Abstract

Experimental high-pressure investigations on benitoite in the diamond-anvil cell reveal a second-order phase transition at a critical transition pressure Pc = 4.24(3) GPa, as determined from synchrotron powder diffraction, single-crystal X-ray diffraction, and Raman spectroscopy. Diffraction experiments indicate a non-isomorphous transition from Pc2 to P31c space-group symmetry with a′ = a√3 and c′ = c relative to the P6̄2c subcell below Pc. The high-pressure polymorph is characterized by a larger compressibility compared to the compressional behavior of benitoite below Pc. Fitting second-order Birch-Murnaghan equations of state to the experimental data sets, the parameters obtained are V0 = 372.34(4) Å3, K0 = 117.9(7) GPa, with a0 = 6.6387(3) Å, Ka = 108.1(7) GPa, and c0 = 9.7554(4) Å, Kc = 143.3(1.1) GPa for the low-pressure form (P < Pc), and V0 = 376.1(4) Å3, K0 = 88.9(1.6) GPa, with a0 = 11.516(4) Å, Ka = 95.4(1.8) GPa, and c0 = 9.826(4) Å, Kc = 77.2(1.6) GPa for the high-pressure form (P > Pc). One of the most significant structural changes is related to the coordination of Ba atoms, changing from an irregular [6+6] coordination to a more regular ninefold. Simultaneously, the Si3O9 rings are distorted due to no longer being constrained by mirror-plane symmetry, and the Si atoms occupy three independent sites. The higher compressibility along the c-axis direction is explained by the relative displacement of the Ba position to the Si3O9 rings, which is coupled to the lateral displacement of the non-bridging O2-type atoms of the ring unit. A symmetry mode analysis revealed that the transition is induced by the onset of a primary order parameter transforming according to the K6 irreducible representation of Pc2.

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