高压下LiYF4晶体光学性质的第一性原理研究

本文采用第一性原理方法,在100 GPa的压力范围内, 计算了LiYF4理想晶体和含空位点缺陷晶体的光学性质.吸收谱数据表明,在100 GPa范围内,压力和相变因素的存在不会改变LiYF4晶体在250-1000 nm的波段内没有光吸收的事实. 氟、钇空位点缺陷的出现会使得LiYF4的吸收边蓝移,而锂空位点缺陷将导致它的吸收边微弱红移(但在250-1000 nm的波段内它仍不具有光吸收行为).波长在532 nm处的折射率数据显示, 在LiYF4的三个结构相区,其折射率均随压力的增加而增大. LiYF4从白钨矿结构到褐钇铌矿结构的相变会使得其折射率略微增加,但从褐钇铌矿结构到类黑钨矿结构的相变将导致其折射率显著降低. 同时,空位缺陷的存在将引起LiYF4的折射率明显增大. 分析指明,LiYF4有成为冲击窗口材料的可能. 本文所获得的信息对未来的实验研究有参考作用.     

First principles study on the structural and optical properties of the high-pressure ZnO phases

A new high-pressure tetragonal phase (B10) of ZnO is investigated with an ab initio calculation based on density functional theory and is compared with the cubic B1 (rocksalt structure) and B2 (CsCl structure) phases at high pressure. It is found that the B10 phase has a more covalent nature than the B2 phase. The B1, B2, and B10 phases are semiconductors and their band gap energies are determined to be 3.73, 3.15, and 3.27 eV, respectively. The B10 phase has a similar optical response to the B2 phase, but not the B1 phase. The similarity of dielectric function between B10 and B2 phases are the result of the similar profiles of electronic density of state.     

Thermoelasticity of CaO from first principles

The thermoelastic properties of CaO over a wide range of pressure and temperature are studied using density functional theory in the generalized gradient approximation. The transition pressure taken from the enthalpy calculations is 66.7GPa for CaO, which accords with the experimental result very well. The athermal elastic moduli of the two phases of CaO are calculated as a function of pressure up to 200GPa. The calculated results are in excellent agreement with existing experimental data at ambient pressure and compared favourably with other pseudopotential predictions over the pressure regime studied. It is also found that the degree of the anisotropy rapidly decreases with pressure increasing in the B1 phase, whereas it strongly increases as the pressure increases in the B2 phase. The thermodynamic properties of the B1 phase of CaO are predicted using the quasi-harmonic Debye model; the heat capacity and entropy are consistent with other previous results at zero pressure.     

Electronic and thermodynamic properties of B2-FeSi from first principles

The electronic and thermodynamic properties of B2-FeSi have been investigated using the first-principles method based on the plane-wave basis set. The calculated equilibrium lattice constant is in good agreement with available experimental and theoretical data. Our results have shown that B2-FeSi was a narrow gap semiconductor of above 0.055 eV and exhibited metallic characteristics. The density of states (DOS) can also describe orbital mixing. Using the quasi-harmonic Debye model, the thermodynamic properties of B2-FeSi have been analyzed. Variations of the Debye temperature Θ_D, thermal expansion α, heat capacity C_v, entropy S and the Grüneisen parameter γ on temperature T and pressure P were obtained successfully in the ranges of 0-2400 K and 0-140 GPa.     

相变及空位缺陷对高压下BeO光学性质的影响

为了探究BeO晶体能否成为冲击波实验中的候选窗口材料,本文采用密度泛函理论(DFT)的第一性原理方法,计算了150 GPa的压力范围内BeO理想晶体和含氧空位点缺陷晶体的光学性质.吸收谱数据显示,BeO高压结构相变对其吸收谱的吸收边几乎没有影响.并且,在150 GPa压力范围内,BeO理想晶体在可见光区没有光吸收行为.氧空位点缺陷的存在将使得其吸收边出现明显的红移现象,但在可见光区仍然没有光吸收(是透明的).波长在532 nm处的折射率数据表明:在BeO的WZ和RS结构相区,其折射率会随着压力增加而缓慢降低,而高压结构相变和氧空位缺陷将使得其折射率显著增大.计算数据分析表明BeO有成为冲击窗口材料的可能,并且本文所获信息将对未来进一步的实验有重要参考价值.     

