Ba0.5Sr0.5TiO3电子结构和光学性质的第一性原理研究

利用第一性原理研究了Ba_0.5Sr_0.5TiO₃的能带结构和光学性质.结果表明,导带和价带都来源于钛原子3d轨道和氧原子2p轨道的杂化.导带主要由钛原子的3d轨道贡献,价带主要由氧原子的2p轨道贡献.吸收系数为10⁵ cm^-1量级,且吸收主要集中在低能区.折射率为n（0）=2.1,结果与实验符合. 关键词： 第一性原理 能带结构 光学性质     

Ba0.5Ca0.5ZrO3电子结构和光学性质的第一性原理研究

从第一性原理出发,利用密度泛函理论体系下的广义梯度近似,研究了Ba0.5Ca0.5ZrO3的电子结构和光学性质.计算得到该晶体的晶格常数为4.1823 A,且此材料是一种间隙的半导体材料,价带和导带都来源于Ba原子、Ca原子、Zr原子的d态和O原子的p态电子间的杂化.吸收系数为105 cm-1量级,且吸收主要集中在低能区.静态折射率为1.79,能量损失峰出现在10.8 eV处.该研究结果为Ba0.5Ca0.5ZrO3光电材料的设计和应用提供了理论依据.     

BaHf0.5Ti0.5O3电子结构与光学性质的第一性原理研究

基于密度泛函理论体系下的广义梯度近似（GGA）,利用第一性原理方法研究了BaHf_0.5Ti_0.5O₃的电子结构和光学性质.计算结果表明,BaHf_0.5Ti_0.5O₃是一种间接带隙半导体材料,其导带底主要由Ba、Hf和Ti的d态电子构成,价带顶则主要由O的p态、Hf和Ti的d态电子构成;理论计算的介电函数最高峰的峰位与实验结果吻合较好,相对误差小于4%;吸收系数最大峰值为2.43×10⁵cm^-1,且吸收主要集中在低能区,静态折射率为2.01,能量损失峰出现在13.24 eV处.研究结果为BaHf_0.5Ti_0.5O₃光电材料设计与应用提供了理论依据.     

Ba0.5Sr0.5TiO3有序构型的第一性原理研究

采用第一性原理计算了Ba_0.5Sr_0.5TiO₃三种有序构型的晶格结构和对应的电子结构,晶格结构的详细分析结果表明BST{100}有序构型为四方相,Ti-O八面体中Ti原子和Ba-Sr平面上的O原子沿[100]方向分别偏心位移0.040 Å和0.065 Å,八面体畸变导致反平行自发极化出现,构型处于反铁电态. BST{110}构型也是四方相,并且(110)和(1 关键词： 钛酸锶钡 第一性原理 有序结构 铁电性     

0.5NdAlO3-0.5CaTiO3电子结构及光学性质的第一性原理计算

采用基于第一性原理密度泛函理论的平面波赝势方法,对0.5NdAlO₃-0.5CaTiO₃晶体进行结构优化,并对其能带结构,态密度和光学性质进行了理论计算.结构优化后晶格参数与实验数据相符合,误差小于1%;能带计算结果表明0.5NdAlO₃-0.5CaTiO₃为间接带隙,带隙值为0.52eV;费米面附近的能带由Nd-4f,O-2p,Nd-4p,Al-3p,Ti-4d层的电子态密度确定.同时也计算了该结构的介电函数,反射率和复折射率等光学性质.     

BaHf0.5Ti0.5O3电子结构与光学性质的第一性原理研究

基于密度泛函理论体系下的广义梯度近似（GGA）,利用第一性原理方法研究了BaHf0.5Ti0.5O3的电子结构和光学性质．计算结果表明,BaHf0.5Ti0.5O3是一种间隙的半导体材料,其导带底主要由Ba、Hf和Ti的d态电子构成,价带顶则主要由O的p态、Hf 和Ti的d态电子构成;理论计算的介电函数最高峰的峰位与实验结果吻合较好,相对误差小于4%;吸收系数最大峰值为2.43×105cm-1,且吸收主要集中在低能区,静态折射率为2.01,能量损失峰出现在13.24eV处．研究结果为BaHf0.5Ti0.5O3光电材料设计与应用提供了理论依据．     

