High-pressure synthesis, crystal structure, and properties of the first ternary hafniumborate β-HfB2O5

The first ternary hafniumborate β-HfB2O5 was synthesized under high-pressure and high-temperature conditions in a Walker-type multianvil apparatus at 7.5 GPa and 1100 °C. The monoclinic hafnium-diborate crystallizes with four formula units in the space group P21/c with lattice parameters a=438.48(9), b=690.60(2), c=897.60(2) pm, and β=90.76(3)°. Due to the fact that high-pressure conditions favour the fourfold-coordination of boron, a structure is observed, which is built up exclusively from layers of BO4 tetrahedra; between the layers, the hafnium ions are coordinated square-antiprismatically by eight oxygen atoms. A structural comparison of β-HfB2O5 with minerals of the gadolinite–homilite–hingganite–datolite family proved this compound to be the simplest structural variant in this structure type, known so far. Along with a structural discussion, temperature-programmed X-ray powder diffraction data are shown, demonstrating the metastable character of this compound.