Mechanism of H2O insertion and chemical bond formation in AlPO(4)-54·xH2O at high pressure.

The insertion of H2O in AlPO4-54·xH2O at high pressure was investigated by single-crystal X-ray diffraction and Monte Carlo molecular simulation. H2O molecules are concentrated, in particular, near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H2O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H2O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process.

Carbon enters silica forming a cristobalite-type CO2-SiO2 solid solution.

Extreme conditions permit unique materials to be synthesized and can significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct with respect to its heavier homologues in forming oxides. Here we report the synthesis of a crystalline CO2-SiO2 solid solution by reacting carbon dioxide and silica in a laser-heated diamond anvil cell (P = 16-22 GPa, T>4,000 K), showing that carbon enters silica. Remarkably, this material is recovered to ambient conditions. X-ray diffraction shows that the crystal adopts a densely packed α-cristobalite structure (P4(1)2(1)2) with carbon and silicon in fourfold coordination to oxygen at pressures where silica normally adopts a sixfold coordinated rutile-type stishovite structure. An average formula of C0.6(1)Si0.4(1)O2 is consistent with X-ray diffraction and Raman spectroscopy results. These findings may modify our view on oxide chemistry, which is of great interest for materials science, as well as Earth and planetary sciences.

Confined H2O molecules as local probes of pressure-induced amorphisation in faujasite.

Confined H2O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials.

Study of Ga(3+)-induced hydrothermal crystallization of an α-quartz type Ga(1-x)Fe(x)PO4 single crystal by in situ X-ray absorption spectroscopy (XAS).

The dissolution of α-FePO(4) and the α-Ga(0.75)Fe(0.25)PO(4) solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO(3) aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO(4) increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe(3+) cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga(0.75)Fe(0.25)PO(4) as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe(3+) with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO(4) by the hydrothermal method is not possible due to the formation of very stable Fe(3+) hexa-aquo complexes [Fe(H(2)O)(6)](3+) and to the absence of FeO(4) tetrahedra in solution. Ga(3+) cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe(3+) substitutes Ga(3+) with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga(0.77)Fe(0.23)PO(4) single crystal.

Partially collapsed cristobalite structure in the non molecular phase V in CO2.

Non molecular CO(2) has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO(2) includes several non molecular phases above 30 GPa. Among these, the first discovered was CO(2)-V which appeared silica-like. Theoretical studies suggested that the structure of CO(2)-V is related to that of ß-cristobalite with tetrahedral carbon coordination similar to silicon in SiO(2), but reported experimental structural studies have been controversial. We have investigated CO(2)-V obtained from molecular CO(2) at 40-50 GPa and T > 1500 K using synchrotron X-ray diffraction, optical spectroscopy, and computer simulations. The structure refined by the Rietveld method is a partially collapsed variant of SiO(2) ß-cristobalite, space group I42d, in which the CO(4) tetrahedra are tilted by 38.4° about the c-axis. The existence of CO(4) tetrahedra (average O-C-O angle of 109.5°) is thus confirmed. The results add to the knowledge of carbon chemistry with mineral phases similar to SiO(2) and potential implications for Earth and planetary interiors.

Enhanced mechanical strength of zeolites by adsorption of guest molecules.

We report a molecular simulation study of the mechanical properties of microporous zeolites filled with guest molecules. We show that the adsorption of molecules in the micropores of the material increases its bulk modulus. These results provide a microscopic picture of the deactivation of pressure-induced amorphization by incorporation of molecules.

Silicon carbonate phase formed from carbon dioxide and silica under pressure.

The discovery of nonmolecular carbon dioxide under high-pressure conditions shows that there are remarkable analogies between this important substance and other group IV oxides. A natural and long-standing question is whether compounds between CO(2) and SiO(2) are possible. Under ambient conditions, CO(2) and SiO(2) are thermodynamically stable and do not react with each other. We show that reactions occur at high pressures indicating that silica can behave in a manner similar to ionic metal oxides that form carbonates at room pressure. A silicon carbonate phase was synthesized by reacting silicalite, a microporous SiO(2) zeolite, and molecular CO(2) that fills the pores, in diamond anvil cells at 18-26 GPa and 600-980 K; the compound was then temperature quenched. The material was characterized by Raman and IR spectroscopy, and synchrotron X-ray diffraction. The experiments reveal unique oxide chemistry at high pressures and the potential for synthesis of a class of previously uncharacterized materials. There are also potential implications for CO(2) segregation in planetary interiors and for CO(2) storage.

Brillouin spectroscopy, calculated elastic and bond properties of GaAsO4.

Experimental and theoretical studies have been performed to demonstrate the high performance of the novel piezoelectric material GaAsO(4). Hydrothermally grown single crystals of α-quartz phase GaAsO(4) were studied by Brillouin spectroscopy to determine elastic constants. Experimentally obtained values of C(11), C(66), C(33), C(44), C(14) and C(12) are 59.32, 19.12, 103.54, 30.70, 1.7, and 21.1 GPa, respectively. Elastic and piezoelectric tensors were also calculated by a first principles method in this work, leading to a very good agreement with experimental results and confirming the values of elastic components obtained indirectly such as C(14) and the negligible piezoelectric correction for C(11). The thermal behavior of the elastic constant corresponding to the [100] longitudinal L mode (C(11)) was studied up to 1137 K to estimate potential piezoelectric performance. It was found that the thermal behavior is linear up to 1273 K which is just below the thermal decomposition temperature of 1303 K. High thermal stability can be linked to the higher polarizability of large cations Ga and As because of neighboring oxygen atoms. On the basis of thermal behavior, GaAsO(4) is a promising material for high temperature piezoelectric applications.

Deactivation of pressure-induced amorphization in silicalite SiO2 by insertion of guest species.

The incorporation of carbon dioxide or argon stabilizes the structure of the microporous silica polymorph silicalite well beyond the stability range of tetrahedrally coordinated SiO(2) and, in fact, beyond even the metastability range of low-pressure silica polymorphs such as quartz and cristobalite at room temperature. The bulk modulus of silicalite strongly increases as a result of the incorporation of CO(2) or Ar and is equivalent to that of quartz. The insertion of these species deactivates the normal compression and pressure-induced amorphization mechanisms in this material, impeding the softening of low-energy vibrations, amorphization, and the eventual increase in silicon coordination up to at least 25 GPa.