On the P-induced behavior of the zeolite phillipsite: an in situ single-crystal synchrotron X-ray diffraction study

The elastic behavior and the structural evolution at high pressure of a natural phillipsite have been investigated by in situ single-crystal X-ray diffraction up to 9.44 GPa, using a diamond anvil cell and the nominally penetrating P-transmitting fluid methanol:ethanol:water (16:3:1) mix. Although no phase transition was observed within the P-range investigated, two different compressional regimes occur. Between 0.0001 and 2.0 GPa, the refined elastic parameters, calculated by a second-order Birch-Murnaghan equation of state (BM-EoS) fit, are V (0) = 1005(1) a<<(3), K (0) = 89(8) GPa for the unit-cell volume; a (0) = 9.914(7) a<<, K (a) = 81(12) GPa for the a-axis; b (0) = 14.201(9) a<<, K (b) = 50(5) GPa for the b-axis; and c (0) = 8.707(2) a<<, K (c) = 107(8) GPa for the c-axis (K (a) :K (b) :K (c) similar to 1.62:1:2.14). Between 2.0 and 9.4 GPa, a P-induced change in the configuration of H2O molecules, coupled with a change in the tilting mechanisms of the framework tetrahedra, gives rise to a second compressional regime, in which the phillipsite structure is softer if compared to the first compressional range. In the second compressional regime, the refined elastic parameters, calculated by a second-order BM-EoS fit, are V (0) = 1098 (7) a<<(3), K (0) = 18.8(7) GPa for the unit-cell volume; a (0) = 10.07(3) a<<, K (a) = 30(2) GPa for the a-axis; b (0) = 14.8(1) a<<, K (b) = 11(1) GPa for the b-axis; and c (0) = 8.94(2) a<<, K (c) = 21(1) GPa for the c-axis (K (a) :K (b) :K (c) similar to 2.72:1:1.90). The evolution of the monoclinic beta angle with pressure shows two distinct trends in the two compressional regimes: with a negative slope between 0.0001 and 2.0 GPa, and a positive slope between 2.0 and 9.4 GPa. The mechanisms, at the atomic scale, that govern the two compressional regimes of the phillipsite structure are described.