Structural study of a ${\mathrm{Ag}}_{3.4}{\mathrm{In}}_{3.7}{\mathrm{Sb}}_{76.4}{\mathrm{Te}}_{16.5}$ quadruple compound utilized for phase-change optical disks

The behavior of a quadruple compound, ${\mathrm{Ag}}_{3.4}{\mathrm{In}}_{3.7}{\mathrm{Sb}}_{76.4}{\mathrm{Te}}_{16.5},$ is investigated at various temperatures using the large Debye-Sherrer camera installed in BL02B2 at SPring-8, to elucidate its crystal structure. The low-temperature phase of this crystal has an $A7\mathrm{}$ structure with atoms of Ag, In, Sb, or Te randomly occupying the $6\left(c\right)\mathrm{}$ site in the $R3\mathrm{}¯m$ space group. The crystal lattice thermally expands almost linearly with increasing temperature between 81 K and around 850 K. The crystal structure shows little change between 81 K and around 600 K however, above 600 K, marked changes with increased temperature are observed with respect to the thermal vibration of atoms, interatomic distances and bond angles. The $A7\mathrm{}$ structure transforms at approximately 780 K to a rhombohedral structure that includes only one atom in each unit cell. This change of structure is thought to be a second-order phase transition. The compound ${\mathrm{Ag}}_{3.4}{\mathrm{In}}_{3.7}{\mathrm{Sb}}_{76.4}{\mathrm{Te}}_{16.5}$ can maintain its crystalline phase up to approximately 850 K, but at higher temperatures it changes into an amorphous phase.