Nicola Rotiroti

Elastic behavior and phase stability of pollucite, a potential host for nuclear waste

Abstract 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) Å3, KT0 = 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) Å3, KT0 = 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, KT0(a) = 25.5(17) GPa, and K′T(a) = 6.8(6) for the a axis; b0 = 13.728(12) Å, KT0(b) = 23.2(15) GPa, and K′T(b) = 7.7(7) for the b axis; c0 = 13.710(7) Å, KT0(c) = 25.2(10) GPa, and K′T(c) = 6.8(4) for the c axis [KT0(a):KT0(b):KT0(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 (Ks) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are Ks = 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.

Leucite at high pressure: Elastic behavior, phase stability, and petrological implications

Abstrac 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 P1), 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 (fe) vs. normalized stress (Fe) curves were calculated for the low- and high-P polymorphs, providing Fe(0) = 42(1) and Fe(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.

Structural evolution of a 3T phengite mica up to 10 GPa : an in-situ single-crystal X-ray diffraction study

Abstract The high-pressure structural evolution of a natural 3T-phengite [(K0.90Na0.05)S = 0.95(Al1.51Mg0.32Fe0.18Ti0.03)S = 2.04(Si3.40Al0.60)O10(OH)2, a = b = 5.2279(11) and c = 29.752(7) Å, space group: P3112] from Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to about 10 GPa. Nine structural refinements were performed at selected pressures within the P-range investigated. The compressional behavior of the same phengite sample was previously studied up to about 27 GPa by synchrotron X-ray powder diffraction, and the corresponding P–V curve was modeled by a third-order Birch–Murnaghan Equation of State (BM-EoS). The significant elastic anisotropy of the 3T-phengite (i.e. β(c) > β(a)) is mainly controlled by the compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P is monotonic, without any evidence of discontinuity. Fitting the P–V data with a truncated second-order BM-EoS, we obtain a bulk modulus value of K0(K-polyhedron) = 35(3) GPa. The tetrahedra and octahedra in the 3T-phengite structure are significantly less compressible than the K-polyhedron, and behave similarly to rigid units within the P-range investigated. The main P-induced effect on the tetrahedral sheet consists in a cooperative rotation of the tetrahedra, describable by the evolution of the “tetrahedral rotation angle” (or “ditrigonal rotation angle”, α) as a function of P. The value of the ditrigonal rotation angle increases significantly with P: α (°) = αP0 + 0.57(2)P (GPa) [R about 99%]. The volume of the K-polyhedron and the value of ditrigonal rotation parameter (α) are not independent of one another, showing a correlation of about 99%.

New insights into the crystal structure and crystal chemistry of the zeolite phillipsite

Abstract The crystal-structure, crystal-chemistry, and low-temperature behavior of a natural phillipsite-Na from the “Newer Volcanic Suite,” Richmond, Melbourne district, Victoria, Australia [K0.75(Na0.88Ca0.57)Σ1.45 (Al2.96Ti0.01Si5.07)Σ8.04O16·6.2H2O (Z = 2), a = 9.9238(6), b = 14.3145(5), c = 8.7416(5) Å, β = 124.920(9)°, and V = 1018.20(9) Å3, 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 Fo > 4σ(Fo) and 195 parameters, and R1(F)100K = 0.035, 3855 unique reflections, Fo > 4σ(Fo) and 195 parameters. In both refinements, the residuals in the final difference Fourier maps are <1 e-/Å3. 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 (αj = lj-1⋅∂lj/∂T, αV = V-1⋅∂V/∂T) between 100 and 298 K, calculated by weighted linear regression, yield the following values: αa = 1.8(1) × 10-5, αb = 1.2(1) × 10-5, αc = 1.1(1) × 10-5, and αV = 3.7(1) × 10-5K-1. The thermal expansion of phillipsite is significantly anisotropic (αa:αb:αc = 1.64:1.09:1).

New insights into the crystal chemistry of epididymite and eudidymite from Malosa, Malawi: A single-crystal neutron diffraction study

Abstract The crystal chemistry of two dimorphic hydrated sodium beryllium silicates, epididymite [a = 12.7334(4), b = 13.6298(5), c = 7.3467(3) Å, V = 1275.04 Å3, space group Pnma)] and eudidymite [a = 12.6188(10), b = 7.3781(5), c = 13.9940(9) Å, β = 103.762(5)°, V = 1265.47 Å3, space group C2/c] from Malosa, Malawi, has been reinvestigated by means of energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, inductively coupled plasma-optical emission spectroscopy and single-crystal neutron diffraction. Two anisotropic structure refinements have been performed with final agreement index R1 = 0.0317 for 137 refined parameters and 2261 unique reflections with Fo > 4σ(Fo) for epididymite, and R1 = 0.0478 for 136 refined parameters and 1732 unique reflections with Fo > 4σ(Fo) for eudidymite. The analysis of the difference-Fourier maps of the nuclear density of the two dimorphs confirms the presence of extra-framework water molecules in both, and not hydroxyl groups as wrongly reported in previous studies and in several crystal-structure databases. The correct chemical formula of edipidymite and eudidymite is Na2Be2Si6O15·H2O (Z = 4). The configuration of the water molecules and the hydrogen bonds are fully described for both the dimorphs. The chemical analyses show that a small, but significant, amount of Al and Fe (most likely substituting for Si in the tetrahedral sites) and K (substituting for Na as an extra-framework cation) occurs in both dimorphs

