APSe₆ (A = K, Rb, and Cs):  Polymeric Selenophosphates with Reversible Phase-Change Properties

The ternary alkali selenophosphates KPSe6 and RbPSe6 crystallize in the polar orthorhombic space group Pca2(1) with a = 11.7764(17) A, b = 6.8580(10) A, c = 11.4596(16) A, and Z = 4 for RbPSe6. CsPSe6 crystallizes in the monoclinic space group P2/n with a = 6.877(3) A, b = 12.713(4) A, c = 11.242(4) A, beta = 92.735(7) degrees, and Z = 4. All compounds feature the one-dimensional infinite chain of [PSe2(Se)4-], where each P atom is connected with Se4(2-) bridge. These compounds show reversible glass-crystal transition, and 31P NMR data suggest that crystallization and infinite [PSe(6-)] chain formation are coupled processes.