Olivier Cambon

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.

Diffraction studies under in situ electric field using a large-area hybrid pixel XPAD detector

A new experimental approach to perform in situ electric field diffraction on single crystals using an on-then-off pump–probe mode in situ (i.e. the field-switching method) with a synchrotron or a laboratory X-ray source is presented. Taking advantage of the fast readout of the XPAD hybrid pixel two-dimensional detector and its programmable functionalities, the operation mode of the detector has been customized to significantly increase the efficiency of the method. The very weak electric field-induced structural response of a piezoelectric crystal can be accurately measured. This allows the piezoelectric tensor to be precisely obtained from Δθ shifts while the structural variations can be modelled using a full set of ΔI/I data. The experimental method and methodology are detailed and tested as a case study on pure piezoelectric compounds belonging to the α-quartz family (SiO2 and GaAsO4 single crystals). Using the two scan modes developed, it is demonstrated that tiny Bragg angle shifts can be measured as well as small field-induced Bragg intensity variations (<1%). The relevance of measurements performed with an X-ray laboratory source is demonstrated: partial data sets collected at synchrotrons can be completed, but more interestingly, a large part of the study can now be realized in the laboratory (medium to strong intensity reflections) in a comparable data collection time.

The effects of pressure, temperature and composition on the crystal structures of α-quartz homeotypes

Abstract α-Quartz and its homeotypes are of great importance for both materials and Earth sciences. The properties of these materials depend strongly on their crystal structures and particularly the intertetrahedral bridging angle and the tetrahedral tilt angle. These angles are highly dependent on composition and the external parameters pressure and temperature. The behavior of the eleven known α-quartz homeotypes, along with examples of α-quartz-type solid solutions, are compared. The distortion in α-quartz-type structures decreases as a function of temperature and increases as a function of pressure. Thermal stability depends on initial structural distortion and on the electronic configuration of the cation. Pressure stability also depends on the former and on cation size. Transitions to new crystalline and/or amorphous forms, often with increased cation coordination number, are commonly observed at high-pressure. The combined use of high-pressure and high-temperature can be used to synthesize novel α-quartz homeotypes in compounds with small cations.

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.

Crystal structures of α-quartz homeotypes boron phosphate and boron arsenate: structure-property relationships

Abstract The structures of single crystals of the α-quartz homeotypes boron phosphate and boron arsenate prepared under high-pressure, high-temperature conditions in a belt-type apparatus were determined by X-ray diffraction. Both structures are more distorted with respect to the β-quartz structure type than α-quartz itself with average tetrahedral tilt angles δ of 19.7° and 24.6° for BPO4 and BAsO4, respectively. The structure of BAsO4 is one of the most highly distorted found among α-quartz homeotypes. It is shown that for the known α-quartz homeotypes, the density, which can be related to the frequency constant for piezoelectric resonators, scales with the intertetrahedral bridging angle θ.

Vibrational origin of the thermal stability in the high-performance piezoelectric material GaAsO4.

Theoretical calculations and experiments show the absence of libration modes of the tetrahedra in GaAsO(4), the most α-quartz-type distorted material. In consequence, the degree of dynamic disorder at high temperature is very low, making GaAsO(4) of high interest for high-temperature applications. This paper shows the importance of the theoretical calculations of vibration in oxide materials. In this way, it could be possible to extend this result to other materials and predict the thermal stability of the materials and their potential applications at high temperature.

Hydrothermal growth and structural studies of Si(1-x)Ge(x)O2 single crystals.

The substitution of germanium in the α-quartz structure is a method investigated to improve the piezoelectric properties and the thermal stability of α-quartz. Growth of α-quartz type Si(1-x)Ge(x)O(2) single crystals was performed using a temperature gradient hydrothermal method under different experimental conditions (pressure, temperature, nature of the solvent, and the nutrient). To avoid the difference of dissolution kinetics between pure SiO(2) and pure GeO(2), single phases Si(1-x)Ge(x)O(2) solid solutions were prepared and used as nutrients. The influence of the nature (cristobalite-type, glass) and the composition of this nutrient were also studied. Single crystals were grown in aqueous NaOH (0.2-1 M) solutions and in pure water. A wide range of pressures (95-280 MPa) and temperatures (315-505 °C) was investigated. Structures of single crystals with x = 0.07, 0.1, and 0.13 were refined, and it was shown that the structural distortion (i.e., θ and δ) increases with the atomic fraction of Ge in an almost linear way. Thus, the piezoelectric properties of Si(1-x)Ge(x)O(2) solid solution should increase with x, and this material could be a good candidate for technological applications requiring a high piezoelectric coupling factor or high thermal stability.

Mechanism of H2O Insertion and Chemical Bond Formation in AlPO4-54·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.