层状铁电晶体MBiP2X6(M=Cu,Ag;X=S,Se)拉曼光谱的第一性原理研究

基于密度泛函理论的第一性原理计算,本文首次研究了过渡金属卤代磷酸酯MBiP2X6(M=Cu,Ag;X=S,Se)三维晶体和二维晶体的声子色谱和拉曼散射谱.以反铁电晶体AgBiP2Se6为例,根据声子谱的计算结果,验证了低温反铁电相是动力学稳定的.结合声子总态密度可以得到:声子谱光学支的低频部分主要来源于阳离子M+和Bi...     

镍基超导母体材料EuNi_2Si_2的结构和热力学性质研究

应用Chen-Mbius晶格反演获得的原子间相互作用势,对镍基超导母体材料EuNi2Si2不同空间群的结构进行结构弛豫、切变拉伸、随机扰动和X射线衍射谱的分析.研究表明,空间群号为139结构的EuNi2Si2母体材料能量最低,结构最稳定.另外,还计算了空间群号为139稳定晶格结构的声子态密度和热力学性质.计算结果表明:对于声子态密度,原子质量较大的稀土元素Eu在低频范围内贡献最大,随着频率的升高,原子质量较小的元素Si的贡献越来越突出;对于比热容和振动熵,在低温区元素Eu和Ni的贡献较大,随着温度的升高,元素Si的贡献越来越突出.     

Ti_3AC_2相(A=Si,Sn,Al,Ge)电子结构、弹性性质的第一性原理研究

采用第一性原理的密度泛函理论平面波赝势法,通过广义梯度近似研究了Ti_3AC_2相(A=Si,Sn,Al,Ge)的相结构、能量、电子结构和弹性性质.首先对六方晶相结构的Ti_3AC_2(A=Si,Sn,Al,Ge)四个相进行几何优化,对其能带结构、总态密度、分态密度和电荷密度分布以及弹性性质进行研究,并计算各相的内聚能与形成能.计算结果表明:Ti_3GeC_2较其他三相稳定,Ti_3AlC_2的形成能最低,说明Ti_3AlC_2较Ti_3SiC_2,Ti_3SnC_2和Ti_3GeC_2更易生成;Ti_3AC_2(A=Si,Sn,Al,Ge)各相在费米能级处的电子态密度较高,材料表现出较强的金属性,同时各相的导电性为各向异性.Ti_3AC_2(A=Si,Sn,Al,Ge)各相的导电性主要由Ti的3d电子决定,A(A=Si,Sn,Al,Ge)的p态电子和C的2p态电子也有少量贡献.决定材料电学性质的主要是Ti的3d,A的P和C的2P态电子的P-d电子轨道杂化,而P-d电子轨道杂化成键则使材料具有比较稳定的结构;对Ti_3AC_2相(A=Si,Sn,Al,Ge)弹性性质的研究表明Ti_3AlC_2的原子间结合力较弱,而Ti_3GeC_2的原子间结合力相对较强,材料的强度较大.     

Si3Xn (X=C,O,N;n=1,2)团簇的密度泛函研究

使用密度泛函理论(DFT)的杂化密度泛函B3LYP方法在6-31G*基组水平上对Si3Xn(X=C,O,N;n=1,2)团簇各种可能的构型进行几何结构优化,预测了各团簇的最稳定结构.并对最稳定结构的电子结构、振动特性、成键特性和电荷特性等进行了理论研究.结果表明团簇的几何结构都是平面结构,通常Si3X2出现是Si-X键,较少出现X-X键;而Si3X中出现Si-X键和Si-Si键共存,Si3Xn(X=C,O,N;n=1,2)团簇的电荷布局分布表明这种电荷转移的作用使得团簇中所有X原子呈负电性,Si原子显正电性.处于不同位置的Si原子呈不同大小布局数,而且由于Si3X2的对称性,2个X负电性相同.     

Cr3+掺杂的Cd3 Al2 Ge3O12光谱特性及晶场参数计算

采用高温固相法合成了Cd3Al2 Ge3O12Cr3 多晶材料,利用X射线衍射对其结构进行了分析,通过Cr3 的室温吸收光谱、室温和77 K发射光谱分别对其光谱特性和晶场参数进行了分析和计算.结果表明:在450 nm的蓝光激发下,Cd3Al2 Ge3O3 室温发射光谱主要由三个宽带及附加其上的弱R线构成,分别对应于C3 的4T1,T2,2T2,4T2到4A2能级跃迁.低温时宽带发射变弱,而R线变得强而锐.Cr3 在Cd3Al2Ge3O12中晶场强度Dq/B为2.32,Stokes位移AE,为1919 cm1,黄昆-里斯因子S为5.58.表明在Cd3Al2Ge3O12中Cr3 处于弱的中等晶场强度,电子-声子耦合较强.4T2零声子能级与2能级的间距△E仅为325.5 cm-1,对Cr3 可调谐激光的输出非常有利.     

