J. Haines

A High‐Temperature Molecular Ferroelectric Zn/Dy Complex Exhibiting Single‐Ion‐Magnet Behavior and Lanthanide Luminescence

Multifunctional molecular ferroelectrics are exciting materials synthesized using molecular chemistry concepts, which may combine a spontaneous electrical polarization, switched upon applying an electric field, with another physical property. A high-temperature ferroelectric material is presented that is based on a chiral Zn(2+) /Dy(3+) complex exhibiting Dy(3+) luminescence, optical activity, and magnetism. We investigate the correlations between the electric polarization and the crystal structure as well as between the low-temperature magnetic slow relaxation and the optical properties.

Large breathing of the MOF MIL-47(VIV) under mechanical pressure: a joint experimental–modelling exploration

A joint experimental–modelling study has demonstrated a large flexibility of the MIL-47(VIV) upon mechanical pressure which strongly deviates from its rigid behaviour in presence of guest molecules. A structural transition suspected by mercury intrusion and further confirmed by X-ray powder diffraction and molecular dynamics simulations, leads to a closed MIL-47(VIV) form never observed so far corresponding to a cell contraction of up to 43%. The microscopic key features that govern this transition are then elucidated from complementary Raman experiments.

Pa Modified Fluorite-Type Structures in Metal Dioxides at High Pressure

Rutile-structured SnO2, PbO2, and RuO2 have long been known to transform to cubic high-pressure phases, for which a fluorite structure has been assumed. Rietveld refinement results from x-ray diffraction studies indicated that these phases have a modified fluorite structure (space group Pa). Thus, for metal dioxides, all known cubic, postrutile phases have the Pa structure, thereby providing experimental examples of the high-pressure structure predicted from theoretical calculations for stishovite (rutile-structured silica). High-pressure transitions in stishovite may have profound implications for the geochemistry of the core-mantle boundary.

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.

The high-pressure phase transition sequence from the rutile-type through to the cotunnite-type structure in

Four phase transitions were observed in under pressure up to 47 GPa using x-ray diffraction in a diamond anvil cell. At close to 4 GPa, rutile-structured underwent a second-order transition to an orthorhombic, -type phase. Above 7 GPa, this -type phase transformed to a cubic phase with a modified fluorite structure. A transition to an orthorhombic phase was observed at 11.4 GPa. The intensities of the diffraction lines in this orthorhombic phase indicate a displacement of the lead ions from the fcc positions occupied in the cubic phase. This orthorhombic phase has similar cell constants (a = 10.027(2) A, b = 5.246(1) A, and c = 5.116(1) A at 26 GPa) to the orthorhombic I phase of and also that of , and could be isostructural. A further transition began above 29 GPa to a cotunnite-type phase, with space group Pnam, and Z = 4, and with a = 5.443(18) A, b = 6.346(17) A, and c = 3.368(8) A at 47 GPa. The coordination number of the lead ion is 6 in the first two phases, 6 + 2 in phase III, most probably 7 in phase IV and 9 in phase V. The volume decreases observed in the three first-order transitions are 6.9, 1.4, and 7.5% at 7, 11.4, and 29 GPa, respectively. The two higher-pressure transitions were reversible, whereas the cubic phase transformed to upon decompression, and this was retained down to ambient pressure. This is the first time a cotunnite-type structure has been adopted by a group IVb dioxide, which has implications for the high-pressure behaviour of the homologous compounds with smaller cations: and .

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.

Structural disorder and loss of piezoelectric properties in α-quartz at high temperature

The piezoelectric properties of α-quartz-based resonators, characterized by the mechanical quality factor, Q, are found to degrade beginning above 300 °C. This is well below the transition at 573 °C to the β phase, which in principle limits the piezoelectric response of this material. This gradual loss of piezoelectric response can be linked to the increase in structural disorder in α-quartz found in total neutron scattering measurements. Analysis of these data by reverse Monte Carlo modeling indicates that between 200 and 400 °C, the local disorder in the instantaneous structure of α-quartz becomes comparable to that of β-quartz.

Growth and dielectric characterization of large single crystals of GaAsO4, a novel piezoelectric material

AbstractGallium arsenate (GaAsO 4 )isanew α -quartz-type piezoelectric material. Large single crystals (8 mm along the c -direction) were grownfor thefirst timeby hydrothermal methods. The different crystal faces wereindexed by X-ray diffraction. The crystal quality was characterizedby infrared measurements. X -and Z -cut crystals were prepared in order to perform dielectric measurements. The dielectric constants ofGaAsO 4 ( e 11 =8 . 5, e 33 = 8 . 6) are the highest found among α -quartz type materials. A linear relationship has been established between astructural property, the tetrahedral tilt angle δ , and the dielectric constants for the well-known α -quartz homeotypes. These results indicatethat gallium arsenate should have the highest piezoelectric coupling coefficient of any material of this family.  2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved. 1. IntroductionQuartz material is currently the most used piezoelectricmaterial. Nevertheless, its properties are limited for certainapplications due to its low electromechanical couplingcoefficient. GaAsO