Claire Levelut

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.

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.

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 CO2 and SiO2 are possible. Under ambient conditions, CO2 and SiO2 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 SiO2 zeolite, and molecular CO2 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 CO2 segregation in planetary interiors and for CO2 storage.

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

Non molecular CO2 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 CO2 includes several non molecular phases above 30 GPa. Among these, the first discovered was CO2-V which appeared silica-like. Theoretical studies suggested that the structure of CO2-V is related to that of β-cristobalite with tetrahedral carbon coordination similar to silicon in SiO2, but reported experimental structural studies have been controversial. We have investigated CO2-V obtained from molecular CO2 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 SiO2 β-cristobalite, space group , in which the CO4 tetrahedra are tilted by 38.4° about the c-axis. The existence of CO4 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 SiO2 and potential implications for Earth and planetary interiors.

Amorphization of faujasite at high pressure: an X-ray diffraction and Raman spectroscopy study

In situ X-ray diffraction and Raman spectroscopy measurements of synthetic powdered samples of faujasite 13X were carried out at high pressure using diamond anvil cells. Structural changes are detected, linked to a progressive reduction in crystallinity, before complete amorphization of the material. Three distinct compressibility regions are clearly observed, delimited by two discontinuities in the pressure dependence of the faujasite volume around 2 and 3.5 GPa. The transition from the crystal to the amorphous state is incomplete and partially reversible below pressures of between 8 and 12 GPa, depending on the pressure-transmitting medium used. This partial recovery of the initial structure, at least on a local level, could be related to the presence of hydrated sodium ions in the faujasite framework. In addition, the position of the first sharp diffraction peak in the X-ray diffraction pattern of a fully amorphous sample recovered from 24 GPa is consistent with the presence of 4-membered rings of tetrahedra and the persistence of a significant number of larger rings as compared to a dried amorphous faujasite precursor.

Temperature independence of pressure-induced amorphization of the phase-change memory alloy Ge2Sb2Te5

In the temperature range from room temperature to about 150°C, the prototypic phase-change material Ge2Sb2Te5 becomes amorphous upon hydrostatic compression. In the studied temperature range, the onset of amorphization is at about 15GPa and the material completely amorphizes at 25GPa; these values do not depend on temperature. Upon decompression, the amorphous phase is stable at lower temperatures, yet at higher temperatures (145°C), the initial fcc phase is recovered upon decompression. A possible mechanism of pressure-induced amorphization and its implications for phase-change memories are discussed.

Multiple scattering calculations of the XANES Si K-edge in amorphous silica

Abstract X-ray absorption spectra in the near-edge region (XANES) contain information about the structural organization of materials over distances up to about 10 A. In glasses, the structural information over such a large distance is rather complex and difficult to obtain. One possible method for extracting this information is to use multiple scattering (MS) calculations in real space to mimic X-ray absorption spectra in the XANES region. Such calculations for disordered systems require that a good structural model be assumed. We present full MS calculations at the silicon K-edge of amorphous silica, performed using atomic co-ordinates taken from molecular dynamics (MD) calculations. The spectra corresponding to several clusters are averaged in order to account for the disorder. The experimental spectrum is well reproduced by the calculations, showing the validity of the combination of MD and MS calculations. The results show an influence of medium-range order up to more than 5 A. Two different structural models taken from ab initio and classical MD are compared for this study.

In situ measurements of density fluctuations and compressibility in silica glasses as a function of temperature and thermal history

In this paper, small-angle x-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility {chi}{sub T}{sup 0} in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain, respectively, the temperature and fictive temperature dependences of {chi}{sub T}{sup 0}. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations.

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 (P41212) 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.

Influence of thermal aging on density fluctuations in oxide glasses measured by small-angle X-ray scattering

Small-angle X-ray measurements of the density fluctuations as a function of temperature and thermal history are performed in silica and float glass. Structural relaxation in the glass transition range is observed for the samples stabilized at high temperature. The amplitude of the density fluctuations in the glassy state depends on the thermal history for samples of same composition. It increases with fictive temperature for silica as well as for float glass. Our results are compared to previous results in oxide glasses or polymers and discussed in relation with the concept of fragility i.e. the temperature sensitivity of viscosity in the liquid state.