On the elastic behaviour of zeolite mordenite : a synchrotron powder diffraction study

The high-pressure elastic behaviour of a synthetic zeolite mordenite, Na6Al6.02Si42.02O96 ·19H2O [a = 18.131(2), b = 20.507(2), c = 7.5221(5) Å space group Cmc21], has been investigated by means of in situ synchrotron X-ray powder diffraction up to 5.68 GPa. No phase transition has been observed within the pressure range investigated. Axial and volume bulk moduli have been calculated using a truncated second-order Birch-Murnaghan equation-of-state (II-BM-EoS). The refined elastic parameters are: V0 = 2801(11) Å3, KT0= 41(2) GPa for the unit-cell volume; a0 = 18.138(32) Å, KT0(a) = 70(8) GPa for the a-axis; b0 = 20.517(35) Å, KT0(b) = 29(2) GPa for the b-axis and c0 = 7.531(5) Å, KT0(c) = 38(1) GPa for the c-axis [KT0(a): KT0(b): KT0(c) = 2.41:1.00:1.31]. Axial and volume Eulerian finite strain versus "normalized stress" plots (fe-Fe plot) show an almost linear trend and the weighted linear regression through the data points yields the following intercept values: Fe(0) = 39(4) GPa for V; Fea(0) = 65(18) GPa for a; Feb(0) = 28(3) GPa for b; Fec(0) = 38(2) GPa for c. The magnitudes of the principal Lagrangian unit-strain coefficients, between 0.47 GPa (the lowest HP-data point) and each measured P > 0.47 GPa, were calculated. The unit-strain ellipsoid is oriented with ε1 ∥ b, ε2 ∥ c, ε3 ∥ a and ε1 > ε2 > ε3 . Between 0.47 and 5.68 GPa the relationship between the unit-strain coefficient is ε1 : ε2: ε3 = 2.16:1.81:1.00. The reasons of the elastic anisotropy are discussed.