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. Three isostructural layered gallium phosphites, A[Ga(HPO3)2] (A = K, NH4, Rb), have been successfully prepared under hydrothermal conditions and their structure solved by single crystal X-ray diffraction. Thermogravimetric analyses carried under air atmosphere indicate that alkali cations containing compounds are stable up to 750K. AGaP2O7 (with A = K or Rb) is identified as major constituent after thermal treatment. For NH4 phosphite compound, the decomposition leads to an amorphous phase accompanied with orthorhombic GaPO4. These thermal studies evidence the oxidation reaction of phosphite to phosphate anions. [Display omitted] •Three new gallium phosphites have been synthesized hydrothermally.•The layered structure of the compounds is related to other AIMIII(HPO3)2 compounds.•The thermogravimetric studies show a complex thermal behavior of gallium phosphites.•Oxidation of the phosphite group into phosphate is observed whatever the atmosphere.•Under nitrogen atmosphere hydrogen release is evidenced by TGA-MS studies.