The elastic behavior and the phase stability of natural pollucite, (Cs, Na)₁₆Al₁₆Si₃₂O₉₆·nH₂O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P-induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic (Ia3̅d) to triclinic symmetry (P1̅). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V₀ = 2558.3(4) ų, K_T0 = 41(2) GPa, and K_T′ = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V₀ = 2577.5(40) ų, K_T0 = 25.1(9) GPa, and K_T′ = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order “linearized” BM-EoS. The EoS parameters are a₀ = 13.699(12) Å, K_T0(a) = 25.5(17) GPa, and K_T′ (a) = 6.8(6) for the a axis; b₀ = 13.728(12) Å, K_T0(b) = 23.2(15)GPa, and K_T′ (b) = 7.7(7) for the b axis; c₀ = 13.710(7) Å, K_T0(c) = 25.2(10) GPa, and K_T′ (c) = 6.8(4) for the c axis [K_T0(a):K_T0(b):K_T0(c) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus (K_s) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are K_s = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T) and 1470 K (at room P) in elastic regime, the large amount of Cs hosted in this material (Cs₂O ~ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste.

Elastic and structural behavior of a natural tetragonal leucite from the volcanic Lazium district (Italy) were investigated at high pressure by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. A first-order phase transition, never reported in the literature, was observed at P = 2.4 ± 0.2 GPa from tetragonal (I4₁/a) to triclinic symmetry (analysis of diffraction intensities suggests the space group P1̅), accompanied by a drastic increase in density of about 4.7%. The transition pressure was bracketed by several measurements in compression and decompression. No further phase-transition has been observed up to 7 GPa. Fitting a second-order Birch-Murnaghan equation of state (BM-EoS) to the pressure-volume data of the tetragonal polymorph, we obtain K₀ = 41.9(6) GPa and K′ = 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives K₀ = 33.2(5) GPa. The eulerian finite strain (f_e) vs. normalized stress (F_e) curves were calculated for the low- and high-P polymorphs, providing F_e(0) = 42(1) and F_e(0) = 33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearized BM-EoS, are K₀(a) = 34.5(5) and K₀(c) = 78(1) GPa. For the triclinic polymorph, we obtain K₀(a) = 35.9(5), K₀(b) = 34.9(7), and K₀(c) = 35.5(7) GPa. The elastic behavior of the low-P polymorph appears to be more anisotropic than that of the high-P polymorph. The HP-crystal structure evolution of the tetragonal polymorph of leucite was studied on the basis of six structural refinements at different pressures between 0.0001 and 1.8 GPa. The main deformation mechanisms at high-pressure are due to tetrahedral tilting, giving rise to an increase of the ellipticity of the four- and six-membered rings of the tetrahedral framework. The T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal → triclinic phase-transition, and the low quality of the diffraction data at pressure above the phase-transition, did not allow the refinement of the crystal structure of the triclinic polymorph.

The crystal-structure, crystal-chemistry, and low-temperature behavior of a natural phillipsite-Na from the “Newer Volcanic Suite,” Richmond, Melbourne district, Victoria, Australia [K_0.75(Na_0.88Ca_0.57)_∑1.45 (Al_2.96Ti_0.01Si_5.07)_∑8.04O₁₆· 6.2H₂O (Z = 2), a = 9.9238(6), b = 14.3145(5), c = 8.7416(5) Å, β = 124.920(9)°, and V = 1018.20(9) ų, space group P2₁/m], have been investigated by means of in situ single-crystal X-ray diffraction, thermogravimetric analysis, and electron microprobe analysis in the wavelength dispersive mode. Two accurate structural refinements have been obtained on the basis of single-crystal X-ray diffraction data collected at 298 and 100 K, with: R₁(F)_298K = 0.035, 3678 unique reflections with F_o > 4σ(F_o) and 195 parameters, and R₁(F)_100K = 0.035, 3855 unique reflections, F_o > 4σ(F_o) and 195 parameters. In both refinements, the residuals in the final difference Fourier maps are <1 e⁻/ų. A configuration of the extra-framework population different from that reported in previous studies is found at room temperature, with two possible sites for potassium (K1 and K2), one sodium/calcium site (Ca), and seven independent sites partially occupied by water molecules (W1, W2, W3, W4, W4′, W5, and W6). The low-temperature refinement shows that the framework component of the phillipsite structure is maintained within the T-range investigated. However, a change in the configuration of the extra-framework content occurs at low temperature: the occupancy of site K2 drastically decreases, while that of site K1 increases, the Ca site is split into two sub-sites (Ca1 and Ca2) and the number of water molecule sites decreases to six (W1, W2, W3, W4, W5, and W6). The rearrangement of the extra-framework population at low temperature is likely due to the change in shape (and size) of the micropores by tetrahedral tilting. The evolution of the “free diameters” with temperature shows that an “inversion” of the ellipticity of the eight-membered ring channel along [010] occurs. The evolution of the unit-cell parameters with T (measured at 298, 250, 200, 150, and 100 K) shows a continuous and linear trend, without evident thermo-elastic anomalies. The axial and volume thermal expansion coefficients (α_j = l_j⁻¹·∂l_j/∂T, α_V = V⁻¹·∂V/∂T) between 100 and 298 K, calculated by weighted linear regression, yield the following values: α_a = 1.8(1) × 10⁻⁵, α_b = 1.2(1) × 10⁻⁵, α_c = 1.1(1) × 10⁻⁵, and α_V = 3.7(1) × 10⁻⁵K⁻¹. The thermal expansion of phillipsite is significantly anisotropic (α_a:α_b:α_c = 1.64:1.09:1).