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 V0 = 1005(1) Å3, K0 = 89(8) GPa for the unit-cell volume; a0 = 9.914(7) Å, Ka = 81(12) GPa for the a-axis; b0 = 14.201(9) Å, Kb = 50(5) GPa for the b-axis; and c0 = 8.707(2) Å, Kc = 107(8) GPa for the c-axis (Ka:Kb:Kc ~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 V0 = 1098 (7) Å3, K0 = 18.8(7) GPa for the unit-cell volume; a0 = 10.07(3) Å, Ka = 30(2) GPa for the a-axis; b0 = 14.8(1) Å, Kb = 11(1) GPa for the b-axis; and c0 = 8.94(2) Å, Kc = 21(1) GPa for the c-axis (Ka:Kb:Kc ~2.72:1:1.90). The evolution of the monoclinic β 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.