Microscopic analysis of AgCl polymorphism

Abstract

A detailed theoretical study of the structural properties of the phases involved in the pressure-induced polymorphic sequence of AgCl is reported. As many other binary ionic compounds, AgCl crystallizes in the B1 (NaCl-type) structure at ambient pressure and shows a high-pressure B2 (CsCl-type) structure under pressure. However, unlike alkali halides, there is not a direct transformation from one to the other. The greater polarizability of the silver chloride structure enables the stabilization of two other phases in between B1 and B2: the monoclinic (KOH-type) and the orthorhombic (TlI-type, B33) structures. This polymorphic sequence is specially interesting, since a concerted transition pathway within the monoclinic KOH-type symmetry has also been proposed as a transition pathway in the B1 → B2 transformation. This would point at a connection between symmetries that are thermodynamically and kinetically stabilized, with phases that appear under pressure being good candidates for mechanistic analyses. Our calculations yield cell parameters and structural behavior under pressure in overall good agreement with available experimental data for all these phases and extend the information previously observed for the four polymorphs (e.g., compressibilities). Computed transition pressures confirm the B1→KOH-type→B33 sequence. In order to analyze the occurrence of the KOH-type phase between the B1 and B33 phases, we have resorted to the Atoms In Molecules approach to obtain the evolution of charges with pressure. We find that the descent to the KOH-type symmetry provides a continuous change in ionic charges between B1 and B33 phases, which implies a smaller stress. Thus, coupling group–subgroup relationships to chemical bond analysis can provide microscopic insight into why a given symmetry is favored.