First Principles Calculations On Transition Metal Nitrides And Borides

Here we use the first principles calculations based on density function theory to study the crystal structure, stability, elastic constants and electronic properties of tuansition metal nitrides and transition metal borides. It will be helpful to predict the properties of materials and design new materials. The contents are as follows:1. Investigated the crystal structure, stability, elastic constants and electronic properties of PdN2 for four polymorph structures:pyrite, marcasite, CoSb2 and STAA, using first-principles calculations. At zero pressure all four polymorphs are metallic and thermodynamically unstable but mechanically stable. Pyrite PdN2 is found to be the lowest energy phase at high pressure. Good agreement between calculated and observed Raman frequencies was found, indicating that the recently synthesized palladium nitride at high pressure is likely to have the pyrite structure. Pyrite PdN2 is phononically stable but thermodynamically unstable at low pressure, and may decompose into metallic Pd and solid N2.2. Studied the stability, elasticity and electronic properties of 4d and 5d transition metal nitrides with three structural types (pyrite, marcasite and CoSb2 structure) by first principles calculations. In agreement with experiments and previous theoretical predictions, the crystal structures synthesized in the experiments for OsN2 is marcasite, for PtN2 is pyrite, and for IrN2 the CoSb2 structure. It is found that these three compounds are thermodynamically metastable but mechanically and dynamically stable. OsN2 is found to be metallic material, while IrN2 and PtN2 are both semiconductor. The formation energy of AuN2 is found to be very high as compared with other three nitrides studied here. This underlies the experimental difficulty in the synthesis for this compound.3.Studied the electronic and elastic properties of 4d and 5d transition metal mononitrides by first-principles calculations. The calculated results fit well with the available experimental data. All metal mononitrides studied in our work are metallic, rather than semiconductor. As a valence shell (the d shell in this case) starts to be filled, the equilibrium lattice constant decreases and bulk modulus increases because bonding states are being filled while lattice constant increases and bulk modulus decreases as the anti-bonding states are filled. This leads to a minimum in the lattice constant and maximum in the bulk modulus for compounds near a half-filled shell.4. Investigated the structure, elastic, and electronic properties of 4d transition metal diborides MB2 (M=Zr, Nb, Mo, Tc, Ru, Rh).It is found that both TcB2 and MoB2 are ultrahard materials.Among 4d transition metal diborides, hexagonal ReB2-type TcB2 has the highest C33 value of 947 GPa. Both highly directional covalency and a zigzag topology of interconnected bonds are the origin of the lower compressibility.5. Studied crystal structures, thermodynamic stability, electronic and elastic properties of transition metal borides. NaCl-type ZrB,NbB, MoB, HfB, TaB, WB, WC-type TcB, RuB, ReB, OsB, IrB, and anti-NiAs-type RhB, PdB are thermodynamically stable at zero pressure. The Vickers hardnesses of these monoborides are very low. The presence of B-vacancies is the origin for the difference of lattice parameters between theoretical data and experimental results for WC-type IrB and anti-NiAs-type PtB. At ambient pressure, WC-type IrB with stoichiometry is mechanically unstable, while anti-NiAs-type PtB with stoichiometry is dynamically unstable. This indicates that IrB and PtB synthesized in experiments are nonstoichiometry. The most stable structures studied here change from NaCl-type, WC-type to anti-NiAs-type structure in the order of left to right for 4d and 5d transition metal monoborides.And we also synthesized ZnO2 nanoparticles by an organometallic precursor method. The structure, structural stability, magnetic and optical properties of ZnO2 nanoparticles have been investigated by experiments and first-principles calculations. It is found that ZnO2 nanoparticles decompose into ZnO at about 230℃,and is stable up to 36 GPa at ambient temperature. The cubic ZnO2 phase has a bulk modulus of Bo=174 GPa at zero pressure. Nanocrystalline ZnO2 material is in-direct semiconductor with an energy gap of about 4.5 eV and paramagnetic down to 5 K.