三方和四方相PbZr0.5Ti0.5O3 的结构稳定性和电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理超原胞方法和虚晶近似方法, 在局域密度近似和广义梯度近似下系统研究了三方相和四方相 PbZr_0.5Ti_0.5O₃的能量稳定性、原子结构以及电子结构. 计算结果表明三方相的能量比四方相低, 说明三方相结构更加稳定, 并且发现利用广义梯度近似计算的结构参数与实验值符合得更好. 电子结构表明, 两种相的Ti/Zr的3d电子和O的2p电子间存在明显的轨道杂化, 并且Ti-O之间的作用比Zr-O作用更强;Pb的6s和5d电子与O的2s和2p电子也分别存在轨道杂化. 而三方相中Pb的5d电子与O的2s电子杂化比四方相更强, 进一步说明三方相比四方相结构更加稳定.     

A位等价与非等价取代对(K0.5Na0.5)NbO3陶瓷极化的影响

研究了在铌酸钾钠基陶瓷中,A位等价与非等价取代对陶瓷极化温度和极化电场的影响,结果表明:A位等价取代的陶瓷对极化温度和极化电场没有强烈依赖性,可以使极化足够充分,能有效提高铌酸钾钠基陶瓷的压电性能;相反,A位非等价取代的陶瓷对极化温度和极化电场敏感,容易击穿,极化不充分,限制了铌酸钾钠基陶瓷的压电性能. 关键词： 铌酸钾钠 极化 压电     

四方铁电体PbFe0.5Nb0.5O3精细结构的第一性原理研究

采用全电势线性缀加平面波法(FPLAPW),计算了PbFe0.5Nb0.5O3的总能量,从而确定了它的四方铁电稳态结构,即B位离子(Fe和Nb)在四方铁电相的平衡构型.计算结果表明,Fe相对于氧八面体沿[001]方向的位移为0.022nm,而Nb的偏心位移为0.0020nm,其中铁电非稳性主要归因于前者 关键词： 铁电体 密度泛函理论 平面波法     

SrCoO3电子结构和磁学性质的第一性原理研究

通过基于密度泛函理论的广义梯度近似GGA+U方法对铁磁相SrCoO₃的电子结构和磁学性质进行了系统研究.结果表明:随着U值的增大,对于Co离子,主自旋方向的t_2g和e_g态向低能级移动,而次自旋方向的t_2g和e_g态向高能级移动;O2p电子态的分布基本不随U变化.能带结构表明,U大约在7-8eV之间时,SrCoO₃由金属性转变为半金属性.U值小于7eV时,Co离子的磁矩随着U值的增大几乎成线性增大,而当U大于7eV后基本保持不变.结合实验结果,本文认为U取8eV时得到的计算结果更为合理,Co离子的磁矩为3.19μв,且SrCoO₃表现出半金属特性.     

四方相BaTiO3铁电性的第一性原理研究

在广义梯度近似下，利用超软赝势对立方相和四方相BaTiO3晶胞中Ti原子沿c轴位移时体系的能量、原子间电子云重叠布局数和各原子上的净电荷等进行了自洽计算.结果显示，当Ti原子沿c轴位移0012nm时，四方相BaTiO3体系能量最低，其自发极化强度为0261C/m2，该结果与实验数据相符合；同时表明，O原子的2p轨道和Ti原子的3d轨道的杂 化是BaTiO3晶体出现铁电性的重要原因. 关键词： 四方相BaTiO3 铁电性 位移 态密度     

Co,N共掺杂锐钛矿相TiO2电子结构和光学性质的第一性原理研究

二氧化钛（TiO₂）作为一种性能优良的光催化剂已经被广泛地研究和使用.本研究中利用了第一性原理和GGA+U方法,对锐钛矿结构TiO₂晶体三种可能的（Co,N）共掺杂体系的几何结构、形成能、电子结构和光吸收系数进行了研究,并与单掺杂（Co/N）体系进行了对比.结果表明,在三种共掺杂TiO₂中,Co与N相邻时晶格畸变最小,但掺杂原子周围晶格畸变较大;同时,较低的形成能表明此种共掺杂结构最容易形成;此外,因为C0与N成键,其杂质能级的数目与能量较其他共掺杂结构有较大差异.（Co,N）共掺杂体系与未掺杂TiO₂的相比,其禁带宽度较小,禁带中存在杂质能级,因此其吸收边红移,在可见光波段有较好的光吸收能力.故（Co,N）共掺杂可以很好地提升锐钛矿型TiO₂在可见光波段的光催化性能.     

