Structures And Characteristics Of Typical Nitrogen Containing Molecular Solid Under High Pressure

Triple bond in molecular nitrogen (N2) is the strongest homopolar bond known,which dominates nitrogen chemistry and causing difficulty converting the N2intocompounds. But under the high temperature and high pressure, reactions ofchemically inert N2with other element happen. Because of the exceedingly largedifference in energy between the single N-N and triple N≡N bonds, singly bondedpolymeric nitrogen has the potential to become an excellent high-energy-densitymaterial for energy storage, propellants, and explosives. The search for polymericforms of nitrogen upon compression has attracted great attention. In2004, Eremets etal. made a prominent contribution to successfully synthesize the cubic-gauche (cg)structure at high pressure (110GPa) and high temperature (2000K). Unfortunately,attempts to recover the polymeric nitrogen at ambient conditions have beenunsuccessful to date, and its properties are largely unknown, including the predictedhigh energy content. We focused on nitrogen containing compounds due to the releaseof large amounts of useful energy when the these compounds containing nitrogen burn,explode, or decay back into nitrogen gas. Covalent compounds formed by lightelements, e.g., B, C, N, and O, have the ability to form short and strong3-dimensionalcovalent bonds, which is a necessary condition for superhard materials.Simple molecular solids are bounded by strong covalent intramolecular bonds, yetrelatively weak intermolecular van der Waals and/or hydrogen bonds. The weak intermolecular bonds make these solids highly compressible while the strong covalentbonds make them chemically inert, at least initially. The pressurization reduces theintermolecular distances to such an extent that an increasingly repulsive part of theintermolecular potential is explored. Because of pressure-induced changes in chemicalaffinities, the reactivities of otherwise familiar elements and compounds are totallyaltered, and entirely newclasses of materials with unusual combinations of physicalproperties can be formed. The major work of this thesis is to explore the high-pressurestructure and behavior of nitrogen containing solid molecules such as N2CO, N2O andN-F compounds, and the details are as follows.1. Nitrogen and carbon monoxide are isoelectronic and exhibiting similar crestalstrucutres at low pressure. It has been shown that both CO and N2undergo the tran-sition from a molecular crystal to a polymeric phase at high pressures. While bothhigh-pressure CO and N are potential high-energy density materials (HEDM). N2CO,an intriguing metastable material relevant to the strongest bonded diatomic moleculesN2and CO, has attracted special attention. We have extensively investigated thecrystal structures of N2CO under high pressure using the swarm structure searchingtechnique in combination with density functional theory. Three single-bonded3-dimensional structures are discovered. We show that the P43phase is the moststable structure compared with N2and CO above35.6GPa. In these predictedstructures, all the C atoms are tetrahedrally bonded with clear sp3hybridization andconnected to two N and two O atoms. N atoms are three coordinated (two N N bondsand one C N bond), while O atoms are two coordinated and forming only C O bondsin the lattices. The O atoms form a pair of lone-pair electrons and the N atoms form astable localized lone-pair nonbonding state. The strong covalent bonds and lone-pairare together the driving force for its high bulk and shear modulus. The calculatedhardness values are71,70, and54GPa for these structures at equilibrium. N2CO canbe considered as a potentially interesting high-energy density material since it is unstable at ambient condition and tends to dissociate into CO and N2with releasing agreat deal of heat, corresponding to an energy density of approximately4.6kJg1,which is higher than the modern explosive TNT (4.2kJ g1).2. N2O is one of the most extensively studied molecular crystals for its applicationsin medicine, refrigeration, and combustion reaction. The physical properties of N2Oand CO2are very similar and N2O is therefore expected to form similar structureswith CO2at low pressures. In the solid state the N2O molecules crystallize in the sameconfigurations as CO2, as Pa-3structure at ambient condition and Cmca structureabove~5GPa. Recently, theoretical study of N2O has predicted that N2O forms aone-dimensional polymer with an all-nitrogen backbone analogous tocis-polyacetylene in which alternate N atoms are bonded to O atoms above60GPa.Later on, a new N2O nanotube structure is found to be the most stable form above180GPa. Generally, molecular solids will go through insulator-metal transition atsufficiently high pressures due to the broadening of electronic bands. However, themetallic N2O have so far not been found. Here we perform our swarm structuralsearching method combined with first-principles calculations to explore structures andphysical properties of N2O under high pressure conditions. We find two metallicstructures of N2O which may be observed in high-pressure experiments. The currentcalculations reveal that the C2/m is the most stable structure over a pressure range of177-194GPa, and C2/c is metastable and only10meV/atom higher in energy than theC2/m structure at180GPa. They are both layered structures, and the N atoms of C2/mstructure in the same layer form wrinkled N6rings. At higher pressure, the metallicC2/m phases transforms into an insulating phase with space group of P21/m. Ourresults clarify and correct previous structural assignments at high pressures, andrepresent a significant step forward in understanding the behavior of N2O and othermolecular crystals at high pressures.3. Hypervalency has recently become a new surge of interest due to the potential applications in materials science and its important role played in the discovery ofcompounds for organic synthesis. Scientists have been trying to experimentallyprepare hypervalent nitrogen (N) compounds, however, numerous experimentalattempts to prepare the hypervalent N compounds by a variety of methods were turnedout to be in vain. Theoretical design of various gas molecules of pentacoordinate Nare themodynamically unstable. High pressure can effectively lower the barrier ofchemical reaction and thus makes the chemical reaction possible. We therefore havesystematically explored the structures and phase diagrams of various N-F compoundsin order to find hypervalent N under high pressure using a swarm-structure search. Weidentify three stoichiometric N-F compounds that might be experimentallysynthesizable, including NF, NF3, and NF5. At low pressure (11.2GPa), the NF5shows distinct molecular character with NF3and F2units. With increasing pressure(53.2GPa) the F2unit is found to be dissociation, and the compound finallytransforms to an ionic solid via a strongly pressure dependent charge transferinteraction. At128.1GPa, a novel hypenvalent pentacoordinate nitrogen moleculeappears, N and F atoms show a covalent bonding character. In addition, At ambientpressure, the metastable Cmca structure of NF dissociates into NF3and N2with theestimated energy release of1.23eV, corresponding to an energy density ofapproximate3.69kJg-1. NF has the potential to be an excellent high energy densitymaterial. The predicted NF5opens the possibility of new pathways to the formation ofhigh-pressure hypervalent compounds, possibly with significant applications forchemical reactions and materials science.