第一性原理计算Mo浓度对Mo掺杂BiVO4光催化性能的影响

本文利用基于密度泛函理论的第一性原理研究了不同浓度的Mo掺杂BiVO₄的V位的电子结构、光学性质和光催化性能.缺陷形成能的计算结果说明BiMo_xV_1-xO₄(x=0.0625, 0.125, 0.25)三种掺杂体系都是可以稳定存在的.电子结构计算结果表明：BiMo_xV_1-xO₄(x=0, 0.0625, 0.125, 0.25)四种体系的带隙分别为2.123 eV,2.142 eV,2.160 eV和2.213 eV.掺杂BiVO₄体系的带隙值均大于本征BiVO₄,且带隙随着Mo浓度的增加而增大. BiMo_xV_1-xO₄(x=0.0625, 0.125, 0.25)三种掺杂体系的能带结构全部向低能量区域移动,导致掺杂体系导带底越过费米能级,Mo掺杂BiVO₄后具...     

相变及空位缺陷对CaF2在高压下光学性质的影响

本文采用第一性原理方法, 计算了CaF2的理想晶体和含钙、氟空位点缺陷晶体在100 GPa压力范围内的光吸收谱和折射率. 吸收谱数据表明, 压力因素诱导的两个结构相变对 CaF2的吸收谱均有影响: 第一个相变将导致其吸收边蓝移, 而第二个相变却引起其吸收边红移. 钙空位点缺陷会使得CaF2的吸收边微弱蓝移, 但氟空位点缺陷却导致其吸收边有显著的红移. 然而, 这些红移的行为并未使得CaF2晶体在250-1000 nm的波段范围内出现光吸收的现象（是透明的）. 532 nm处的折射率数据显示, 在 CaF2的三个结构相区（Fm3m、Pnma、P63/mmc相区）, 其折射率均随压力增大而增加; 同时, 高压结构相变以及氟空位点缺陷也使得CaF2的折射率增大, 但钙空位点缺陷却导致其折射率减小. 数据分析表明, CaF2晶体有成为冲击窗口材料的可能, 本文所获得的信息对未来的实验研究有参考价值.     

Elastic properties and phonon dispersions of rhenium in hexagonal-close-packed structure under pressure from first principles

The elastic properties of the hexagonal-close-packed (hcp) structure rhenium (Re) and their behavior under pressure are investigated using the local density approximation (LDA) and the generalized gradient approximation (GGA). The obtained high pressure elastic constants are well consistent with previous theoretical date, while large discrepancies are found between theory and the high pressure experiments. The calculated isothermal bulk modulus B₀ (376 GPa for GGA and 389 GPa for LDA) and its initial pressure derivative (4.52 for LDA and 4.58 for GGA) compare favorably with the experimental values. Moreover, it is found that the value of c/a, B/G, Poisson's ratio, and B_c/B_a are virtually independent of pressure. We also performed calculation for phonon dispersions at high pressure. GGA in our calculation exhibits a same trend as the high pressure experimental curve.     

Electronic and lattice vibrational properties of cubic BaHfO3 from first principles calculations

Through first principles calculations, we investigated the electronic structure and lattice vibrational properties of BaHfO₃. The optimized lattice constant of BaHfO₃ is in agreement with experimental and theoretical results. Our results show that cubic BaHfO₃ is an insulator with an indirect band gap of 3.5 eV. Besides, the calculation using the screened exchange local density approximation (sX-LDA) has been performed with the predicted minimum gap of 5.3 eV. The phonon dispersion curves of BaHfO₃ were also calculated. All positive phonon frequencies in the Brillouin zone were found, indicating the stability of BaHfO₃ structure.     

