Size criterion for amorphization of molecular ionic solids

Abstract

X-ray-diffraction measurements show that ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$,${\mathrm{nK}}_{2}$${\mathrm{ZnCl}}_{4}$, and ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$ become amorphous at pressures exceeding 10 GPa. In contrast, their ${\mathrm{Cs}}_{2}$${\mathrm{ZnCl}}_{4}$ and ${\mathrm{K}}_{2}$${\mathrm{SeO}}_{4}$ isomorphs undergo crystal-crystal phase transitions and do not amorphize at the highest pressures measured \ensuremath{\sim}60 GPa. This indicates that the tendency of ${\mathrm{A}}_{2}$${\mathrm{BX}}_{4}$ molecular ionic solids to amorphize upon compression depends predominantly on the ratio of the size of the anionic tetrahedral ${\mathrm{BX}}_{4}$ groups to that of the interstitial A cations. This criterion is also consistent with the incommensurate phase behavior exhibited by several of these solids at ambient pressure.