The deformation mechanism of a pressure-induced phase transition in dehydrated analcime

Abstract

AbstractThe elastic and structural behaviour of dehydrated analcime in compression in a non-penetrating medium up to 3 GPa was studied in a diamond anvil cell usingin situsynchrotron powder diffraction. A first-order phase transition at 0.4–0.7 GPa is accompanied by a symmetry change from monoclinic (I2/a) to pseudo-rhombohedral (R3) due to trigonalization of the aluminosilicate framework. This is due to the migration of cations to new positions close to the 6-membered rings forming the channels. The reduction of the mean aperture of the structure-forming 6- and 8-membered rings, as a result of tetrahedral tilting, leads to a 7.5% reduction in volume at the phase transition. The bulk modulus values are 38(2) GPa for the low pressure (LP) phase [fitted with a Murnaghan equation of state,K' = 4 (fixed)] and 11(4) GPa for the high pressure (HP) phase [fitted with a third-order Birch–Murnaghan equation of state,K' = 9(1)]. The elastic behaviour of the LPphase is anisotropic, with compressibilities βa:βb:βcin the ratio 1:4:2; the most compressible directionbcoinciding with the orientation of empty 8-membered rings. The compressibility of the HP phase is isotropic. Trigonalization appears to be the most effective (and probably unique) mechanism of radical volume contraction for the ANA structure type.