Theoretical Design And Experimental Synthesis Of Nitrogen Containing High Energy Density Material At High Pressures

High-pressure may change existing state of materials effectively and makematerials show exotic behaviors that wouldn’t appear at ambient pressure, thus, itopened up a new dimension for the research of condensed matter physics. At highpressures, distances between the atoms in materials are expected to decreasesignificantly and electron orbits of adjacent atoms would overlap. These mightinduce the change of electronic properties and even so structural phase transitions.Finding the global minimum of the free-energy surface for crystal at highpressure is of great importance in both study of high-pressure phase transitionand design of new materials. The newly developed “Universal StructurePredictor: Evolutionary Xtallography”（USPEX） is global optimization methodsfor general crystal structure prediction, which is very efficient in finding theglobal minimum valley of the free-energy surface and large sets of competitivelocal minimums. Superhard materials are of great interest due to their widerange of industrial applications, from scratch-resistant coatings to polishing andcutting tools, etc. Now, the commonly used superhard material is still diamond.However, its stability at high temperature limits its application. Over the pastdecades, extensive theoretical and experimental efforts have been devoted tofinding new materials that are harder and thermally more stable than diamond.Scientists mainly focused on the exploration of covalent compounds formed bylight elements, namely, boron, carbon, nitrogen, and oxygen, since these elements have the ability to form short and strong three dimensional covalentbonds, which is a necessary condition for superhard materials. Since Cohen et al.designed-C₃N₄（P63/m） as a new low-compressibility material and estimatedits bulk modulus and hardness to be exceeding that of diamond, manyexperiments were performed to synthesize this material. However, growth ofcrystalline-C₃N₄with a large enough size has not been achieved so far. Instead,several hypothetical structures with C substituted for Si in polymorphs Si3N4areproposed for potential superhard materials, since the incorporation of C intoSi3N4is expected to considerably enhance the hardness. In1997, two crystallinesolids in the ternary Si-C-N systems: SiC₂N₄and Si₂CN₄, synthesized at ambientpressure and high temperature, and their ambient pressure structures have beendetermined by X-ray powder diffraction to be cubic （Pn3m） and orthorhombic（Aba2）, respectively. Linear Si-N=C=N-Si fragments are revealed in the Pn3mstructure of SiC₂N₄and Aba2structure of Si2CN4. Our hardness calculations ofPn3m type SiC₂N₄（16.9GPa） and Aba2-type Si2CN4（28.2GPa） suggest thatthe ambient-pressure structures are not superhard materials. Ab initioevolutionary methodology for crystal structure prediction is performed toexplore the high-pressure structures of two ternary compounds, SiC₂N₄andSi₂CN₄. For SiC₂N₄, we found intriguing high-pressure polymorphs withmonoclinic C2/m and orthorhombic Cmmm symmetries containing tetrahedralCN₄and octahedral SiN6units, respectively. For Si2CN4, two high-pressuremonoclinic C2/m and P21/m structures both consisting of octahedral SiN6unitswere discovered. Thermodynamic study demonstrated that it is energeticallydesirable to synthesize the Cmmm structured SiC₂N₄and P21/m structuredSi2CN4at above29and19GPa, respectively. We have ruled out the earlierproposed high-pressure monoclinic structures for the two ternary compoundsborrowed from known structural information. The newly predicted high-pressurephases of the two ternary compounds contain three-dimensional stacking of CN4tetrahedrons and SiN6octahedrons, small bond volumes comparable to those ofdiamond, and c-BN, and the strong covalent bonds between Si/C and N, whichare altogether responsible for the predicted superior mechanical properties, e.g.,very large bulk and shear modulus. Hardness calculations suggest that Cmmm structured SiC₂N₄and P21/m （C2/m） structured Si2CN4possess superhardness of58.7GPa and51.7GPa （51.6GPa）, respectively, and the ductility of them ismuch improved in comparison with the diamond. The current theoreticalprediction will inevitably stimulate future experimental synthesis and hasillustrated the major role played by high pressure in design of