双能谷效应对N型掺杂Si基Ge材料载流子晶格散射的影响

性能优越的Si基高效发光材料与器件的制备一直是Si基光电集成电路中最具挑战性的课题之一.Si基Ge材料不仅与成熟的硅工艺相兼容,而且具有准直接带特性,被认为是实现Si基激光器最有希望的材料.对Si基Ge材料N型掺杂的研究有利于提示出其直接带发光增强机理.本文研究了N型掺杂Si基Ge材料导带电子的晶格散射过程.N型掺杂Si基Ge材料具有独特的双能谷(Γ能谷与L能谷)结构,它将通过以下两方面的竞争关系提高直接带导带底电子的占有率:一方面,处于Γ能谷的导带电子通过谷间光学声子的散射方式散射到L能谷;另一方面,处于L能谷的导带电子通过谷内光学声子散射以及二次谷间光学声子散射或者直接通过谷间光学声子散射的方式跃迁到Γ能谷.当掺杂浓度界于10~(17)cm~(-3)到10~(19)cm~(-3)时,适当提高N型掺杂浓度有利于提高直接带Γ能谷导带底电子占有率,进而提高Si基Ge材料直接带发光效率.     

压力作用下Mg_2X(X=Si,Ge)相热力学性质的第一性原理研究

采用基于密度泛函理论的第一性原理方法,研究了压力作用下Mg_2Si和Mg_2Ge的结构、弹性和热力学性质。计算结果表明:0GPa压力作用下两者的晶格参数与实验值以及其他理论值吻合较好,且相对晶格常数a/a_0和晶胞体积V/V_0均随压力的增大而减小;在0~25GPa压力作用下,Mg_2Si和Mg_2Ge相体模量B、剪切模量G、杨氏模量E均随压力的增大而增大,材料的刚度和塑性均增强,当压力达到15GPa时,材料由脆性转变为延性。最后借助准谐德拜模型和Gibbs软件,研究了温度与压力对Mg_2Si和Mg_2Ge的德拜温度、体模量、热容和热膨胀系数的影响。     

YPd3X(X=B,C)晶体结构、弹性性质与电子结构的第一性原理研究

在广义梯度近似(GGA)和GGA+ U(在位库仑势)下,采用第一性原理方法系统地研究了三元过渡金属硼碳化合物YPd3 X(X=B,C)的晶体结构、弹性性质、电子结构和成键特性.计算的晶格参数和体弹性模量均与报道的实验结果吻合,而YPd3 X(X=B,C)的弹性参数计算值则表明YPd3C的硬度大于YPd3B.根据晶体机械稳定标准得到YPd3B和YPd3C的失稳临界压强分别约为16.5 GPa和23 GPa.由Pugh经验关系可知YPd3X(X=B,C)均属于韧性材料,且YPd3B的韧性略高于YPd3C.电子能带结构分析表明YPd3B和YPd3C均具有金属特性,且导电能力相当.由态密度和电荷密度分析得知,X与Pd之间形成较强的共价键,而Y与Pd3X之间形成离子键,化学键键能的不同是两种材料的弹性参数存在差异的内在原因.上述的研究结果为YPd3X(X=B,C)的力电材料的设计和应用提供了一定的理论依据.     

YPd3X（X=B,C）晶体结构、弹性性质与电子结构的第一性原理研究

在广义梯度近似（GGA）和GGA+U（在位库仑势）下,采用第一性原理方法系统地研究了三元过渡金属硼碳化合物YPd3X(X＝B,C)的晶体结构、弹性性质、电子结构和成键特性．计算的晶格参数和体弹性模量均与报道的实验结果吻合,而YPd3X(X＝B,C) 的弹性参数计算值则表明YPd3C的硬度大于YPd3B．根据晶体机械稳定标准得到YPd3B和YPd3C的失稳临界压强分别约为16.5GPa和23GPa．由Pugh经验关系可知YPd3X(X=B,C)均属于韧性材料,且YPd3B的韧性略高于YPd3C．电子能带结构分析表明YPd3B和YPd3C均具有金属特性,且导电能力相当．由态密度和电荷密度分析得知,X与Pd之间形成较强的共价键,而Y与Pd3X之间形成离子键,化学键键能的不同是两种材料的弹性参数存在差异的内在原因．上述的研究结果为YPd3X(X=B,C)的力电材料的设计和应用提供了一定的理论依据．     