单斜相HfO2薄膜弹性常数的第一性原理计算

用电子束蒸发沉积在K9玻璃基底上镀制HfO2薄膜,沉积温度为200℃,蒸发速率为0.03nm/s。由X射线衍射谱可知薄膜出现明显结晶,且为单斜相和正交相混合结构,其中单斜相占明显优势。用Jade5软件分析得到单斜相HfO2的晶格常数a,b,c以及晶格矢量a和c之间的夹角β。基于得到的晶格常数建立了单斜相HfO2薄膜的晶体结构模型。同时建立固态单斜相HfO2的晶体结构模型进行对比。通过密度泛函理论(DFT)框架下的平面超软赝势法,采用两种不同的交换关联函数:局域密度近似(LDA)中的CA-PZ和广义梯度近似(GGA)中的质子平衡方程(PBE),计算了薄膜态和固态单斜晶相HfO2的弹性刚度系数矩阵Cij和弹性柔度系数矩阵Sij,Reuss模型、Voigt模型和Hill理论下的体积模量和剪切模量,材料平均杨氏模量和泊松比。此外还计算得到薄膜态和固态单斜晶相HfO2在不同方向上的杨氏模量。     

β-Si3N4电子结构和红外和拉曼频率的第一性原理研究

分别用PW91,B3LYP两种密度泛函方法和全电子高斯基组对β-Si₃N₄的几何结构进行全优化(包括晶格参数和原子坐标),结果和实验值符合良好. 同时计算了能带结构和态密度.在此基础上分别用上述两种方法计算了Γ点拉曼振动频率,并按对称性进行分类,将得到的11种拉曼活性模式的频率值与实验值以及其他文献值进行了比较,进一步确定了A_g模式为中等频率,值约459cm^-1. 计算结果表明 关键词： 振动频率 密度泛函理论 电子结构 3N₄')" href="#">β-Si₃N₄     

高压下BeP2N4结构相变和电子结构的第一性原理计算

 基于密度泛函理论的第一性原理方法,计算了硅铍石型和尖晶石型结构BeP₂N₄的总能量随体积的变化关系。利用Brich-Murnaghan状态方程,通过能量和体积拟合,得到了2种结构的体变模量及其对压强的一阶导数。在压力作用下,BeP₂N₄的相变是从硅铍石型结构（空间群R-3,No.148）转变到尖晶石型结构（空间群Fd-3m,No.227）,计算出的相变点与其它理论值符合得非常好。同时计算了BeP₂N₄的相对晶格常数a/a₀和相对体积V/V₀的压缩率,在低压下发现,尖晶石结构BeP₂N₄的压缩率接近金刚石,进一步计算了不同压力下的体弹模量B_H、剪切模量G_H、B_H/G_H和杨氏模量E。此外,对两种结构的BeP₂N₄的电子态密度和带隙随压强的变化关系进行了计算和分析。结果表明：在压力作用下,上价带顶向费米能级移动,并有一定的展宽。Be—N、P—N键缩短,电子转移增加,导致电荷发生重新分布。     

Co,Zn共掺铌酸锂电子结构和吸收光谱的第一性原理研究

利用基于密度泛函的第一性原理的计算方法,研究了Co单掺及Co和Zn共掺LiNbO_3晶体的电子结构和吸收光谱.研究显示,各掺杂体系铌酸锂晶体的带隙均较纯铌酸锂晶体变窄. Co:LiNbO_3晶体禁带宽度为3.32 eV; Co:Zn:LiNbO_3晶体, Zn的浓度低于阈值或达到阈值时,禁带宽度分别为2.87或2.75 eV. Co:LiNbO_3晶体在可见-近红外光波段2.40, 1.58, 1.10 eV处形成吸收峰,这些峰归结于Co 3d分裂轨道的跃迁;加入抗光折变离子Zn~(2+),在1.58, 1.10 eV处的吸收峰增强,可以认为Zn~(2+)与Co~(2+)之间存在电荷转移,使e_g轨道电子减少,但并不影响t_(2g)轨道电子.结果表明,晶体中的Co离子在不同共掺离子下可充当深能级中心(2.40 eV),或可充当浅能级中心(1.58 eV),两种情况下,掺入近阈值的Zn离子均有助于实现优化存储.     

