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

beta-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 beta-BiB3O6 retains its structure below 9.0 GPa and undergoes a structural phase transition that starts at similar to 11.5 GPa and finishes at similar to 18.5 GPa. No other phase transition occurred with increasing pressure up to similar 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 zeta-BiB3O6 which could be indexed by an orthorhombic unit cell (a approximate to 12.5 angstrom, b approximate to 6.7 angstrom, c approximate to 4.0 angstrom) from the powder XRD pattern collected at 22.2 GPa. Moreover, zeta-Bib(3)O(6) can be quenched to ambient conditions. The investigation of the pressure dependence of the lattice parameters reveals that both beta-BiB3O6 and zeta-Bib(3)O(6) exhibit a large amount of crystallographic anisotropy. An unusual expansion of the c-axis of zeta-Bib(3)O(6) was observed. Assignments for the Raman spectra of beta-Bib(3)O(6), gamma-Bib(3)O(6), and delta-Bib(3)O(6) under ambient conditions were also performed. Currently, we cannot solve the crystal structure of zeta-Bib(3)O(6) but give some speculations based on its relationship with beta-BiB3O6.