Observation of the Sixth Polymorph of BiB₃O₆: In Situ High-Pressure Raman Spectroscopy and Synchrotron X‑ray Diffraction Studies on the β‑Polymorph

β-BiB₃O₆ was compressed in a diamond anvil cell at room temperature and studied using a combinstion of in situ high-pressure Raman scattering and angle-dispersive synchrotron X-ray diffraction. The results reveal that β-BiB₃O₆ retains its structure below 9.0 GPa and undergoes a structural phase transition that starts at ∼11.5 GPa and finishes at ∼18.5 GPa. No other phase transition occurred with increasing pressure up to ∼46 GPa. It was also found that the high-pressure phase is different from the already-reported five polymorphs of BiB₃O₆. Therefore, the new phase is denoted as ζ-BiB₃O₆, which could be indexed by an orthorhombic unit cell (a ≈ 12.5 Å, b ≈ 6.7 Å, c ≈ 4.0 Å) from the powder XRD pattern collected at 22.2 GPa. Moreover, ζ-BiB₃O₆ can be quenched to ambient conditions. The investigation of the pressure dependence of the lattice parameters reveals that both β-BiB₃O₆ and ζ-BiB₃O₆ exhibit a large amount of crystallographic anisotropy. An unusual expansion of the c-axis of ζ-BiB₃O₆ was observed. Assignments for the Raman spectra of β-BiB₃O₆, γ-BiB₃O₆, and δ-BiB₃O₆ under ambient conditions were also performed. Currently, we cannot solve the crystal structure of ζ-BiB₃O₆ but give some speculations based on its relationship with β-BiB₃O₆.