外压下VN相变和力学性质的第一性原理研究

运用基于赝势平面波基组的密度泛函程序VASP并结合Quantum ESPRESSO,Phonopy软件包对压力下VN的结构、力学性质、声子色散关系进行了第一性原理的研究.分别对NaCl型（B1）,CsCl型（B2）,WC型（Bh）三种构型的VN进行了计算,三种结构的体积能量曲线、焓压关系和声子谱表明在常压下六角WC结构与立方结构相比更稳定.随着压力增加VN由Bh结构到B1结构的相变点发生在30GPa左右,而B1结构到B2结构的相变点可能发生在150GPa左右.常压下三种结构的VN是力学稳定的,其弹性常数和弹性模量都有随压强的增大而增加的趋势,三者都是脆性材料.B1结构和B2结构坐标基矢方向上的杨氏模量数值与体对角线方向上的差距较大,体现出明显的各向异性.随压力的增加B1结构各向异性程度增大而B2结构各向异性程度减小     

M- (Sm, Pr, Ga) 掺杂TiO2带隙及电子结构的第一原理研究

采用密度泛函理论对M-（Sm、Pr、Ga）掺杂锐钛矿型TiO2能带和电子性质进行了系统的理论研究. 计算结果表明,通过Sm和Pr的掺杂可以降低TiO2的带隙进而使其产生吸收边红移,通过Ga的掺杂能使带隙稍增加. 这主要是由于Sm和Pr的掺杂使Sm和Pr上的4f层电子与原子相邻O原子上的2p层电子相互作用,形成的杂质能级影响了Ti-O的能带结构,从而降低带隙,提高TiO2的可见光吸收性能.     

二维g-AlN材料中n型掺杂探索：基于第一性原理的带电缺陷计算

电荷转移能级和缺陷形成能的计算对探索半导体材料的n型和p型掺杂效率具有重要的指导意义。基于第一性原理方法,结合二维带电缺陷计算理论,系统计算了二维类石墨烯氮化铝(graphene-like AlN, g-AlN)中四种（C_Al,Si_Al,Ge_Al,Sn_Al）可能的n型掺杂体系的结构、磁学、电学以及缺陷性质。结果表明,四种体系的最稳定价态均为+1价和0价,Sn_Al具有较深的施主能级,不具备为二维g-AlN提供n型载流子的条件,而C_Al,Si_Al,Ge_Al表现为浅能级施主特性,均能在一定条件下成为理想的施主杂质,其中Si_Al具有最浅的施主特性以及最低的缺陷形成能,因而是二维g-AlN中实现n型掺杂的首选掺杂剂。另外,在p型二维g-AlN中,四种掺杂原子都会成为有效的空穴捕获中心,严重降低p型载流子导电率。研究数据将会为实验上实现二维g-AlN n型掺杂提供理论解释和指导。     

Au掺杂CNT吸附NO的第一性原理研究

碳纳米管(CNT)对于气体有超强的敏感性,可用于制备基于CNT的有害气体传感器.本文采用基于密度泛函理论的第一性原理研究Au掺杂CNT对NO和O_2的吸附特性.对吸附能、最终吸附距离、电荷转移量、态密度等的分析显示,Au掺杂使得CNT与NO间的交互作用明显增强,其中N原子端靠近CNT交互作用更强.禁带宽度和电荷密度分析表明,相比于NO分子中O原子端或者O2吸附,NO分子中N原子端与CNT发生交互作用会使体系导电性变化更为明显.说明Au掺杂能够很好地屏蔽空气中O_2对CNT导电性的影响,Au掺杂CNT作为NO气敏材料是可行的.     

Atomic and electronic structures of Mg-doped perfect SrTiO3 and crystals containing oxygen vacancies from first principles

The electronic properties of Mg-doped perfect SrTiO₃ and crystals containing oxygen vacancies systems are investigated by first principles calculation. Dopant formation energy results show that the Mg atoms preferentially enter the Sr site in SrTiO₃. Substitution of Ti by Mg brings some acceptor levels, which introduces the p-type conductivity of SrTiO₃. Creation of oxygen vacancies in SrTiO₃ introduces donor levels, which can contribute to the n-type conductivity. In SrTiO₃ containing oxygen vacancies system, a self-compensation effect will occur when Ti is substituted by Mg, and the system undergoes n-type to p-type transition.     

