Polymorphs of the Gadolinite‐Type Borates ZrB₂O₅ and HfB₂O₅ Under Extreme Pressure

Abstract

Based on the results from previous high‐pressure experiments on the gadolinite‐type mineral datolite, CaBSiO4(OH), the behavior of the isostructural borates β‐HfB2O5 and β‐ZrB2O5 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‐ZrB2O5 features a rearrangement of the corner‐sharing BO4 tetrahedra, while still maintaining the four‐ and eight‐membered rings. The new phase γ‐HfB2O5 contains ten‐membered rings including the rare structural motif of edge‐sharing BO4 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 Hf4+, and tenfold coordinated Zr4+, 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.

Recent Progress in Crystalline Borates with Edge-Sharing BO4 Tetrahedra

“…γ-HfB 2 O 5 . In 2021, the γ -phase of HfB 2 O 5 , which incorporates edge-sharing [BO 4 ] tetrahedra, was obtained under extreme pressure (120 GPa) by Huppertz [ 74 ]. γ -HfB 2 O 5 crystallizes in the centrosymmetric monoclinic space group, P 2 1 / c (no.…”

Sr2[B5O8(OH)]2 ⋅ [B(OH)3] ⋅ H2O: A Strontium Borate That Shows Deep‐Ultraviolet‐Transparent Nonlinear Optical Properties

“…They are the ideal system to better understand the structural evolution under external stimuli, and the structure-NLO property relationships. [18][19][20][21][22][23][24][25] For example, β-HfB 2 O 5 transform to γ-HfB 2 O 5 under extreme pressure of 120 GPa, new edge-sharing [B 2 O 6 ] 6À units with extremely short BÀ O distances, as well as the increased coordination number of the Hf 4 + cations are observed in high pressure phase. [18] K 3 Sb 4 BO 13 undergoes an unusual symmetry increase during the cooling process, two adjacent triangular [BO 3 ] groups transform to edge-sharing [B 2 O 6 ] 6À units due to the shrinkage of the unit cell.…”

“…Polymorphs have an identical chemical composition, but different crystal structures and properties. They are the ideal system to better understand the structural evolution under external stimuli, and the structure‐NLO property relationships [18–25] . For example, β ‐HfB 2 O 5 transform to γ ‐HfB 2 O 5 under extreme pressure of 120 GPa, new edge‐sharing [B 2 O 6 ] 6− units with extremely short B−O distances, as well as the increased coordination number of the Hf 4+ cations are observed in high pressure phase [18] .…”