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Recent synthesis of platinum nitride has provoked considerable interest on account of the compound's anomalously high bulk modulus, which is more than 30% higher than that of the parent metal. Numerous theoretical studies have since offered contradicting hypotheses on the structure and properties of this compound. Here we show, based on first-principles calculations, that the recently synthesized phase of platinum nitride has the pyrite structure. In the PtN 2 pyrite structure single-bonded N 2 units occupy the octahedral interstitial sites of the Pt close-packed lattice, giving rise to strong, directional Pt-N bonds and to an insulating character. Excellent agreement with x-ray, Raman, and compressibility measurements is obtained.

Understanding the structural transformations of solid CO2 from a molecular solid characterized by weak intermolecular bonding to a 3-dimensional network solid at high pressure has challenged researchers for the past decade. We employ the recently developed metadynamics method combined with ab initio calculations to provide fundamental insight into recent experimental reports on carbon dioxide in the 60 -80 GPa pressure region. Pressure-induced polymeric phases and their transformation mechanisms are found. Metadynamics simulations starting from the CO 2-II (P42/mnm) at 60 GPa and 600 K proceed via an intermediate, partially polymerized phase, and finally yield a fully tetrahedral, layered structure (P-4m2). Based on the agreement between calculated and experimental Raman and X-ray patterns, the recently identified phase VI solid CO2 ͉ first-principles molecular dynamics ͉ metadynamics ͉ phase transition ͉ density functional theory T he search for high-pressure structures of CO 2 has resulted in numerous experimental reports and theoretical predictions over the last several decades (1-14). There are many reasons for the large number of studies on this molecular system characterized by weak intermolecular bonding in the solid state at low pressures. There is the distinct possibility that CO 2 may convert to a 3-dimensional network solid that is extraordinarily hard and light, at high pressures. It has also been suggested that CO 2 may be present in the Earth's mantle with structures that are similar or identical to structures of SiO 2 having either 4-or 6-coordinated carbon atoms (12). There are many remaining unresolved issues relating to the detailed structure of solid CO 2 and changes in the chemical bonding that may occur at high pressures. These include questions about the transformation from a quadrupolar molecular solid to an extended network structure, the onset of the bending of the CO 2 linear molecule, and the structural relationship to other materials such as SiO 2 .At moderate temperatures up to Ϸ700 K, and pressures of 50-80 GPa, transformations from the molecular phases are reported yielding a stishovite-like P4 2 /mnm structure (1) starting with molecular phase II of CO 2 . However, the stishovite-like structure is energetically unfavorable and mechanically unstable according to first-principles calculations (15). There is also a recent report indicating that CO 2 transforms at moderately high temperatures from the Cmca phase III to a network-forming amorphous phase (2, 3) with mixed 3-and 4-coordinated carbon atoms (15). At high temperatures, the Cmca phase III was reported to transform to a ''superhard'' material (7, 8). Theoretical investigations have predicted and examined a variety of competing phases with structures ranging from those found in SiO 2 to a layered HgI 2 -type structure (7,9,11,12). Theoretical studies have generally used density-functional methods such as total-energy calculations and relaxations and constant-pressure molecular dynamics. Total-energy calculations provide accura...

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 β-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 β-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.carbon dioxide | material science C O 2 is a simple system of paramount importance for a broad range of scientific problems. Carbon dioxide is found in the atmospheres of Earth-like planets, in the ices of outer planets and asteroids, and plays a role in volcanic and seismic activity. The double bonds of the molecule are dramatically altered at high pressures as revealed by the discovery of non molecular CO 2 that forms above 30 GPa in crystalline and amorphous forms, which has attracted great interest in the last decade (1-7). The high degree of metastability in these materials makes the study of the phase diagram of this archetypal substance difficult (8). The first non molecular phase discovered was CO 2 -V, obtained by laser heating molecular CO 2 -III to 1,800 K at about 40 GPa (3), but its structure has been the subject of controversy and has never been solved so far (1). Raman spectroscopy indicates that CO 2 -V is non molecular and most probably characterized by C-O single bonds and carbon in tetrahedral coordination by oxygen as is silicon in silica (3, 9, 10).Over the same period, several density functional theory (DFT) studies were performed. All of them support the existence of non molecular, tetrahedrally coordinated CO 2 (11-15). These studies took into account a variety of potential silica-like structures, including different types of quartz, tridymite, and cristobalite. A β-cristobalite-like structure was predicted to be the most stable among these structures and most likely the thermodynamically stable phase in the P-T range where CO 2 -V is formed and temperature quenched (13-15). This structure is tetragonal, space group I42d, built up by corner sharing CO 4 tetrahedra with the carbon atoms forming a diamond network. CO 2 -V was predicted to be denser than molecular CO 2 (12, 16) and hard with bulk modulus of 149.1 GPa (14). The I42d structure is adopted by tetrahedrally co...