Computational Discovery and Property Study of Novel Hydrides and Nitrides

Author:Yu Shu Yin

Supervisor:zhang li tong zeng qing feng


Degree Year:2017





With the rapid development of modern science and technology,new functional crystal materials with particular properties have been widely concerned.Understanding the relationship between macroscopic properties and microstructures of materials has important academic value in accelerating the development and synthesis of new materials.In this paper,we combine structural prediction method with first-principles calculations to predict several new materials which are used for national defense,aerospace and advanced manufacturing,such as beryllium and chromium-rich hydrogen compounds with excellent superconductivity,and the IVB transition metal nitrides with non-stoichiometric phenomenon,molybdenum pernitride with excellent mechanical and catalytic properties,and magnesium nitrides which can be used as novel green high energy density materials.The main contents and results are as follows:(1)The hydrogen-rich compounds and their superconductivity of Be-H and Cr-H systems under high pressure have been studied.The results show that transition metal chromium has more abundant chemical behavior than alkaline earth metal beryllium.The Be-H system has only BeH2 between 0 and 400 GPa,while Cr-H system has a series of compounds between 0and 300 GPa,i.e.,Cr2Hn(n=2-8,16).The electronic properties of Cr2Hn compounds show that they have metallic properties under ambient conditions,while the metallization of BeH2needs more than 200 GPa.The calculated superconductivity shows that the metallized BeH2and Cr2Hn have superconductivity,and superconductivity of the former is obviously superior than the latter one.Free electrons involved in the superconductivity of beryllium hydride mainly come from the metallic hydrogen,but in the chromium hydrides,they mainly come from the chromium metal atoms.It can be seen that the metallic hydrogen can significantly improve the superconductivity of compounds.(2)The influence of non-stoichiometric phenomenon on the mechanical properties of transition metal nitride(IVB)have been studied.The results of crystal structure predictions of Ti-N and Zr-N systems show that non-stoichiometric ratios include 2:1,3:2,4:3,6:5,8:7,15:16,4:5 and 7:8.Those non-stoichiometric compounds usually have a rock salt structure,and there is a certain concentration of vacancies.The compounds not only have a high mechanical strength,but also a certain toughness,and the mechanical strength decreases with the increase of the vacancy concentration.It was found that the excellent mechanical properties were attributed to the strong covalent interaction between metal and nitrogen atoms,and the slight metallic character.It is worth mentioning that the two compounds ZrNx(1.06<x<1.23)synthesized by Juza et al.are probably Zr4N5 and Zr15N16,found in this paper.(3)The formation mechanism of“N2”unit in MoN2 system and its influence on the electronic and mechanical properties has been studied.First-principles calculations show that the previously proposed MoS2-type MoN2 structure can not be stable because it does not meet the thermodynamic,dynamical and mechanical stability criteria.Our crystal structure prediction found the structure of MoN2 is P63/mmc and P4/mbm between 0 and 100 GPa.Their structures contain molecule like“N2”unit.And the valence state of the“N2”unit significantly affects the electronic properties:when the HOMO level of the“N2”unit is completely filled,the system exhibits semiconductor properties,if semi-filled,then it would be metallic.At last,the influence of“N2”unit on the mechanical properties of MgN2,MoN2and TiN2 was studied.The results show that the higher the number of valence electrons of the“N2”unit,the higher the bulk modulus.Thus,mechanical properties of the transition metal pernitrides are better than alkaline earth metals.It is worth mentioning that our predicted TiN2has been experimentally confirmed by Venkata et al.(73 GPa,2400 K).(4)The nitrogen-rich compounds in the Mg-N system under high pressure and the corresponding nitrogen lattice evolution have been studied.The results of crystal structure predictions show that besides the known Mg3N2,the Mg-N system also contains MgN4,MgN3,MgN2,Mg2N3,Mg5N7,Mg5N3,Mg4N3 and Mg5N4 in the pressure range of 0300 GPa.Nitrogen atoms in the their structures hold many lattice forms,including one-dimensional chain,two-dimensional layer,“N2”,“N3”,“N4”and isolated atoms.Their formation mechanism can be explained by molecular orbital methods and VSEPR theory.It was found that the lattice dimension of nitrogen atoms increased with the increase of nitrogen content.The nitrogen atoms only existed in the isolated state in the Mg-rich compounds;when increasing the nitrogen content,it will form“N2”,“N3”,“N4”molecular groups;Further increase of the nitrogen content would result in the formation of one-dimensional chain and two-dimensional layered structures.It is worth mentioning that MgN4 is a potential high-energy density material,the synthesis pressure only needs 18 GPa,while the traditional synthesis of nitrogen-rich compounds usually require more than 100 GPa.