Fig. 1.

Crystal structure and phase transition in ${\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{r}\mathrm{S}\mathrm{e}}_2$. ($A$) Crystal structure of ${\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{r}\mathrm{S}\mathrm{e}}_2$. Ag, Cr, and Se atoms are shown in red, blue, and green, respectively. Half-red–half-white spheres indicate 50% occupancy of Ag atoms. The blue dashed lines indicate the Se tetrahedron. Below $T_{od}\hspace{0.17em}=$ 475 K, the $\alpha$ sites are fully occupied. Above $T_{od}$, both $\alpha$ and $\beta$ sites are occupied with 50% probability. ($B$) X-ray powder diffraction pattern at 300 K (blue) and 600 K (orange), obtained from APS sector 11-ID-C ($E_i\hspace{0.17em}=$ 105.708 keV). Inset shows the integrated intensities of (003), (006), and (015) Bragg peaks as a function of temperature on heating, plotted as blue, orange, and green dots, respectively. The intensities are suppressed around nominal temperature 500 K.