STRUCTURAL PHASE TRANSITIONS IN IONIC MOLECULAR SOLIDS*. John R. Hardy, Department of Physics & Astronomy, 260 Behlen Lab University of Nebraska-Lincoln Lincoln, NE 68588-0111

A review will be presented of first-principles theoretical predictions of structural instabilities in ionically bonded solids in which one component is a molecular ion. This approach employs modified electron gas density-functional theory, together with a quantum chemical treatment of the molecular ions by the Gaussian computer code and tabulated free ion wave functions for the monatomic constituents, in order to generate parameter-free interionic potentials. These are then employed in molecular dynamics simulations which reproduce the observed structural phase transitions in a wide variety of moderately complex systems.

Specific examples to be discussed will include incommensurate behavior in the A₂BX₄ compound K₂SeO₄ and its isomorphs, and order-disorder transitions in KNO₃ and NaNO₂. These findings will be used to illustrate the degree to which transitions in these and other similar solids are governed by ionic size, geometry, and overall symmetry considerations relating to the packing of molecular groups into spatially periodic arrays.

*This material is based upon work supported by the U.S. Army Research Office under Grant Number DAAH04-93-G-0165.