高压下金属Ba的结构稳定性以及热动力学的第一原理研究

采用第一原理方法计算了高压下金属Ba的三个高压相 Ba-I, Ba-Ⅱ和Ba-V的稳定性及热动力学性质.结果表明, Ba的三个高压相在0 K时在其压力范围内都是动力学和力学稳定的;但随压力增加, Ba-I 和Ba-Ⅱ 的声子谱频率出现异常"软化",而Ba-V则出现"硬化".虽然 Ba-Ⅱ 和 Ba-V 同为六方密堆(hcp)结构,计算表明它们在高压下表现出了不同的弹性各向异性.计算同时发现 Ba-Ⅱ 在更高的压力下仍满足力学稳定条件,但声子谱有虚频存在, 表明动力学失稳是Ba-Ⅱ在压力下向Ba-I!V相转变的原因. 计算和比较了同为六方密堆(hcp)结构的Ba-Ⅱ和Ba-V在高压下的声速、 德拜温度、体模量、剪切模量等力学和热学性质, 展现了金属Ba在压力下的稳定机制和热动力学性质.     

Structural, elastic constants, hardness, and optical properties of pyrite-type dinitrides (CN2, SiN2, GeN2)

The crystal structures, band structures, elastic constants, hardness, and optical properties of pyrite-type dinitrides (CN₂, SiN₂, and GeN₂) are obtained from the density functional theory using the plane-wave pseudopotential (PWP) method within the local density and generalized gradient approximations. The formation enthalpies for AN₂ (A=C, Si, and Ge) compounds suggest the three structures that are stable. The calculated band structures show the indirect gaps (Γ−R) in CN₂, SiN₂, and GeN₂. The intrinsic hardnesses of AN₂ (A=C, Si, and Ge ) compounds are calculated. Our results show that the cubic CN₂ and SiN₂ are superhard materials. Furthermore, we studied the optical properties such as the complex dielectric function and the electron energy loss spectra.     

First-principle investigation on the thermodynamics of X_2N_2O(X = C,Si, Ge) compounds

The structures under different pressures, elastic properties, electronic structures and lattice vibrations of the X_2N_2O(X = C, Si, Ge) compounds are investigated by using the first-principle method. Based on the phonon density of state,the thermodynamic properties of the present compounds are studied under different pressures and at different temperatures. The structural parameters including the bond lengths and bond angles are in agreement with available experimental measurements and theoretical calculations. We employ the elastic theory to calculate the nine independent elastic constants(C_(ij)) and the derived elastic moduli(B, G, E, v). Results indicate that these X_2N_2O(X = C, Si, Ge) compounds are mechanically stable and show the brittle behaviors. The electronic properties of the present compounds are analyzed by using the band structure and density of states. The phonon dispersion calculations imply that the present compounds are dynamically stable. Based on the quasi-harmonic approximation, the calculations of the specific heat indicate that the temperature in a range of 0 K–1500 K and pressure in a range of 0 GPa–40 GPa have a large effect on the thermal quantities of Ge_2N_2O,compared with on those of the C_2N_2O and Si_2N_2O compounds.     

Theoretical study on stability,mechanical properties and thermodynamic parameters of the orthorhombic-A2N2O (A=C,Si and Ge)

The structural stability, mechanical properties and thermodynamic parameters such as Debye temperature, minimum thermal conductivities of orthorhombic-A₂N₂O (A=C, Si and Ge) are calculated by first principles calculations based on density functional theory. The calculated lattice parameters, elastic constants of Si₂N₂O and Ge₂N₂O using PBEsol function are consisted with the experimental data and other calculated values. The full set elastic constants of the orthorhombic-A₂N₂O (A=C, Si and Ge) are calculated by stress–strain method. The mechanical moduli (bulk modulus, shear modulus and Young's modulus) are evaluated by the Voigt–Reuss–Hill approach. The orthorhombic-C₂N₂O exhibits larger mechanical moduli than the other two structures. The hardness of orthorhombic-A₂N₂O (A=C, Si and Ge) is evaluated according to the intrinsic hardness calculation theory of covalent crystal relying on Mulliken overlap population. The results indicate that the orthorhombic-C₂N₂O is a super hard material. Furthermore, the mechanical anisotropy, Debye temperature and minimum thermal conductivity of the orthorhombic-A₂N₂O (A=C, Si and Ge) have been estimated by empirical methods. The orthorhombic-Ge₂N₂O shows the lowest thermal conductivity, which may have useful applications as gas turbine engines and diesel engines.     

