On the P-Induced Behavior of the Zeolite Phillipsite: an in Situ Single-Crystal Synchrotron X-Ray Diffraction Study

Abstract 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 secondorder Birch–Murnaghan equation of state (BM-EoS) fit, are V$_0$ = 1005(1) Å$^3$, K$_0$ = 89(8) GPa for the unitcell volume; a$_0$ = 9.914(7) Å, K$_a$ = 81(12) GPa for the $a$-axis; b$_0$ = 14.201(9) Å, K$_b$ = 50(5) GPa for the $b$-axis; and c$_0$ = 8.707(2) Å, K$_c$ = 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 H$_2$O 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) Å$^3$, K$_0$ = 18.8(7) GPa for the unit-cellvolume; a$_0$ = 10.07(3) Å, K$_a$ = 30(2) GPa for the $a$-axis; b0 = 14.8(1) Å, K$_b$ = 11(1) GPa for the $b$-axis; and c$_0$ = 8.94(2) Å, K$_c$ = 21(1) GPa for the $c$-axis (K$_a$:K$_b$:K$_c$~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