Siliceous zeolite-derived topology of amorphous silica.

The topology of amorphous materials can be affected by mechanical forces during compression or milling, which can induce material densification. Here, we show that densified amorphous silica (SiO2) fabricated by cold compression of siliceous zeolite (SZ) is permanently densified, unlike densified glassy SiO2 (GS) fabricated by cold compression although the X-ray diffraction data and density of the former are identical to those of the latter. Moreover, the topology of the densified amorphous SiO2 fabricated from SZ retains that of crystalline SZ, whereas the densified GS relaxes to pristine GS after thermal annealing. These results indicate that it is possible to design new functional amorphous materials by tuning the topology of the initial zeolitic crystalline phases.

Effect of Extra-Framework Cations on Negative Linear Compressibility and High-Pressure Phase Transitions: A Study of KCd[Ag(CN)2]3.

The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN)2]3, is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN)2]3, this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitions-to very large unit cells involving octahedral tilting and shearing of the structure-below 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application.

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.

Homologous critical behavior in the molecular frameworks Zn(CN)2 and Cd(imidazolate)2.

Using a combination of single-crystal and powder X-ray diffraction measurements, we study temperature- and pressure-driven structural distortions in zinc(II) cyanide (Zn(CN)2) and cadmium(II) imidazolate (Cd(im)2), two molecular frameworks with the anticuprite topology. Under a hydrostatic pressure of 1.52 GPa, Zn(CN)2 undergoes a first-order displacive phase transition to an orthorhombic phase, with the corresponding atomic displacements characterized by correlated collective tilts of pairs of Zn-centered tetrahedra. This displacement pattern sheds light on the mechanism of negative thermal expansion in ambient-pressure Zn(CN)2. We find that the fundamental mechanical response exhibited by Zn(CN)2 is mirrored in the temperature-dependent behavior of Cd(im)2. Our results suggest that the thermodynamics of molecular frameworks may be governed by considerations of packing efficiency while also depending on dynamic instabilities of the underlying framework topology.

Giant negative linear compressibility in zinc dicyanoaurate.

The counterintuitive phenomenon of negative linear compressibility (NLC) is a highly desirable but rare property exploitable in the development of artificial muscles, actuators and next-generation pressure sensors. In all cases, material performance is directly related to the magnitude of intrinsic NLC response. Here we show the molecular framework material zinc(II) dicyanoaurate(I), Zn[Au(CN)(2)](2), exhibits the most extreme and persistent NLC behaviour yet reported: under increasing hydrostatic pressure its crystal structure expands in one direction at a rate that is an order of magnitude greater than both the typical contraction observed for common engineering materials and also the anomalous expansion in established NLC candidates. This extreme behaviour arises from the honeycomb-like structure of Zn[Au(CN)(2)](2) coupling volume reduction to uniaxial expansion, and helical Au…Au 'aurophilic' interactions accommodating abnormally large linear strains by functioning as supramolecular springs.

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.

Topologically ordered amorphous silica obtained from the collapsed siliceous zeolite, silicalite-1-F: a step toward "perfect" glasses.

A dense amorphous form of silica was prepared at high pressure from the highly compressible, siliceous zeolite, silicalite-1-F. Reverse Monte Carlo modeling of total X-ray scattering data shows that the structure of this novel amorphous form of SiO(2) recovered under ambient conditions is distinct from vitreous SiO(2) and retains the basic framework topology (i.e., chemical bonds) of the starting crystalline zeolite. This material is, however, amorphous over the different length scales probed by Raman and X-ray scattering due to strong geometrical distortions. This is thus an example of new topologically ordered, amorphous material with a different intermediate-range structure, a lower entropy with respect to a standard glass, and distinct physical and mechanical properties, eventually approaching those of an "ordered" or "perfect" glass. The same process in more complex aluminosilicate zeolites will, in addition, lead to an amorphous material which conserves the framework topology and chemical order of the crystal. The large volume collapse in this material may also be of considerable interest for new applications in shock wave absorption.