The high pressure behaviour of the 10 A phase: a spectroscopic and diffractometric study up to 42 GPa

High-pressure Raman and X-ray powder diffraction data of the 10 Å phase are collected at several pressures up to 42 GPa by using a diamond anvil cell. The 10 Å phase remains stable throughout this pressure range at room temperature. Indeed, in the quasihydrostatic experiments with compression and decompression paths, the behaviour is completely reversible. Fitting with a thirdorder Birch-Murnghan equation of state the P–V data yields values of V0=492.9(3), K0=39(3) GPa and K′=12.5(8). No significant differences are obtained if the Vinet EoS is used instead. The linear compressibility coefficients of the a, b and β parameters are 1.20(16) 10−3 GPa−1, 1.72(9) 10−3 GPa−1 and 3.6(7) 10−4 GPa−1, respectively. For the c lattice parameter, an exponential decay is used to describe the evolution with P: c/c0=0.876(2)+0.116e−P/6.7(5). The Raman frequencies of all lattice modes (low frequency region <1200 cm− 1) are observed to increase monotonically with increasing pressure and decrease with decompression as well as the OH stretching vibration modes of hydroxyls, at 3626 cm− 1. On the other hand, the stretching bands of the water molecules in the 10 Å phase are found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between non-equivalent water H atoms and the oxygen atoms of the tetrahedral layer. The different evolution of the two OH stretching bands of the water molecules together with the lattice parameters compressibility is interpreted by a rotation of the water molecules with pressure that leads to a hydrogen bond formation due to the tetrahedral distortion. This distortion is a consequence of the larger compressibility of the octahedral layer with respect to tetrahedral layer, as observed in several phyllosilicates when pressure increases. This mechanism could explain how the pressure may stabilise this phase and give a rationale to the rarity of the natural occurrence at ambient conditions when compared with its production in the laboratory under high P/T gradients.