Cordierite under hydrostatic compression: Anomalous elastic behavior as a precursor for a pressure-induced phase transition

Abstract The high-pressure behavior of cordierite was investigated by means of in situ experiments using piston-cylinder press and diamond-anvil cell. Static compression in diamond-anvil cells was conducted with various penetrating and non-penetrating pressure media (H2O up to 2 GPa, argon and 4:1-methanolethanol up to 7 GPa). The measurement of lattice parameters revealed neither a significant influence on the elasticity nor any indication for effects in analogy to over-hydration within the experimental pressure ranges. Volumetric compression experiments at constant rates up to 1.2 GPa in a piston-cylinder apparatus insinuate subtle irregularities in the low-pressure range at around ~0.35 and ~0.85 GPa. The ΔV/V contribution related to the anomalous compression behavior in that pressure range is of the order of 5 × 10-4. The results obtained from single-crystal X-ray diffraction between 10-4 and 7 GPa revealed an unexpected and anomalous linear volume decrease, corresponding to KT,298 = 131±1 GPa for the bulk modulus and K′ = -0.4±0.3 for its pressure derivative for a third-order Birch-Murnaghan equation of state. The compressional behavior of the main axis directions is anisotropic with βa-1 ≈ βb-1 > βc-1 for an initial pressure regime up to ~3 GPa. At pressures above ~4 GPa, the compression of the a- and b-axis starts to differ significantly, with the b-axis showing elastic softening as indicated by negative values for ∂(βb-1)/∂P. The diversification between the a- and b-axis is also expressed by the pressure-depending increase of the distortion parameter Δ. The pronounced elastic softening in both the b-axis and c-axis directions ∂(βb-1)/∂P = -4.3±0.9, ∂(βc-1)/∂P = -1.2±0.8) are responsible for the apparent linear bulk compression, which indicates the structural instability and precedes a so far not reported ferroelastic phase transition to a triclinic polymorph, following a primitive lattice above the critical transition at ~6.9 GPa.

Synthesis and crystal structure of the feldspathoid CsAlSiO4 : an open-framework silicate and potential nuclear waste disposal phase

Abstract Crystalline CsAlSiO4 was synthesized from a stoichiometric mixture of Al2O3 + SiO2 + Cs2O (plus excess water) in Ag-capsules at hydrostatic pressure of 0.1 GPa and temperature of 695 °C. The duration of synthesis was 46 h. The crystal structure of CsAlSiO4 was investigated by single-crystal X-ray diffraction. The structure is orthorhombic with Pc21n space group and lattice parameters: a = 9.414(1), b = 5.435(1), and c = 8.875(1) Å. Because of the orthohexagonal relation between b and a (a ≈ b√3), within the standard uncertainty on the lattice parameters, a hexagonal superlattice exists, which is responsible for twinning. The crystals are twinned by reflection, with twin planes (110) and (310): twinning in both cases is by reticular merohedry with twin index 2 and hexagonal twin lattice (LT). The transformation from the lattice of the individual (Lind) to LT is given by: aT = aind - bind, bT = 2bind, and cT = cind. The refinement was initiated using the previously published atomic coordinates for RbAlSiO4. The final least-square cycles were conducted with anisotropic displacement parameters. R1 = 3.04% for 66 parameters and 2531 unique reflections. For a more reliable crystallographic comparison the crystal structure of RbAlSiO4 is reinvestigated here adopting the same data collection and least-squares refinement strategy as for CsAlSiO4. The crystal structure of the CsAlSiO4 feldspathoid is built on an ABW framework type, showing a fully ordered Si/Al-distribution in the tetrahedral framework. The only extra-framework site is occupied by Cs, lying off-center in the 8mR-channels. CsAlSiO4 is more likely to retain Cs when immersed in a fluid phase, relative to several other Cs-bearing zeolites. The topological configuration of the Cspolyhedron (and its bonding environment), the small dimension of the pores and the high flexibility of the ABW framework type would imply a better thermal and elastic stability of CsAlSiO4 than those of the zeolitic Cs-aluminosilicates. In this light, CsAlSiO4 can be considered as a functional material potentially usable for fixation and deposition of radioactive isotopes of Cs and can also be considered as a potential solid host for a 137Cs γ-radiation source to be used in sterilization applications.