First-principles study of structural,electronic and optical properties of ZnF_2

The structural, electronic, and optical properties of rutile-, CaC12-, and PdF2-ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile-, CaCl2-, and PdF2-ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.     

First-principles study of the structural, elastic, electronic and optical properties of the monoclinic BiScO3

The structural, elastic, electronic and optical properties of the monoclinic BiScO₃ are investigated in the framework of the density functional theory. The calculated structural parameters are in agreement with the experimental values. Moreover, the structural stability of BiScO₃ system has been confirmed by the calculated elastic constants. The band structure, density of states, charge transfers and bond populations are also given. The results indicate that BiScO₃ has a direct band gap of 3.36 eV between the occupied O 2p states and unoccupied Bi 6p states, and its bonding behavior is a combination of covalent and ionic nature. Finally, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss function and dielectric function of the monoclinic BiScO₃ are calculated. In addition, the variation of the static dielectric constants ε₁(0) as a function of pressure for BiScO₃ is also discussed.     

New crystallographic dielectric phase boundary in K0.5Na0.5NbO3‐based lead‐free ceramics

0.979K_0.5Na_0.5Nb_1‐xSb_x O₃‐0.021Bi_0.5Na_0.5TiO₃ (KNNSx ‐BNT) lead‐free piezoelectric ceramics were fabricated by conventional solid state reaction technique, and their phase transition and electrical properties were studied. With the increase of x, the rhombohedral‐orthorhombic phase transition temperature of the ceramics increases. Finally, both the rhombohedral‐orthorhombic and orthorhombic‐tetragonal phase transitions of the ceramics were modified to be around room tempera‐ ture when about 6% Sb were substituted for the Nb site, resulting in the formation of a new phase boundary separating rhombohedral and tetragonal phases. The formation of the new phase boundary results in excellent properties for the ceramics, that is, the KNNS0.05‐BNT ceramic shows an enhancement in piezoelectric properties: d₃₃ = 380 pC/N and k_P = 0.438. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First-principles study of structural,electronic and optical properties of orthorhombic SrZrO3

We have investigated the structural parameters, electronic structure and optical properties of orthorhombic SrZrO₃ using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). Our calculated structural parameters are in good agreement with the previous theoretical and experimental data. Band structure, density of states and chemical bonding have been systematically studied. Furthermore, the complex dielectric function, refractive index, extinction coefficient, optical reflectivity, absorption coefficient, loss function and optical conductivity are calculated, which show an optical anisotropy in the components of polarization directions (100), (010) and (001).     

掺Cd氧化锌的电子结构及相结构稳定性的第一性原理研究

采用基于密度泛函理论结合投影缀加平面波方法的VASP软件包,在考虑所有掺杂原子构型的前提下,对Cd掺杂ZnO合金的晶格常数、禁带宽度、电子态密度和形成焓进行了计算,分析了Cd含量和掺杂原子构型对纤锌矿wz-Zn_1-xCd_xO合金的电子结构和结构稳定性的影响.计算结果表明:随着Cd含量的不断增加,纤锌矿ZnCdO合金的平均晶格常数a,c均线性增加,但c/a的比值不会发生显著的变化;纤锌矿ZnCd 关键词： 密度泛函理论 ZnCdO合金 电子结构 形成焓     

First-principles study of the electronic structure and optical properties of cubic Perovskite NaMgF_3

The structural, electronic, and optical properties of cubic perovskite NaMgF3 are calculated by plane-wave pseudopo- tential density functional theory. The calculated lattice constant a0, bulk modulus B0, and the derivative of bulk modulus B~ are 3.872/~, 78.2 GPa, and 3.97, respectively. The results are in good agreement with the available experimental and theo- retical values. The electronic structure shows that cubic NaMgF3 is an indirect insulator with a wide forbidden band gap of Eg = 5.90 eV. The contribution of the different bands is analyzed by total and partial density of states curves. Population analysis of NaMgF3 indicates that there is strong ionic bonding in the MgF2 unit, and a mixture of ionic and weak covalent bonding in the NaF unit. Calculations of dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, optical reflectivity, and conductivity are also performed in the energy range 0 to 70 eV.