G. Diego Gatta

Effective hydrostatic limits of pressure media for high-pressure crystallographic studies

The behavior of a number of commonly used pressure media, including nitrogen, argon, 2-propanol, a 4:1 methanol–ethanol mixture, glycerol and various grades of silicone oil, has been examined by measuring the X-ray diffraction maxima from quartz single crystals loaded in a diamond-anvil cell with each of these pressure media in turn. In all cases, the onset of non-hydrostatic stresses within the medium is detectable as the broadening of the rocking curves of X-ray diffraction peaks from the single crystals. The onset of broadening of the rocking curves of quartz is detected at ∼9.8 GPa in a 4:1 mixture of methanol and ethanol and at ∼4.2 GPa in 2-propanol, essentially at the same pressures as the previously reported hydrostatic limits determined by other techniques. Gigahertz ultrasonic interferometry was also used to detect the onset of the glass transition in 4:1 methanol–ethanol and 16:3:1 methanol–ethanol–water, which were observed to support shear waves above ∼9.2 and ∼10.5 GPa, respectively, at 0.8–1.2 GHz. By contrast, peak broadening is first detected at ∼3 GPa in nitrogen, ∼1.9 GPa in argon, ∼1.4 GPa in glycerol and ∼0.9 GPa in various grades of silicone oil. These pressures, which are significantly lower than hydrostatic limits quoted in the literature, should be considered as the practical maximum limits to the hydrostatic behavior of these pressure media at room temperature.

Does porous mean soft? On the elastic behaviour and structural evolution of zeolites under pressure

This is a comparative study on lattice compressibility, pressure (P)-induced structural deformation mechanisms and influence of the framework and extra-framework content on the elastic behaviour of zeolites, based on previously published data obtained by in situ HP-single crystal and powder diffraction experiments. The elastic data of zeolites reported so far allow us to infer that: 1) the peculiar characteristics of the zeolitic structure, with large channels and a flexible framework built of rigid units (i.e. the tetrahedra), implies that the main deformation mechanisms at high-pressure (HP) are controlled by rigid (Si,Al)O4-tetrahedral tilting; 2) the structural rearrangement at HP is mainly driven by framework geometry and its topological symmetry; 3) the compressibility of zeolites appears not to be directly related to the microporosity, represented by the “framework density”; 4) the elastic parameters available for natural zeolites demonstrate that microporosity does not necessarily imply high compressibility. Several zeolites appear to be less compressible than many rock-forming minerals. A high compressibility is generally expected for open-framework structures due to the tetrahedral tilting, which produces inter-tetrahedral angle variations and accommodates the effect of pressure. However, the bonding between the host zeolitic framework and the stuffed guest species (cations and H2O molecules) affect the overall compression behaviour, making this class of porous material unexpectedly less compressible than other silicates.

Elastic behavior, phase transition, and pressure induced structural evolution of analcime

Abstract Elastic and structural behavior of a natural cubic analcime (space group: Ia3 . d) from Sardinia (Italy) was investigated at high pressure by in situ single-crystal X-ray diffraction. A first-order phase transition is observed in the pressure range between 0.91(5) and 1.08(5) GPa. Unit-cell constants and reflection conditions confirm that the space group of the HP-polymorph is P1̄. No further phasetransition has been observed at least up to 7.14 GPa. Fitting the volume data of the cubic polymorph with a second-order BM-EoS we obtain: V0 = 2571.2(4) Å3, KT0 = 56(3) GPa, and K’ = 4 (fixed). For the triclinic polymorph, a third-order EoS gives: V0 = 2607(9) Å3, KT0 = 19(2) GPa, and K’ = 6.8(7). Axial bulk moduli of the triclinic polymorph, calculated with a linearized BM-EoS, are: KT0(a) = 29(2) GPa, with K’(a) = 4.9(6) and a0 = 13.727(10) Å; KT0(b) = 20(1) GPa, with K’(b) = 5.2(5), and b0 = 13.751(15) Å; KT0(c) = 11(1) GPa, with K’(c) = 12.6(6) and c0 = 13.822(31) Å. The elastic behavior of the HP-polymorph appears to be strongly anisotropic, being KT0(a):KT0(b):KT0(c) = 2.64:1.82:1.00. The relevant structural variations in response to the cubic → triclinic phase transition are due to tetrahedral tilting. The tetrahedral framework distortion gives rise to a change of the eight- and six-ring channels ellipticity and of the extra-framework topological configuration: it appears in fact that for the high-pressure triclinic polymorph the coordination number of some of the Na atoms is seven (2H2O + five framework O atoms) instead of six (2H2O + four framework O atoms).

