High-pressure behaviour and atomic-scale deformation mechanisms in inyoite, CaB 3 O 3 (OH) 5 ·4H 2 O

The compressional behaviour of inyoite, ideally CaB 3 O 3 (OH) 5 ·4H 2 O, has been studied by an in-situ high-pressure single-crystal X-ray experiment, at the ESRF large-scale facility, up to 19.80(5) GPa. Inyoite undergoes a first-order phase transition to inyoite-II, bracketed between 8.25(5) and 8.86(5) GPa, with a large volume discontinuity (Δ V ⁓ 7.5%). The structure of the high-pressure polymorph has not been solved due to a significant decrease in the number of Bragg reflections. The isothermal bulk modulus ( K V 0 = β −1 P 0, T 0 , where β P 0, T 0 is the volume compressibility coefficient) of inyoite was found to be K V 0 = 26.9(8) GPa, whereas in inyoite-II, the K V 0 value increases to 52(5) GPa. The increase of the bulk modulus is paired with a sharp decrease of the anisotropy of compressibility, as shown by the magnitude of the Eulerian finite unit-strain ellipsoid with: ε 1: ε 2: ε 3 = 3.5:2.1:1 in inyoite and ε 1: ε 2: ε 3 = 1.5:1.1:1 in inyoite-II. The P -induced deformation mechanisms controlling, at the atomic scale, the bulk compression of inyoite are here described on the basis of a series of structure refinements.