superhardmaterials.igh energy density material （HEDM） is widely used in space industry, weaponand so on. Nearly all the HEDM used now is based on the carbon framework.The traditional HEDM composing of C, H, O, N, include TNT, HMX, RDX,TATB, CL-20and so on. Unfortunately, the production process of mostenergetic compounds containing nitro is accompanied by serious environmentalpollution. On the other hand, with developing industry, these HEDM can’tsatisfy the demand of application. Therefore, exploring new HEDM has alwaysbeen the focus of the condensed matter physics.Nitrogen can be considered as an inert material because the N≡N triplebond is one of the most stable chemical bonds known. However, nitrogen atomsconnected with single bonds into a polymeric network, if metastable, will form ahigh-energy density material （HEDM）. There is a large difference in averageenergy between the nitrogen single bond （0.83eV/atom） and triple bond （4.94eV/atom）. Therefore, a very large energy should be released at thetransformation from polymerized nitrogen to diatomic molecular nitrogen.Nitrogen may form a high-energy density material with energy content higherthan that of any known nonnuclear material. Therefore, search for thenonmolecular single-bonded （polymeric） form of solid nitrogen under pressurehas attracted much attention. The formation of such singly bonded nitrogeninvolves the dissociation of the extremely strong triple N≡N bond into weakersingle N-N bonds upon ultrahigh compression and high temperature. Until2004,Eremets et al. made a breakthrough contribution by successfully synthesizingthe cubic gauche structure at high pressure （110GPa） and high temperature（2000K）. Eremets et al. observed that cubic gauche nitrogen is at leastmetastable at room temperature and under42GPa, or at140K and25GPa. However, the potential application depends first on whether or not the materialis metastable under the usual conditions. So, it is significant to stabilize singlebonded nitrogen to ambient conditions. As follows from calculations of Mattson,amorphous nitrogen passivated by hydrogen should be more stable than itsunpassivated form. Zhang et al. suggest that hydrogen can be used to “heal”unsaturated nitrogen bonds at the surface and other defects and stabilizepolymeric nitrogen after recovering to atmospheric pressure. Nitrogen andhydrogen do not chemically react at ambient conditions but at temperatures of300-550°C with the aid of a catalyst and pressures of200atm they formammonia in the Haber-Bosch process–a basis for production of fertilizers.In the present work, optical spectroscopy techniques, including Raman,Infrared, X-ray diffraction measurement and theoretical simulation have beenperformed to study N₂:H₂mixtures at high pressure and high temperature. Anew route for nitrogen and hydrogen reaction was found. At room temperatureand high pressures of38GPa, mixture of nitrogen and hydrogen moleculars startto dissociate and transform into N-H compound. This N-H compound is apolymer consisting of chains of nitrogen atoms connected each other with singlebonds and the rest bonds terminated with hydrogen atoms. The reaction wascompleted at about50GPa. The polymer is stable at releasing pressure, andbelow10GPa it decomposes to hydrazine and ammonia. At about1-2GPabefore opening the cell, hydrazine decomposes to nitrogen and hydrogen.Theoretical simulations at room temperature further certify that nitrogen andhydrogen molecular transformed into polymeric nitrogen amorphous at highpressure, and four kinds of structure units are included in polymer: N2H4, N4H6,N₄H₃and N6H8unit. In the N₂H₄, N₄H₆and N6H8unit, N atoms polymerizedtogether with single bond and the rest bonds are saturated with H atom. N₄H₃unit is a special unit, which can be seen as a hydrogen azide bonded with a NH₂unit. With the temperature increase from285K to565K, the transformedpressure greatly reduced from62GPa to18.9GPa. Extrapolation shows that at700K, the pressure of the synthesis should approach to ambient pressure. More extended polymeric structure was obtained at high temperature according toinfrared spectra and high temperature theoretical simulation.Our finding likely opens a way of synthesis of this new energetic hydrogen-rich material in a practical scale. Our results might have an implication to theinterior of the Earth as well as gas planets where N-H compounds can form.