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.

Stability and transformation mechanism of weddellite nanocrystals studied by X-ray diffraction and infrared spectroscopy.

This study is focused on the stability of weddellite, the dihydrate phase of calcium oxalate [CaC(2)O(4)·(2 + x)H(2)O], mainly detected in kidney stones and in oxalate films found on the surfaces of several ancient monuments. Its occurrence is a critical issue since, at environmental conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of calcium oxalate (CaC(2)O(4)·H(2)O). New single crystal X-ray diffraction experiments have been carried out, which confirm the structural model of weddellite previously published. Synthesised nanocrystals of weddellite have been kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the influence of humidity on their stability. Moreover, the mechanism of transformation of weddellite nanocrystals has been investigated by infrared spectroscopy using D(2)O as a structural probe.

The real topological configuration of the extra-framework content in alkali-poor beryl: A multi-methodological study

Abstract The crystal structure of alkali/water-poor beryl (H2O + Na2O + Cs2O < 1.2 wt%) was reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction, and polarized infrared spectroscopy to determine the real topological configuration of the extra-framework content in the six-membered ring channels. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the sixfold axis at the 2a site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogen atoms are distributed in 6 × 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) Å and the H-O-H bond angle is 106.9(2.2)°. The H···H vector is oriented at -4° from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H···H vector is perpendicular to [001]. Na is probably located, with the H2O oxygen, at the 2a site. According to the chemical analysis, which shows that the amounts of other alkali and earth-alkali cations are negligible (Rb, K, Mg, Mn ≤ 110 ppm, Ca ≤ 225 ppm, Cs ≤ 430 ppm), no effect of other cations on the extra-framework population was observed in the structural refinement. The final agreement index (R1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with Fo > 4σ(Fo). The topological configuration of the H2O molecule into the channel is confirmed by the spectroscopic investigation. Polarized single-crystal IR spectra show that the H2O molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H···H vector parallel (or quasi-parallel) to [001].

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.

Single-crystal elastic properties of (Cs,Na)AlSi2O6⋅H2O pollucite: A zeolite with potential use for long-term storage of Cs radioisotopes

The single-crystal and aggregate elastic properties of the zeolite pollucite (Cs,Na)AlSi2O6⋅H2O, a potential host for Cs radionucleides in geological repositories, have been determined by Brillouin scattering spectroscopy at ambient conditions. The three nonzero individual elastic constants of cubic pollucite are: C11=105.0(1.3) GPa, C44=27.0(3) GPa, and C12=25.7(6) GPa. The Voigt–Reuss–Hill average of the aggregate bulk, shear modulus, Youngs modulus and Poisson’s ratio are KS=52.2(1.0) GPa, G=31.5(7) GPa, E=78.6(1.0) GPa, and ν=0.248(4), respectively. The bulk modulus of pollucite is 12.7% lower than that of the all-Na isotypic mineral analcime NaAlSi2O6⋅H2O whereas the shear moduli G are identical within mutual uncertainties. The higher compressibility of pollucite results from the weaker Cs–O bonds compared to Na–O bonds, suggesting strong control of the nature and configuration of the extraframework content on the behavior of the structure. The elastic properties of pollucite reported here will help...

On the crystal structure and crystal chemistry of pollucite, (Cs,Na)16Al16Si32O96·nH2O: A natural microporous material of interest in nuclear technology