Si，Ge，Zr和Sn掺杂SrTiO3的电子结构和光催化性能第一性原理研究

使用QUANTUM ESPRESSO(QE)软件包实现的密度泛函理论研究了Si, Ge, Zr和Sn掺杂SrTiO₃的结构,电子结构和光催化性能.使用广义梯度近似(GGA)获得SrTiO₃的晶格常数与先前的实验数据非常一致.同时,获得了SrTi_0.875X_0.125O₃(X=Si, Ge, Zr, Sn)四种掺杂体系的晶格常数. SrTiO₃和SrTi_0.875X_0.125O₃(X=Si, Ge, Zr, Sn)四种掺杂的带隙值分别1.853 eV、1.849 eV、1.916 eV、1.895 eV和1.925 eV.在研究五种SrTiO₃体系的光催化性能时,采用剪刀算符对五种SrTiO₃体系的带隙值进行修正.计算本征SrTiO₃和SrTi_0.875X_0.125O₃     

Molecular bonding in the conduction states of the Mg2X (X=Si,Ge, Sn) semiconductors

Summary The chemical bond in the electron-deficient semiconductor series Mg₂X (X=Si, Ge, Sn) is analysed in the pseudopotential self-consistent local-density scheme. Hard-core atomic potentials are used to investigate the physical properties of these compounds and to show the regularities peculiar to the periodic table. The ionic character of these materials is studied and related to the scaling electronegativities of their anions. Our results, in good agreement with the experiment, are discussed in order to explain the molecular behaviour of the bonding conduction states.

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Riassunto Si studia il legame chimico nella serie dei semiconduttori mancanti di elettroni Mg₂X, dove X=Si, Ge e Sn, con il metodo dello pseudopotenziale autocoerente nello schema del funzionale densità locale. Le proprietà fisiche dei composti in esame sono studiate utilizzando potenziali atomici che permettono di evidenziarne le caratteristiche di periodicità. Il carattere ionico di questi materiali è correlato all'elettronegatività degli anioni. I nostri risultati, in buon accordo con l'esperimento, sono discussi in modo da spiegare il comportamento molecolare degli stati di conduzione.

     

Ti_3(Ge_(1-x)Si_x)C_2固溶体力学和热力学性能的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,系统研究了Ti_3(Ge_(1-x)Si_x)C_2(x=0, 0.5, 1)固溶体的晶体结构、弹性性质以及热力学性能.研究结果表明,Ti_3(Ge_(1-x)Si_x)C_2体系均具有力学和热力学稳定结构,并且为脆性材料;Ti_3(Ge_(1-x)Si_x)C_2固溶体的力学性能随Si含量的增加而提高;Ti_3(Ge_(1-x)Si_x)C_2固溶体在室温下具有稳定的晶格结构和较高的晶格热导率,有望用于一些需要良好散热性能电子元器件的封装材料.     

Density functional theory study of structural,electronic,and thermal properties of Pt,Pd, Rh,Ir, Os and PtPdX(X= Ir,Os, and Rh) alloys

The structural, electronic, mechanical, and thermal properties of Pt, Pd, Rh, Ir, Os metals and their alloys Pt Pd X(X= Ir, Os and Rh) are studied systematically using ab initio density functional theory. The groundstate properties such as lattice constant and bulk modulus are calculated to find the equilibrium atomic position for stable alloys. The electronic band structure and density of states are calculated to study the electronic behavior of metals on making their alloys. The electronic properties substantiate the metallic behavior for all studied materials. The firstprinciples density functional perturbation theory as implemented in quasi-harmonic approximation is used for the calculations of thermal properties.We have calculated the thermal properties such as the Debye temperature, vibrational energy, entropy and constant-volume specific heat. The calculated properties are compared with the previously reported experimental and theoretical data for metals and are found to be in good agreement. Calculated results for alloys could not be compared because there is no data available in the literature with such alloy composition.     

Graphene-like Be_3X_2(X=C,Si, Ge,Sn):A new family of two-dimensional topological insulators

Using first-principle calculations, we predict a new family of stable two-dimensional(2 D) topological insulators(TI),monolayer Be_3 X_2(X = C,Si, Ge, Sn) with honeycomb Kagome lattice. Based on the configuration of Be_3 C_2, which has been reported to be a 2 D Dirac material, we construct the other three 2 D materials and confirm their stability according to their chemical bonding properties and phonon-dispersion relationships. Because of their tiny spin-orbit coupling(SOC)gaps, Be_3 C_2 and Be_3 Si_2 are 2 D Dirac materials with high Fermi velocity at the same order of magnitude as that of graphene.For Be3 Ge2 and Be_3 Sn_2,the SOC gaps are 1.5 meV and 11.7 meV, and their topological nontrivial properties are also confirmed by their semi-infinite Dirac edge states. Our findings not only extend the family of 2 D Dirac materials, but also open an avenue to track new 2 DTI.