote tragonal cell of a(pt)=b(pt)=5.694 Angstrom, c(pt)=9.750 Angstrom, alp ha(pt)=beta(pt)=88.70 degrees, and gamma(pt)=89.82 degrees on the basi s of the conventional chalcopyrite-type unit cell. The high-pressure p hase is assumed to have a distorted chalcopyrite-type structure. The p hase transition is reversible. A transformation twinning was observed in the high-pressure phase. The four possible twin components are rela ted by the twofold axes present in the symmetry of the ambient-pressur e phase.