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The counterintuitive phenomenon of negative linear compressibility (NLC) is a highly desirable but rare property exploitable in the development of artificial muscles1, actuators2 and next-generation pressure sensors3. In all cases, material performance is directly related to the magnitude of intrinsic NLC response. Here we show the molecular framework material zinc(II) dicyanoaurate(I), Zn[Au(CN)2]2, exhibits the most extreme and persistent NLC behaviour yet reported: under increasing hydrostatic pressure its crystal structure expands in one direction at a rate that is an order of magnitude greater than both the typical contraction observed for common engineering materials4 and also the anomalous expansion in established NLC candidates3. This extreme behaviour arises from the honeycomb-like structure of Zn[Au(CN)2]2 coupling volume reduction to uniaxial expansion5, and helical Au...Au 'aurophilic' interactions6 accommodating abnormally large linear strains by functioning as supramolecular springs.

The dissolution of α-FePO4 and the α-Ga0.75Fe0.25PO4 solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO3 aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO4 increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe3+ cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga0.75Fe0.25PO4 as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe3+ with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO4 by the hydrothermal method is not possible due to the formation of very stable Fe3+ hexa-aquo complexes [Fe(H2O)6]3+ and to the absence of FeO4 tetrahedra in solution. Ga3+ cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe3+ substitutes Ga3+ with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga0.77Fe0.23PO4 single crystal.

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 I42d , 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.