Polymorphs of the Gadolinite-Type Borates ZrB O and HfB O Under Extreme Pressure.

Based on the results from previous high-pressure experiments on the gadolinite-type mineral datolite, CaBSiO (OH), the behavior of the isostructural borates β-HfB O and β-ZrB O have been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction experiments. On compression to 120 GPa, both borate layer-structures are preserved. Additionally, at ≈114 GPa, the formation of a second phase can be observed in both compounds. The new high-pressure modification γ-ZrB O features a rearrangement of the corner-sharing BO tetrahedra, while still maintaining the four- and eight-membered rings. The new phase γ-HfB O contains ten-membered rings including the rare structural motif of edge-sharing BO tetrahedra with exceptionally short B-O and B⋅⋅⋅B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf , and tenfold coordinated Zr , respectively. These findings remarkably show the potential of cold compression as a low-energy pathway to discover metastable structures that exhibit new coordinations and structural motifs.