Synchrotron x-ray study of the monoclinic high-pressure structure ofAgGaS2

Abstract

The structure of the monoclinic high-pressure phase of ${\mathrm{AgGaS}}_{2}$ was determined at 5.5 GPa by single-crystal x-ray diffraction using synchrotron radiation and a miniature diamond-anvil cell. The high-pressure phase has the space group symmetry of Cc with cell dimensions $a=8.031(4)\AA{},$ $b=8.014(1)\AA{},$ $c=6.221(2)\AA{},$ and $\ensuremath{\beta}=128.42(3)\ifmmode^\circ\else\textdegree\fi{}$ at 5.5 GPa. The monoclinic unit cell can be converted into a pseudotetragonal unit cell with $a=b=5.673(2)\AA{},$ $c=9.754(3)\AA{},$ $\ensuremath{\alpha}=\ensuremath{\beta}=88.75(3)\ifmmode^\circ\else\textdegree\fi{},$ and $\ensuremath{\gamma}=89.87(3)\ifmmode^\circ\else\textdegree\fi{},$ similar to the chalcopyrite-type tetragonal unit cell. The ${\mathrm{AgS}}_{4}$ tetrahedra undergo a relatively large geometrical deformation reflecting weaker force constants of Ag-S compared to those of Ga-S. On the other hand, changes in the geometrical shape of ${\mathrm{GaS}}_{4}$ are less significant and can be approximated as a rotation rather than a distortion. The two crystallographically independent ${\mathrm{S}}_{6}$ octahedral interstices in the monoclinic high-pressure phase experience discrete degrees of compression, which contributes to a further decrease of the $c/a$ ratio above the phase transition pressure of 4.2 GPa.