New insights into the high-pressure behaviour of the cyclosilicate benitoite, BaTiSi 3 O 9

The high-pressure behaviour of benitoite (ideally BaTiSi 3 O 9 , a ∼6.64 Å, c ∼9.76 Å, space group P6‾c2) was studied at ambient temperature by single-crystal X-ray diffraction up to 22.3 GPa using a diamond anvil cell and He as a quasi-hydrostatic pressure-transmitting medium. High-pressure data revealed three phase transitions, occurring between 3.93(5)–4.40(5) GPa (from benitoite to benitoite-II), between 12.44(5)–13.05(5) GPa (from benitoite-II to benitoite-III), and between 16.36(5)–17.08(5) GPa (from benitoite-III to benitoite-IV). The P – V data were fitted with a second-order Birch–Murnaghan equation, allowing the determination of the elastic parameters for benitoite, benitoite-II, and benitoite-III. The benitoite-to-benitoite-II transition presents a second-order character, consistent with previous literature, whereas the transitions from benitoite-II to benitoite-III and from benitoite-III to benitoite-IV are accompanied by a volume discontinuity, more pronounced in the latter. The structures of benitoite-III (space group Pbn 2 1 ) and benitoite-IV (space group R 32) have been solved. A progressive deformation of the Si 3 O 9 rings is observed, with the tilting angle of tetrahedral edges gradually increasing from benitoite to benitoite-II and to benitoite-III. In benitoite-IV the topology of one Si 3 O 9 ring is significantly different from the lower-pressure polymorphs in that the basal faces of the three tetrahedra are almost parallel; these mechanisms are primarily correlated with the overall increase in density, along with the deformation of the TiO 6 octahedra and an overall increase in the Ba coordination number. Understanding the structural characteristics of benitoite and its behaviour under high-pressure conditions provides valuable insights into the stabilization mechanisms of the three-fold ring in various polymorphs, with broader implications for other chemical systems, such as the CaSiO 3 system. Additionally, the findings may offer meaningful insights into the behaviour of high-pressure polymorphs of carbonates with sp 3 -hybridized carbon.