Abstract The crystal structure and crystal chemistry of two natural pollucite samples, from Buckfield, Oxford County, Maine (sample M3), and from Kanzit Mawaie, Laghman, Nooristan, Afghanistan (sample N5), have been investigated by means of wavelength-dispersive X-ray microanalysis, thermogravimetric analysis, single-crystal X-ray and neutron diffraction, and single-crystal Fourier-transform infrared spectroscopy. The X-ray and neutron diffraction patterns of the two pollucite crystals show a metrically cubic unit cell [with aM3 = 13.6914(6) Å and aN5 = 13.6808(6) Å by neutron diffraction data; the deviation from isometry is <1.5σ(li), where li is the unrestrained unit-cell length] and the reflection conditions are consistent with the space group Ia3d. Anisotropic neutron structural refinements gave final agreement indices: R1 = 0.0543 for 32 refined parameters and 372 unique reflections with Fo > 4σ(Fo) for M3 and R1 = 0.0693 for 31 refined parameters and 331 unique reflections with Fo > 4σ(Fo) for N5. The structure refinements show a disordered Si/Al-distribution in the tetrahedral framework. The analysis of the difference-Fourier maps of the nuclear density confirms the presence of extraframework water molecules with oxygen sharing the Cs site (at 1/8, 1/8, 1/8, Wyckoff-16b position). However, the minima, ascribable to the proton sites, are very weak in density. Two possible proton positions, leading to a reasonable H2O configuration, are given, and the possible hydrogen bonding is described. Sodium is located at 1/4, 1/8, 0 (Wyckoff-24c position). The main IR absorption bands in the regions typical of H2O are assigned, and the presence of hydroxyls in the studied samples is ruled out. Neutron diffraction and FTIR data agree with the presence of very weak hydrogen bonds in the structure. The detailed description of the crystal structure and crystal chemistry of pollucite (e.g., Si/Al-distribution, configuration of the extra-framework content, possible hydrogen bonding scheme) reported in this study is the key to understand the high thermo-elastic stability of pollucite, the immobility of Cs at non-ambient conditions, and the extremely low leaching rate of Cs, which make this open-framework silicate a promising material with potential use for fixation and deposition of Cs radioisotopes

Elastic behavior and pressure-induced structural evolution of nepheline : Implications for the nature of the modulated superstructure

Abstract The elastic behavior and the pressure-induced structural evolution of a natural nepheline (K0.54 Na3.24Ca0.03Al4Si4O16) were investigated by in-situ single-crystal X-ray diffraction up to 7.5 GPa with a diamond anvil cell under hydrostatic conditions. As observed in previous studies, at room conditions the diffraction pattern of nepheline includes satellite reflections, whereas the structure refinement to the Bragg reflections confirms that the O1 site is displaced from the triad at (2/3, 1/3, z). The reflection conditions confirm that the space group of the average structure of nepheline remains as P63 throughout the pressure range investigated, and no significant compression of the T-O bonds was measured up to 7.5 GPa. As pressure was increased to around 1 GPa the integrated intensities of the satellites decreased slightly, and at 1.8 GPa no significant intensity of the satellites was detected. Over the same pressure range the O1 site moved toward the triad and thus the tilts of the T1 and T2 tetrahedra decreased. The presence of the subsidiary non-Bragg reflections is therefore related to the split of the O1 site. When the satellites disappear at pressures above 2 GPa, the O1 site is on the triad at (2/3, 1/3, z), corresponding to a straight T1-O1-T2 bond. Below 2 GPa the structure responds to increased pressure by tilting of all four tetrahedra and above 2 GPa by tilting of the T3 and T4 tetrahedra alone. The change in compression mechanism arising from the changes in the O1 position is associated with changes in the compression of the unit-cell axes and the unit-cell volume. The volume can be described by fourth-order Birch-Murnaghan equation-of-state with parameters V0+ = 723.57(4) Å3, KT0 = 47.32(26) GPa, K’ = 2.77(24), and K’’ = 0.758(79) GPa-1. The elastic behavior along the a- and c-axis can be described with a “linearized” fourth-order Birch-Murnaghan equations-of-state, with the following refined parameters: a0 = 9.9911(2) Å, KT0(a) = 43.1(3) GPa, K’(a) = 2.5(3), and K’’(a) = 0.68(8) GPa-1 for the a-axis and c0 = 8.3700(1) Å, KT0(c) = 58.6(3) GPa, K’(c) = 4.0(3), and K’’(c) = 0.85(11) GPa-1 for the c-axis. The pressure-induced structural evolution in nepheline up to 7.5 GPa appears to be completely reversible. The recovery of the modulation upon complete pressure release points to the framework of nepheline having an instability corresponding to a rigid-unit mode with a wave vector corresponding to the observed positions of the satellite reflections