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 $a=8.031\mathrm{}\left(4\right)\mathrm{}\AA ,$ $b=8.014\mathrm{}\left(1\right)\mathrm{}\AA ,$ $c=6.221\mathrm{}\left(2\right)\mathrm{}\AA ,$ and $\beta =128.42\mathrm{}\left(3\right)\mathrm{}°$ at 5.5 GPa. The monoclinic unit cell can be converted into a pseudotetragonal unit cell with $a=b=5.673\mathrm{}\left(2\right)\mathrm{}\AA ,$ $c=9.754\mathrm{}\left(3\right)\mathrm{}\AA ,$ $\alpha =\beta =88.75\mathrm{}\left(3\right)\mathrm{}°,$ and $\gamma =89.87\mathrm{}\left(3\right)\mathrm{}°,$ 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.

• Received 2 September 1999

DOI:https://doi.org/10.1103/PhysRevB.61.3310