Six novel transition-metal phosphite compounds, with structure related to yavapaiite: Crystal structures and magnetic and thermal properties of AI[MIII(HPO3)2] (A=K, NH4, Rb and M=V, Fe)

Six new transition metal(III) phosphites AI[VIII(HPO3)2], where A=K (1), NH4 (2) and Rb (3) and AI[FeIII(HPO3)2] where A=K (4), NH4 (5) and Rb (6) have been synthesized under hydrothermal conditions and the solid-state structures were solved from single-crystal X-ray diffraction data. These compounds crystallize in the hexagonal system, space group P63mc (no. 186), with a=5.3294(2) Å and c=12.3130(5) Å for 1, a=5.3330(2) Å and c=12.8760(4) Å for 2, a=5.3459(2) Å and c=12.6850(8) Å for 3, a=5.3256(1) Å and c=12.2362(3) Å for 4, a=5.3229(2) Å and c=12.8562(4) Å for 5, a=5.3393(2) Å and c=12.6913(5) Å for 6, with Z=2 in the six phases. The crystal structures of these compounds are isotypic and exhibit a layered structure stacked along the c-axis with the A+ cations located in the interlayer space. The [MIII(HPO3)2]− sheets are formed by MO6 octahedra interconnected by HPO3 tetrahedral phosphite oxoanions through sharing vertices. Thermal analysis shows a large range of stability for compounds containing potassium and rubidium cations with decomposition starting around 550 K for stable compounds and above 840 K for the most stable compounds leading in general to pyrophosphate compounds. Triangular nets of metallic centers are observed within the layers in which antiferromagnetic interactions are evidenced by magnetic susceptibility measurements suggesting magnetic frustration.