The structure of polymeric carbon dioxide (CO2-V) has been solved using synchrotron x-ray powder diffraction, and its evolution followed from 8 to 65 GPa. We compare the experimental results obtained for a 100% CO2 sample and a 1 mol % CO2/He sample. The latter allows us to produce the polymer in a pure form and study its compressibility under hydrostatic conditions. The high quality of the x-ray data enables us to solve the structure directly from experiments. The latter is isomorphic to the β-cristobalite phase of SiO2 with the space group I42d. Carbon and oxygen atoms are arranged in CO4 tetrahedral units linked by oxygen atoms at the corners. The bulk modulus determined under hydrostatic conditions, B0=136(10)  GPa, is much smaller than previously reported. The comparison of our experimental findings with theoretical calculations performed in the present and previous studies shows that density functional theory very well describes polymeric CO2.

International audienceThe structure and equation of state of the crystalline molecular phase II of carbon dioxide have been investigated at room temperature from 15.5 to 57.5 GPa using synchrotron x-ray diffraction methods. The CO2 samples were embedded in neon pressure medium in order to provide quasihydrostatic conditions. The x-ray diffraction patterns of phase II are best described by a tetragonal structure, with space group P42/mnm and 2 molecules per unit cell, in accordance with a previous study [Yoo et al., Phys. Rev. B 65, 104103 (2002)]. There is however a large (15%) difference in the intramolecular C=O bond length between the present study, 1.14(3) A° , and the latter work (1.329-1.366 A° ). The present value is similar to that of the free molecule and is in very good agreement with predictions based on density functional theory. The compressibility of CO2-II determined here also disagrees with the previous study: our value for the zero-pressure bulk modulus, B0 = 8.5(3) GPa [with B 0 = (∂B/∂P)0 = 6.29], is 15.5 times smaller. These findings oppose the view that CO2-II is an intermediate state between the low-pressure molecular phases and the high-pressure nonmolecular forms, consistent with our previous results for phase IV [Datchi et al., Phys. Rev. Lett. 103, 185701 (2009)]. The x-ray diffraction patterns of CO2-II above 15 GPa indicate the presence of a large orthorhombic microstrain. Carrying out density functional theory calculations of the elastic tensor and stress-strain relation, we interpret this as due to the softness of the crystal against deviatoric stress in the [110] and symmetry-related directions. Unlike the other dioxides of the group-14 elements, there is however no mechanical or dynamical instability of the P42/mnm structure in CO2 up to 57.5 GPa at 295 K, and therefore no symmetry lowering to Pnnm

We report the experimental discovery of a new phase of ammonia ice, stable at pressures above 57 GPa and temperatures above 700 K. The combination of our experimental results and ab initio molecular dynamics simulations reveal that this new phase is a superionic conductor, characterized by a large proton diffusion coefficient (1.0×10(-4) cm(2)/s at 70 GPa, 850 K). Proton diffusion occurs via a Grotthuss-like mechanism, at a surprisingly lower temperature than in water ice. This may have implications for the onset of superionicity in the molecular ice mixtures present in Jovian planets. Our simulations further suggest that the anisotropic proton hopping along different H bonds in the molecular solid may explain the formation of the recently predicted ionic phase at low temperatures.

The high-pressure phases of solid ammonia have been investigated by x-ray diffraction in a diamond anvil cell at room temperature. Despite the first-order solid phase transition at 4 GPa, quasi-single crystals of NH 3 and ND 3 could be obtained and compressed to 123 and 62 GPa, respectively. The observed reflections above 4 GPa are compatible with the structure determined by neutron diffraction on ND 3. We found strong evidence for an isosymmetric transition at 12 GPa in NH 3 and 18 GPa in ND 3. This transition is more likely due to rearrangements of the hydrogen atoms, whereas the N atoms remain on their quasi-hcp sites. The experimental equation of state ͑EOS͒ is compared to the one derived from first-principles calculations. A stiffening of the EOS above 55 GPa could indicate the onset of a quantum-disordered regime for some of the H bonds.

No abstract

We present an extensive analysis of the Raman spectrum of ammonia IV, for both NH3 and ND3, between 6 and 11 GPa. Polarized Raman scattering experiments were performed at low temperature (20–80 K) on single-crystalline samples grown in a diamond anvil cell. This method enabled us to discern very weak features of the Raman spectrum, including previously unobserved bending modes nu2–nu4. In parallel, we used first-principles methods, based on density functional theory, to compute the theoretical Raman spectra at the conditions of our experiments. An overall good agreement is found between experiment and theory. The results of the measurements with different polarizations and a comparison with our calculations allowed an unambiguous determination of the Raman modes. The assignment was further confirmed by examination of the isotopic ratios and Grüneisen parameters. This study thus provides a much more complete picture of the Raman spectrum of phase IV than available before and constitutes a solid basis for the investigation of ammonia at higher pressures

The high pressure(P)-high temperature(T) phase diagram of solid ammonia has been investigated using diamond anvil cell and resistive heating techniques. The III-IV transition line has been determined up to 20 GPa and 500 K both on compression and decompression paths. No discontinuity is observed at the expected location for the III-IV-V triple point. The melting line has been determined by visual observations of the fluid-solid equilibrium up to 9 GPa and 900 K.The experimental data is well fitted by a Simon-Glatzel equation in the covered P-T range. These transition lines and their extrapolations are compared with reported ab initio calculations.

The phase diagram and melting curve of water ice is investigated up to 45 GPa and 1600 K by synchrotron x-ray diffraction in the resistively and laser heated diamond anvil cell. Our melting data evidence a triple point at 14.6 GPa, 850 K. The latter is shown to be related to a first-order solid transition from the dynamically disordered form of ice VII, denoted ice VII 0 , toward a high-temperature phase with the same bcc oxygen lattice but larger volume and higher entropy. Our experiments are compared to ab initio molecular dynamics simulations, enabling us to identify the high-temperature bcc phase with the predicted superionic ice VII 00 phase [