【作者】 陈伟；

【导师】 蒋建中；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2010， 博士

【摘要】 本论文的目的是使用基于密度泛函理论的第一性原理计算来研究过渡金属氮化物和过渡金属硼化物的结构、稳定性、弹性常数和电子性能,为预测材料的新性能和指导新材料的合成提供依据。具体的研究内容归纳如下：1.通过对氮化钯pyrite、marcasite、CoSb2和STAA四种结构的第一性原理研究,我们得出这四种结构的氮化钯在零压下都是金属性的。它们虽然都具有力学稳定性,但在零压时都不具有热力学稳定性。其中,pyrite结构的氮化钯在较高压力下时是最稳相。第一性原理计算得到的拉曼频率和实验得到的拉曼谱图吻合得很好,有力地证明了高压实验合成的氮化钯具有pyrite结构。pyrite结构的氮化钯虽然从动力学稳定性上来说是稳定的,但是它在低压时热力学不稳定,趋向于分解成Pd金属和氮气。2.研究了三种结构(pyrite结构、narcasite结构和CoSb2结构)4d和5d过渡金属氮化物的稳定性、弹性常数和电子结构。我们的第一性原理计算结果同实验和先前理论研究的结果非常一致,证实了实验中合成的氮化锇、氮化铱和氮化铂分别具有narcasite结构、CoSb2结构和pyrite结构。虽然这三种物质在零压下从热力学上来说都是不稳定的,但是它们都具有力学稳定性和动力学稳定性。氮化锇是导体,而氮化铱和氮化铂是半导体。氮化金的形成能相对于其它三种氮化物来说非常高,这可能是氮化金非常难以制得的原因。3.使用第一性原理系统研究了4d和5d过渡金属一氮化物的电子性能和力学性能。我们的计算结果和实验值非常吻合,并且发现所研究的所有的一氮化物都是金属性的。当d电子壳层开始被填充时,由于成键态被占据,晶格常数减小体模量增加。而当反键态开始被占据时,晶格常数增加,体模量减小。这就导致了在半填充壳层附近,会出现晶格常数的极小值和体模量的极大值。4.对4d过渡金属二硼化物MB2 (M=Zr, Nb, Mo, Tc, Ru, Rh)的晶体结构、力学性能和电子结构进行研究。通过计算得出二硼化锝和二硼化钼都是超硬材料。在4d过渡金属二氮化物中,Hex-I结构的二硼化锝具有最高的C33值947GPa。强共价键和由共价键相互作用形成的z字形结构是低压缩率产生的原因。5.我们使用第一性原理计算研究了4d和5d过渡金属一硼化物的结构、弹性性能、热力学稳定性和电子性能。在零压时NaCl结构的ZrB,NbB,MoB, HfB, TaB和WB, WC结构的TcB, RuB, ReB, OsB和IrB以及anti-NiAs结构的RhB,PdB都是热力学稳定的。这些物质的Vickers硬度都很低。由于B空位的存在,造成了WC结构IrB和anti-NiAs结构PtB晶格常数实验值和理论计算值的巨大差异。在常压下,WC结构化学计量比的IrB力学不稳定,同时anti-NiAs结构化学计量比的PtB动力学不稳定。这表明实验中合成出的IrB和PtB都是非化学计量比的。对于4d和5d过渡金属一硼化物,随着价电子数的增加,最稳定结构呈现NaCl结构到WC结构到anti-NiAs结构的转变。同时,我们还使用有机金属先驱法合成出过氧化锌纳米颗粒,并通过实验和第一性原理计算研究了它的结构、结构稳定性、磁性以及光学性能。研究结果发现过氧化锌会在230℃分解成氧化锌,而在常温条件下可以稳定存在至36 GPa。在零压时,立方结构的过氧化锌体模量为174 GPa,间接禁带宽度为4.5 eV,在温度降至5K时仍不具有铁磁性。更多还原

【Abstract】 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.更多还原