Polymorphs of the Gadolinite-Type Borates ZrB 2 O 5 and HfB 2 O 5 Under Extreme Pressure

• Published in: Chemistry : a European journal Vol. 27; no. 19; pp. 6007 - 6014
• Main Authors: Pakhomova, Anna, Fuchs, Birgit, Dubrovinsky, Leonid S, Dubrovinskaia, Natalia, Huppertz, Hubert
• Format: Journal Article
• Language: English
• Published: Germany 01.04.2021
• Subjects: high-pressure chemistry; synchrotron radiation; borates; gadolinite structure; diamond anvil cell
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202005244

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.