PRESSURE-INDUCED DISORDERING OF SODIUM-POTASSIUM SULFATES AND CHROMATES

Raman scattering and energy-dispersive x-ray diffraction in a diamond- anvil cell were used to study the salts NaKSO4, K3Na(SO4)(2), NaKCrO4, and K3Na(CrO4)(2) to pressures of 22.5 GPa. These measurements reveal that the SO4 and CrO4 tetrahedra in the monopotassium salts become di sordered while those in the tripotassium salts stay ordered. This diso rdering occurs without a significant change in the structural arrangem ent of the cations. We suggest that the disordering results from an im peded ferroelastic phase transition, which causes the zone boundaries, and therefore the SO4 and CrO4 tetrahedra, to be distorted, The four salts are related to one of two structures: The tripotassium salts are isostructural, the monopotassium salts are closely related. The two s tructures differ by one symmetry element, which leaves the neighboring tetrahedra inequivalent. It is this difference that causes only one o f the structures to form distorted zone boundaries.