Structure Design And Property Prediction Of Hydrides,Carbides And Suifides At High Pressure

Computational solid-state science has a large and growing role to play in materials science and related disciplines due to its increasingly strong ability to design materials with novel and unusual properties.Such properties,which may arise from a wide range of potential compounds,are generally direct results of the solid's internal atomic-scale structure.In this work,the role of pressure in controlling and modifying the properties and structures of materials has been explored by employing the first principles electronic structure calculations and crystal structure search methods.Materials under pressure often exhibit exotic physical and chemical behaviors.In particular,unusual and new stable compounds may appear.Here,selected hydride,carbide sulfide and sulfide compiunds are used to study and exhibit the relationship between an external driving force(pressure)and an internal consequence(the structure-related properties of these key materials).The role of pressure on the superconducting properties of simple compounds is explored via the structure-property relations of Mg H6.A sodalite-related phase of Mg H6 is designed using first-principles electronic structure calculations,and is found to be stable above 263 GPa against the decomposition into Mg H2 and solid H2.Electron-phonon coupling calculations plus the analysis based on BCS theory indicate that the new Mg H6 phase is superconducting with a critical temperature of 260 K.The results lend insight into the design of new high-temperature superconductors.Results from first-principles calculations of molybdenum polyhdrides under pressure are presented.It is shown that,in addition to the previously-observed experimental ?-phase of Mo H,several novel structures of Mo H2 and Mo H3 are stabilised by increased pressures below 100 GPa.A hexagonal structure of Mo H2 becomes stable with respect to composition in Mo H and H2 at pressures above 9 GPa,and it transforms to an orthorhombic structure at 24 GPa,remaining stable up to 100 GPa.Mo H3 is found to be unstable against the decomposition into Mo H and H2 over the entire pressure range,and electronic structure calculations demonstrate that these molybdenum polyhydrides are metallic under pressure.Structure prediction is further explored in searches of the yttrium dicarbide(YC2)system as a function of pressure.Four new YC2 structures,stable under pressure,are found with varying carbon polymerization.A low-pressures metallic C2/m phase transforms to a Pnma phase(single chain carbide)before further transformation to an Immm structure(double chain carbide)at 54 GPa and a final phase transition to a P6/mmm phase(sheet carbide)at 267 GPa.Electron-phonon coupling calculations suggest that the high-pressure phases of YC2 are superconducting.Similar approaches were applied in searches of beryllium chalcogenides(Be S,Be Se,and Be Te)at high pressures using an swarm intelligence algorithm in conjunction with density functional theory.The predictions reveal modulated polymorphs,unusual for such simple binary compounds.A hexagonal closed packed P63/mmc phase is predicted to transform to an orthorhombic phase for Be S and Be Se under high pressure before a further transition to Cmca in Be Se,accompanied by the onset of modulation of atomic arrangement.Links between the modulated phases of these chalcogenides and the high-pressure modulated phases of the corresponding elementary chalcogens are drawn.Finally,low-density superhard materials are explored through the example of Li-inserted B-substituted closo-carboranes Li BC11 and Li2Bi2C10.Electronic structure calculations show these phases to be stable and semiconducting.The design of these phases,based on the C12 cage starting structure,the filling with a light metal atom(Li)in the cage,and the charge compensation with B,substitution of C,provides a route to new super-hard light element compounds.Through the five examples presented here,the strengths and potential of structure prediction,either via particle swarm algorithms or guided by chemical analogues and intuitive discovery,are highlighted and illustrated.These examples demonstrate a promising future of the materials design based on first principles electronic structure calculations and automatic crystal structure search algorithms,and of the materials discovery assisted by high-pressure techniques.