Response of high-purity titanium to high-pressure impulsive loading

Abstract

Measurements of free surface velocity profiles of high-purity titanium samples under shock-wave loading were performed to study the dynamic strength and phase transition parameters. The peak pressure of the initial compression waves was within the range of 4 to 40 GPa, and the load duration was vaned between 10^?8 and 10^?6 s. An anomalous structure of shock waves was observed at pressures of ～ 2.0 to 5.0 GPa due to the α-ω phase transition. The dynamic strength of pure titanium is lower than that of titanium alloys but exceeds the spall strength of commercial grade titanium.     

MgO高温高压特性及相变的第一性原理研究

应用第一性原理密度泛函理论计算了MgO在零温(0K)下和0~200GPa静水压范围内的晶体结构和弹性模量,以及B1、B4和B8相结构的MgO的声速随压力的变化。利用准简谐近似下的Debye模型,通过拟合三阶Birch-Murnaghan物态方程模拟了高温效应并对三个相在高温高压下的相稳定性做了研究。本工作的计算结果与前人的理论和实验结果符合较好,说明第一性原理结合准简谐Debye模型能够比较准确的模拟矿物如MgO在高温高压下的热力学性质。     

MgO高温高压特性及相变的第一性原理研究

应用第一性原理密度泛函理论计算了MgO在零温(0K)下和0~200GPa静水压范围内的晶体结构和弹性模量,以及B1、B4和B8相结构的MgO的声速随压力的变化。利用准简谐近似下的Debye模型,通过拟合三阶Birch-Murnaghan物态方程模拟了高温效应并对三个相在高温高压下的相稳定性做了研究。本工作的计算结果与前人的理论和实验结果符合较好,说明第一性原理结合准简谐Debye模型能够比较准确的模拟矿物如MgO在高温高压下的热力学性质。     

第一性原理计算航天新材料金属锆的压缩行为

在密度泛函理论的基础上,运用局域密度近似(LDA)和广义梯度近似(GGA)两种方法,并通过第一性原理等方法对锆晶体在0到50GPa下的物理性质进行了深入、系统的研究。结果表明,相比于广义梯度近似的计算结果而言,局域密度近似的计算结果和实验值更接近,但两者相差不是很大。并通过经典方法对锆晶体模拟计算了高压下的结构性质,得到的结果表明,在讨论的压强范围内,晶格参数与晶胞体积发生了非线性的变化,然而晶胞能量却随压强线性变化。通过对其相应的弹性性质的分析,发现随着压强的增大,锆晶体获得较好的延展性以及力学的稳定性。同时还得到了关于金属锆的能带结构特性及其光学性质,证实压强的改变对金属锆电子性质不会带来太大影响。     

Study of the effects of an adatom Sn on the Cu surface electromigration using a first principles method

It is well known that the doped Sn can effectively suppress the electromigration of Cu thin-film interconnects in integrated circuits. In this paper, the first-principles method was introduced to investigate the suppression mechanism. All the calculations were performed on Sn or Cu adatom/Cu (0 0 1), (1 1 0) and (1 1 1) surface systems within local density approximation. As a Sn adatom was attached to the Cu surface, stable CuSn bonds were formed. The energy calculations show that the Sn/Cu system was more stable than Cu/Cu system with the same structure. Analysis of density of states shows that the nearest neighbor Cu atoms were stabilized by CuSn bond relative to the Cu atoms which are far away from the adatom. The diffusion barrier energies were calculated and found to be consistent with the experimental results. Also, bond population analysis shows that stronger covalent bonds were formed between Sn and Cu relative to that between Cu and Cu. All the results indicate that the CuSn bond plays an important role in the suppression of Cu electromigration.