Crystallographic aspects of the Polymorphism of Silver Chromate and Selenate at high pressures and temperatures

Ag₂CrO₄ II at 25 °C is orthorhombic, space group D-Pnma, with the chrysoberyl structure and a₀, b₀, c₀ = 10.01, 7.01, 5.56 Å. Precipitated Ag₂SeO₄ IV has the thenardite structure. Upon heating to ～600 °C under pressure it transforms sluggishly to Ag₂SeO₄ I, which, upon cooling, then transforms to Ag₂SeO₄ III directly above 8 kbar, or via Ag₂SeO₄ II below 8 kbar. Ag₂SeO₄ III does not easily reconvert to Ag₂SeO₄ IV, and can be quenched to room temperature. It has the same structure as Ag₂CrO₄ II, and the lattice constants a₀, b₀, c₀ = 10.10, 6.95, 5.52 Å. The previously observed transitions in Ag₂SeO₄ at 425 and 537 °C do not involve Ag₂SeO₄ IV and cannot be observed until the IV–I transformation has taken place.