On the anomalous high-pressure phase transition of inderite, MgB3O3(OH)5.5H2O

The high-pressure behaviour of inderite [ideally MgB3O3(OH)(5).5H(2)O, Sp. Gr. P2(1)/c with alpha = 6.8105 (2), b = 13.0955 (5), c = 12.0144 (6) angstrom, beta = 104.556 (4)degrees at room conditions], has been studied by in-situ single-crystal synchrotron X-ray diffraction up to 17 GPa, in order to explore its stability field at high-pressure, the anisotropy of its compressional behaviour and the deformation mechanisms at the atomic scale. Between 6.45 (5) and 6.94 (5) GPa, inderite experiences a first-order phase transition (from P2(1)/c to P21/n, with alpha = 13.4308 (7), b = 11.9970 (5), c = 10.4466 (8) angstrom, and beta = 100.030 (6)degrees at 6.45 (5) GPa, Delta V similar to 7%), reconstructive in character, to the inderite-II polymorph, whose structure was successfully solved and refined. The isothermal bulk modulus (K-V0 = beta(-1)(P0,T0), where beta(P0,T0) is the volume compressibility coefficient) of inderite was found to be K-V0 = 26.5 (7) GPa, whereas in inderite-II the KV0 value increases to 41(5) GPa. The compressional anisotropy of the two polymorphs is drastically different, being the ratio between the principal components of the unit-strain ellipsoid: epsilon 1:epsilon 2:epsilon 3 similar to 4.3:2.7:1 for the low -P polymorph and epsilon 1:epsilon 2:epsilon 3 similar to 3.0:2.7:1 for the high-pressure one. The defor-mation mechanisms at the atomic scale, in inderite and inderite-II, are here described. The pressure at which the phase transition of inderite occurs represents an anomalous and (so far) unique case among the hydrated pol-yionic borates.