α-Na₆Pb₃(PS₄)₄, a Noncentrosymmetric Thiophosphate with the Novel, Saucer-Shaped [Pb₃(PS₄)₄]^6- Cluster, and Its Metastable, 3-Dimensionally Polymerized Allotrope β-Na₆Pb₃(PS₄)₄

Abstract

Discovered in a molten polythiophosphate salt reaction, the compound α-Na6Pb3(PS4)4 is a salt containing the molecular cluster [Pb3(PS4)4].6- The C3v symmetry and the saucer shape of this cluster enable columnar stacking along the c-axis. Interestingly, fast cooling of a melt of the stoichiometric composition "Na6Pb3(PS4)4" results in the putative high-pressure form β-Na6Pb3(PS4)4 in which the molecular clusters have essentially polymerized through intercluster Pb−S bonding. β-Na6Pb3(PS4)4 converts slowly (depolymerizes) to the α-form upon annealing below the melting point.