Hydrothermal synthesis, structural and thermal characterizations of three open-framework gallium phosphites

Three new gallium phosphites A[Ga(HPO3)2], where A = K (1), NH4 (2), Rb (3), have been synthesized by using mild hydrothermal conditions under autogeneous pressure. Their structures have been determined by single-crystal X-ray diffraction. These compounds crystallize in the hexagonal P63mc space group with a = 5.2567 (2)Å and c = 12.2582 (3)Å for 1, a = 5.2576 (2)Å and c = 12.9113 (4)Å for 2, a = 5.27020 (10)Å and c = 12.7619 (5)Å for 3, with Z = 2 in the three phases. The three compounds are isostructural and exhibit the same framework topology, consisting of a layered structure stacked along the c-axis with the A+ cations located in the interlayer spaces. The [Ga(HPO3)2]- sheets contain GaO6 octahedra interconnected by phosphite units through sharing vertices. Thermal analysis under air atmosphere shows a large range stability for alkali cations containing compounds with decomposition starting around 750K leading to phosphate phases. Under nitrogen, a disproportionation of the phosphite into red phosphorus and phosphates is expected, accompanied by a release of H2.