Cu_3N弹性和热力学性质的第一性原理研究

利用基于密度泛函理论的第一性原理全势线性缀加平面波方法,研究了立方反ReO3结构Cu3N在零温(0K)零压下的平衡晶格常数、体弹模量及其对压强的一阶导数,计算结果与其他实验及理论结果基本相符.同时得出Cu3N的弹性常数,Poisson比等,并分析出Cu3N在零温零压下是稳定的.通过准谐Debye模型计算Cu3N的热力学性质,得到了Cu3N的晶格常数、等压比热容、等容比热容、热胀系数与温度和压强之间的关系,同时计算出不同温度不同压强下其体弹模量及Debye温度的值。     

过渡金属铝化物FeAl和CoAl弹性性质的第一性原理研究

本文通过第一性原理计算(密度泛函理论结合均匀形变方法)得到过渡金属铝化物FeAl和CoAl的二阶和三阶弹性常数,这些弹性常数是通过拟合计算出的能量与应力关系得到的。计算结果和理论数据及实验值符合的很好。接下来本文又研究了FeAl和CoAl在不同压强下的弹性性质。不同压强下的弹性常数Cij,体模量B,剪切模量G,泊松比σ 也成功的得到了。B/G比值和柯西压强 PC 都表明在零压下FeAl和CoAl表现出脆性。在压强小于60GPa的情况下,增大压强可以增强它们的韧性,但它们始终表现为脆性。     

立方KCaF_3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波近似法计算研究了立方钙钛矿KCaF_3的弹性、电子和光学性质.基态时,KCaF_3平衡晶格常数、体积弹性模量和实验及其他计算值一致.根据Hooke定律和Christoffel方程,研究了KCaF_3弹性常数Cij、体积弹性模量B、各向同性波速和弹性各向性异性因子随压力的变化关系.从电子能带理论出发,计算得到了KCaF_3电子能带、态密度和Milliken电荷布居数,并对其电子性质进行了详细分析.结果显示:立方钙钛矿KCaF_3为直接带隙绝缘体材料,其禁带宽度为6.22 e V;电荷主要从Ca和K原子向F原子转移;立方钙钛矿KCaF_3属于纯粹的共价型化合物.同时,本文还计算研究了KCaF_3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质.     

立方KCaF3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波近似法计算研究了立方钙钛矿KCaF3的弹性、电子和光学性质。基态时,KCaF3平衡晶格常数、体积弹性模量和其他计实验和算值一致。根据Hooke定律和Christoffel方程,研究了KCaF3弹性常数Cij、体积弹性模量B、各向同性波速和弹性各向性异性因子随压力的变化关系。从电子能带理论出发,计算得到了KCaF3电子能带、态密度和Milliken电荷布居数,并对其电子性质进行了详细分析。结果显示：立方钙钛矿KCaF3为直接带隙绝缘体材料,其禁带宽度为6.22eV;电荷主要从Ca和K原子向F原子转移;立方钙钛矿KCaF3属于纯粹的共价型化合物。同时,本文还计算研究了KCaF3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质。     

Picosecond nonlinear optical response of Ba0.5Sr0.5TiO3 thin films fabricated by pulsed laser deposition

Well-crystallized Ba_0.5Sr_0.5TiO₃ thin films with good surface morphology were prepared on MgO(1 0 0) substrates by pulsed laser deposition technique at a deposition temperature of 800 °C under the oxygen pressure of 2 × 10⁻³ Pa. X-ray diffraction and atomic force microscopy were used to characterize the films. The full width at half maximum of the (0 0 2) Ba_0.5Sr_0.5TiO₃ rocking curve and the root-mean-square surface roughness within the 5 μm × 5 μm area were 0.542° and 0.555 nm, respectively. The nonlinear optical properties of the films were determined by a single beam Z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that Ba_0.5Sr_0.5TiO₃ thin films exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 5.04 × 10⁻⁶ cm²/kW and β = 3.59 × 10⁻⁶ (m/W), respectively.     

First-principles study of the structural,elastic, and optical properties for Sr0.5Ca0.5TiO3

A detailed theoretical study of the structural, elastic, and optical properties for Sr0.5Ca0.5TiO3 is carried out by first- principles calculations. The band structure exhibits a direct bandgap of 2.08 eV at the F point in the Brillouin zone. The bulk modulus, shear modulus, Young＇s modulus, and Poisson＇s ratio are derived based on the calculated elastic constants. The bulk modulus B = 153 GPa and shear modulus G = 81GPa are in good agreement with available experimental data. Poisson＇s ratio v = 0.275 suggests that Sr0.sCa0.sTiO3 should be classified as being a ductile material. Using the electronic band structure and density of states, we analyze the interband contribution to the optical properties. The real and imaginary parts of the dielectric function, as well as the optical properties such as the optical absorption coefficient, refractive index, extinction coefficient, and energy-loss spectrum are calculated. The static dielectric constant ε1 （0） and the refractive index n（0） are also investigated.     

