Theoretical Studies On The Structures And Physical Properties Of Nitrogen-rich Nitrides Under High Pressure

At ambient conditions,molecular nitrogen with typical characteristics of triple bonds(N?N)is the most abundant diatomic molecule,which is hard for it to naturally combine with other elements and participate in chemical reactions.There is a huge bonding energy difference between triple-bonds(954kJ/mol) and single-bonds(160kJ/mol).Exploring the single-bonded polymerization of solid nitrogen has become the focus of scientific research.High pressure,a clean and controllable thermodynamic variable parameter,could supply the sufficient energy to cross the barrier when triple-bonds turn into single-bond polynitrogens.The kinetic energy of electrons exceeds the electrostatic potential in nitrogen molecule above 50 GPa,resulting in partial electrons being transferred to the intermolecular region.At present,the metastable cg-N phase has been directly synthesized from the molecular phase at 110-180 GPa and 2000 K,which can be stabilized to 42 GPa at room temperature.The hexagonal layered polynitrogen,HLP-N,exists stably at a pressure close to 250 GPa and temperature of 4000 K.Using diamond anvil and laser heating technology to compress nitrogen molecules to 140 GPa and 4000 K,a polynitrogen allotrope with black phosphorous structure,bp-N,has been synthesized.The results point out that the previously reported layered LP-N structure similar to Pba2 and C2/c phases are actually a bp-N phase.However,the temperature and pressure required to obtain polynitrogen in pure nitrogen are too high to maintain kinetic stability under ambient conditions,which is a huge challenge in the process of full-nitrogen substance research.In 2004,Ashcroft predicted that adding other elemental alloys to hydrogen substance can produce a chemical"pre-compression"effect,making the pressure much lower than that required for compression of full-hydrogen.This provides new enlightenment for the research of nitrogen-rich compounds:the introduction of non-nitrogen elements can not only reduce the external pressure required but also improve the kinetic energy stability of the polynitrogen structure.The configuration and movement of atoms in the crystal structure are the fundamental factors that determine the properties of materials.It's the basis for understanding the macroscopic properties and mechanisms.In this paper,we use a particle swarm optimization algorithm based on the swarm intelligence theory to search for the crystal structures.The author proposes a new scheme for designing single-bonded polynitrogen.Taking binary nitride systems as an example,a series of basic studies on the structural diversity and chemical properties of polynitrogen in nitrogen-rich compounds under high pressure have been carried out and the following innovative results are obtained.1.The pentazole anions are only stable in acidic environment,which hinders the realization of full-nitrogen salts.Based on the first principles simulation,we report a novel nitrogen-rich Zn(N₅)₂ compound with space group P-1.The pentazole anion has thermodynamic and kinetic stability under the pressure of 25-85 GPa easily accessible by diamond anvil.The results show that the energy density of pentazole reaches 6.57 kJ/g due to the alternate coexistence of single and double bonds in the pentazole anion.The estimated detonation velocity and detonation pressure reach 12 km/s and 75 GPa,which are two times and four times higher than TNT,respectively.It's found that the octahedral pentazole framework centered on Zn is formed by ionic and covalent bond,in which covalent bonds can effectively improve the chemical insensitivity of pentazole anion.This will further prevent the spontaneous decomposition of pentazole anion into N₃^-anion.In addition,the 3D covalent bond network interwoven by Zn-N and N-N endows Zn(N₅)₂ salt with Vickers hardness as high as 34 GPa,which makes it a potential multifunctional material with both high hardness and high energy density.2.The cyclo-N₆ anions have higher nitrogen content than pentazole anions,but it have not been synthesized in the experiment before the publication of our work owning to their low decomposition barrier.Besides,the lack of microscopic study on the structure of cyclo-N₆ anion hinders the recognition of it as a potential high energy density material.In this work,we propose a strategy to maintain the"polynitrogen phase",that is,to design the covalent bonds clamping between cations and cyclo-N₆ anions to enhance the decomposition energy barrier.The tellurium hexanitride characterized by unique armchair cyclo-N₆ anions has been exposed.Theoretical calculations indicated that the stability of cyclo-N₆ anion can be effectively improved by enhancing the covalent component.The unique armchair cyclo-N₆ anions in tellurium hexanitride shape unique ionic and covalent bonds with the surrounding tellurium under high pressure.On the one hand,covalent bonds can partly eliminate the effect of electrons occupying cyclo-N₆ antibonding molecular orbitals,leading to structural instability.On the other hand,these Te-N covalent bonds effectively improve the chemical barrier(96.49 kJ/mol)of cyclo-N₆ anion,which prevent them from breaking down into diatomic molecular nitrogen.The formation of covalent bonds between cyclo-N₆ anion and Te cations also makes tellurium hexanitride possessing high bulk modulus,remarkable detonation performance and high temperature thermodynamic stability.3.We have predicted and identified a AlN₅ salt accompanied by high energy density and hardness via the crystal structure search method and first principles calculation.The delocalization effect of?electrons causing metallization has been proved through the basic principle mechanism.The electronic property is changed from metal phase to insulating phase with pressurization,mainly due to the hybridization configuration mutation and high localization of electrons.Furthermore,the band gap increases abnormally with the pressure mainly originating from the stronger hybridization effects.4.The electronic and bonding features in ground-state structures of germanium nitrides have been systematically studied.The forming essence of weak covalent bonds between the Ge and N atoms is induced by the binding effect of electronic clouds originated from the Ge?p orbitals.It is proved that the highest occupied molecular orbital and the lowest unoccupied molecular orbital are usually separated orbitals of N-?* and N-?*,which are signs of stabilization of the electronic structure.The pressure-induced electrogens transfering to the anti-bonding molecular orbitals of the molecular N₂ cause the triple-bonds into single-bonds,making its energy density as high as 2.32 kJ.g^-1 to reach the magnitude of non-molecular phase in Li N₅.This result breaks the general belief that only non-molecular polynitrogens are regarded as high energy density materials.It also breaks the viewpoint that the distance between atoms will be shortened with pressurization.We found that low nitrogen content tends to absorb visible light combined with previous measurement.Besides,compared with the previous measurement in Ge₃N₄ stoichiometry indicates that low pressure tends to absorb visible light and promotes photon utilization.Thus,tailoring the nitrogen stoichiometry to tune the ground-state band gap in germanium nitrides via high pressure treatment is an efficient way.