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 4 (OH), the behavior of the isostructural borates β ‐HfB 2 O 5 and β ‐ZrB 2 O 5 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 2 O 5 features a rearrangement of the corner‐sharing BO 4 tetrahedra, while still maintaining the four‐ and eight‐membered rings. The new phase γ ‐HfB 2 O 5 contains ten‐membered rings including the rare structural motif of edge‐sharing BO 4 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 4+ , and tenfold coordinated Zr 4+ , 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.