High-pressure study of a natural cancrinite

Abstract The high-pressure elastic behavior and the P-induced structure evolution of a natural cancrinite from Cameroun {Na6.59Ca0.93[Si6Al6O24](CO3)1.04F0.41·2H2O, a = 12.5976(6) Å, c = 5 .1168(2) Å, space group: P63} were investigated by in situ single-crystal X-ray diffraction under hydrostatic conditions up to 6.63(2) GPa with a diamond-anvil cell. The P-V data were fitted with an isothermal Birch-Murnaghan type equation of state (BM EoS) truncated to the third order. Weighted fit (by the uncertainty in P and V) gave the following elastic parameters: V0 = 702.0(7) Å3, KV0 = 51(2) GPa, and K´V = 2.9(4). A linearized BM EoS was used to fit the a-P and c-P data, giving the following refined parameters: a0 = 12.593(5) Å, Ka0 = 64(4) GPa, Ka´ = 4.5(9), for the a-axis, and c0 = 5.112(3) Å, Kc0 = 36(1) GPa, Kc´ = 1.9(3) for the c-axis (elastic anisotropy: Ka0:Kc0 = 1.78:1). A subtle change of the elastic behavior appears to occur at P > 4.62 GPa, and so the elastic behavior was also described on the basis of BM EOS valid between 0.0001-4.62 and 5.00-6.63 GPa, respectively. The high-pressure structure refinements allowed the description of the main deformation mechanisms responsible for the anisotropic compression of cancrinite on (0001) and along [0001]. A comparative analysis of the structure evolution in response of the applied pressure and temperature of isotypic materials with cancrinite-like topology is carried out.

Phase stability, elastic behavior, and pressure-induced structural evolution of kalsilite: A ceramic material and high-T/high-P mineral

Abstract The phase stability, elastic behavior, and pressure-induced structural evolution of a natural metamorphic kalsilite (ideal formula KAlSiO4) from Punalur (Kerala district in southern India), with P31c symmetry and a K/Na molar ratio of ~350, has been investigated by in situ X‑ray single-crystal diffraction up to ~7 GPa with a diamond-anvil cell under hydrostatic conditions. At high-pressure, a previously unreported iso-symmetric first-order phase transition occurs at ~3.5 GPa. The volume compression of the two phases is described by third-order Birch-Murnaghan equations-of-state: V0 = 201.02(1) Å3, KT0 = 59.7(5) GPa, K′ = 3.5(3) for the low-P polymorph, and V0 = 200.1(13) Å3, KT0 = 44(8) GPa, K′ = 6.4(20) for the high-P polymorph. The pressure-induced structural evolution in kalsilite up to 7 GPa appears to be completely reversible. The compression of both phases involves tetrahedral rotations around [0001], which close up the channels within the framework. In addition, compression of the low-pressure phase involves tilting of the tetrahedra. The major structural change at the phase transition is an increase in the tilting of the tetrahedra, but with a reversion of the tetrahedral rotations to the value found at ambient conditions. This behavior is in distinct contrast to that of nepheline, which has a tetrahedral framework of the same topology.

Hydrogen-bond and cation partitioning in muscovite: a single-crystal neutron-diffraction study at 295 and 20 K

Abstract The crystal chemistry of a pegmatitic Fe-bearing muscovite (with FeO ~5.1 wt%) from Val di Crana, Arvogno (Verbania, Nothern Italy) has been investigated by means of wavelength-dispersive X-ray spectroscopy and constant-wavelength [λ = 0.9462(2) Å] single-crystal neutron diffraction at 295 and 20 K (2θmax = 90°). The structure refinement at 295 K shows no significant deviation from the full occupancy of the K site (using the scattering length of potassium alone), and a disordered Si/Aldistribution in the two independent tetrahedral sites (i.e., T1 and T2) and Fe/Al-distribution in the M2 octahedral site. The difference Fourier map of the nuclear density shows that only one independent H site occurs in the muscovite structure, at both room and low temperature. No evidence of disorder in two sub-sites, as observed in a previous study, was found. The thermal displacement of the H site is here described anisotropically. A trifurcated hydrogen bonding scheme was found: the O6-H bond distance corrected for “riding motion” is ~0.984 Å at 295 K, and three significantly weak hydrogen bonds to the O atoms O2, O4, and O6 occur, with H···O2 = 2.635(5), H···O4 = 2.657(4), and H···O5 = 2.647(4) Å and O6-H···O angles all similar in value (~138°). The structure refinements show that the structure configuration of muscovite is maintained at least down to 20 K: the cation disordering in the tetrahedral and octahedral site and the H-bonding scheme are preserved. The structural evolution induced by decreasing temperature is governed by the contraction along a direction perpendicular to (001), mainly due to the compression of the “inner” K-O bonds, which leads to an increase of the tetrahedral rotation angle (α) of the six-membered ring. A further effect in response to lowering the temperature is a significant reduction of the magnitude of the thermal displacement parameters.