Pulsed laser ablation of La0.5Sr0.5CoO3

The ablation process of La_0.5Sr_0.5CoO₃ by a spatial uniform 248 nm excimer laser beam has been characterized. Ablation rates, changes in the target surface morphology and composition have been studied as a function of the laser fluence. The dependence of the ablation rate on the laser fluence has been explained by a model based on plasma absorption. The threshold fluence for complete dissociation of the surface has been determined and is found equal to 0.7 J/cm². Above this threshold, a steady-state target surface condition is obtained within 30 pulses. Below the threshold fluence, CoO nuclei form, which have a pronounced effect on the target composition and morphology. In addition, ellipsometric and reflectance spectra have been measured in the photon energy range of 1.8 to 5.0 eV.     

过渡金属Tc及其氮化物TcN，TcN$lt;sub$gt;2$lt;/sub$gt;，TcN$lt;sub$gt;3$lt;/sub$gt;与TcN$lt;sub$gt;4$lt;/sub$gt;低压缩性的第一性原理计算研究

采用基于密度泛函理论的第一性原理计算方法研究了Tc及其氮化物的弹性性质、电子结构、原子布局数等,并分析和计算了它的氮化物的理论硬度. 结果表明随着Tc中氮的掺入量的渐增,出现层状结构,它们的弹性模量并非单调增加,其中TcN的体弹模量最大而剪切模量最小;层状结构的TcN₃的剪切模量最大而体弹模量最小,TcN₄的理论计算硬度最大. 说明在Tc的氮化物中,其体弹模量与材料的晶体结构、平均每个原子上的电荷密度和材料的质量密度有关;化学键的共价性结构和氮元素的含量对理论计算硬度有正作用;而剪切模量的极大值则与其层状结构及体系中一定量的方向基本一致的N—N键相关. 关键词： 第一性原理 弹性性质 电子结构     

First-principles investigation of the effects of strain on elastic thermal,and optical properties of CuGaTe_2

Based on the density functional theory, the influences of strain on structural, elastic, thermal and optical properties of CuGaTe_2 are discussed in detail. It is found that the tensile strain on CuGaTe_2 is beneficial to the decrease of lattice thermal conductivity by reducing the mean sound velocity and Debye temperature. Moreover, all strained and unstrained CuGaTe_2 exhibit rather similar optical characters. But the tensile strain improves the ability to absorb sunlight in the visible range.These research findings can give hints for designing thermoelectric and photovoltaic devices.     

Photoluminescence at room temperature in Ba0.5Sr0.5TiO3 ceramics

We present in this paper the experimental results of photoluminescence spectra of Ba_0.5Sr_0.5TiO₃ ceramics. An emission band centered at about 920 nm has been observed at room temperature. When we change the amount of oxygen vacancies in these samples by thermal treatment, these samples show an enhancement of luminescence. Our experimental results indicate that the origin of the photoluminescence is related to the oxygen vacancies in these samples.     

几种三元过渡金属碳化物弹性及电子结构的第一性原理研究

基于密度泛函理论平面波方法研究了六方WC型Re_xW_1－xC(x=1, 0.25, 0.75, 0),Re_0.5Os_0.5C和Os_0.5W_0.5C的晶体结构、弹性和电子结构性质.研究发现Re_0.25W_0.75C晶体具有优异的弹性性能及稳定性,其剪切模量(312 GPa)超过了所有其他实验合成和     

First-principles study on the structure instability and electronic structure of cubic Ba0.5Sr0.5TiO3

We present first-principles calculations on the structure instability and the electronic structure properties of cubic Ba_0.5Sr_0.5TiO₃ (BST). The calculated total energy result shows that the Sr ions have a more important effect on the structure instability of BST system than the Ba ions. The off-center displacement of the Sr ions will lower the system energy and makes it instable. In order to understand the interaction between ions, the density of states and the charge density distribution were calculated. From the analysis of the density of states, we conclude that the hybridization between Ba p and O p is stronger than that between Sr p and O p. This is consistent with the analysis of the charge density distribution.