A novel superhard boron nitride polymorph with monoclinic symmetry

In this work, a new superhard material named Pm BN is proposed. The structural properties, stability, mechanical properties, mechanical anisotropy properties, and electronic properties of Pm BN are studied in this work. Pm BN is dynamically and mechanically stable, the relative enthalpy of Pm BN is greater than that of c-BN, and in this respect, and it is more favorable than that of T-B₃N₃, T-B₇N₇, tP24 BN, Imm2 BN, NiAs BN, and rocksalt BN. The Young's modulus, bulk modulus, and shear modulus of Pm BN are 327 GPa, 331 GPa, and 738 GPa, respectively, and according to Chen's model, Pm BN is a novel superhard material. Compared with its original structure, the mechanical anisotropy of Young's modulus of Pm BN is larger than that of C14 carbon. Finally, the calculations of the electronic energy band structure show that Pm BN is a semiconductor material with not only a wide band gap but also an indirect band gap.     

Structural,Elastic, and Electronic Properties of a New Phase of Carbon

Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the local density approximation and the generalized gradient approximation, the structural, elastic, and electronic properties of carbon with P222₁ phase have been systematically studied in this paper. The calculated results show that the P222₁ phase of carbon is mechanically stable and dynamically stable at ambient pressure. The anisotropy studies of Young's modulus, Poisson's ratio, shear anisotropic factor, the percentage of elastic anisotropy for bulk modulus, the percentage of elastic anisotropy for shear modulus and the universal anisotropic index show that P222₁ phase of carbon exhibits anisotropy. In addition, P222₁ phase is an indirect semiconductor with bandgap of 3.423 eV. But, the band gap of P222₁ phase for carbon increase with increasing pressure.     

Elastic properties of CaCO_3 high pressure phases from first principles

Elastic properties of three high pressure polymorphs of CaCO_3 are investigated based on first principles calculations.The calculations are conducted at 0 GPa–40 GPa for aragonite, 40 GPa–65 GPa for post-aragonite, and 65 GPa–150 GPa for the P2_1/c-h-CaCO_3 structure, respectively. By fitting the third-order Birch–Murnaghan equation of state(EOS), the values of bulk modulus K_0 and pressure derivative K~'_0 are 66.09 GPa and 4.64 for aragonite, 81.93 GPa and 4.49 for post-aragonite, and 56.55 GPa and 5.40 for P2_1/c-h-CaCO_3, respectively, which are in good agreement with previous experimental and theoretical data. Elastic constants, wave velocities, and wave velocity anisotropies of the three highpressure CaCO_3 phases are obtained. Post-aragonite exhibits 25.90%–32.10% V_P anisotropy and 74.34%–104.30% V_S splitting anisotropy, and P2_1/c-h-CaCO_3 shows 22.30%–25.40% V_Panisotropy and 42.81%–48.00% V_S splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO_3 high pressure polymorphs have low isotropic wave velocity and high wave velocity anisotropies. These results are important for understanding the deep carbon cycle and seismic wave velocity structure in the lower mantle.     

The structural, elastic, and electronic properties of the pyrite-type phase for SnO2

We have studied some structural, thermodynamic, elastic, and electronic properties of pyrite-type SnO₂ polymorph by performing ab initio calculations within the LDA approximation. The basic physical properties, in particular lattice constant, bulk modulus, second-order elastic constants (C_ij), and the electronic structure, are calculated, and compared with the available experimental data. In order to gain some further information on the mechanical properties, we have also calculated the Young's modulus, Poison's ratio (ν), anisotropy factor (A), sound velocities, and Debye temperature for the same compound.     

Mechanical anisotropy and electronic properties of X2N2 (XH2 ): first-principles calculations*

The mechanical anisotropy, structural properties, electronic band structures and thermal properties of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are detailed and investigated in this work. The novel silicon nitride phase Si₂ N₂ (SiH₂ ) and germanium nitride phase Ge₂ N₂ (GeH₂ ) in the Cmc 2₁ structure are proposed in this work. The novel proposed Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are both mechanically and dynamically stable. The electronic band calculation of the HSE06 hybrid functional shows that C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are all wide band gap semiconductor materials, and C₂ N₂ (CH₂ ) and Si₂ N₂ (SiH₂ ) are direct band gap semiconductor materials, while Ge₂ N₂ (GeH₂ ) is a quasi-direct band gap semiconductor material, the band gap of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) are 5.634 eV, 3.013 eV, and 2.377 eV, respectively. The three-dimensional and plane distributions of Young’s modulus, shear modulus and Poisson’s ratio of C₂ N₂ (CH₂ ), Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) show that these materials have different degrees of mechanical anisotropy. The order of Young’s modulus of Si₂ N₂ (SiH₂ ) and Ge₂ N₂ (GeH₂ ) in different directions is different from that of C₂ N₂ (CH₂ ). When the tensile axis is in a particular direction, the order of the Young’s modulus of Si₂ N₂ (SiH₂ ): E _[110] <E _[120] <E _[111] <E _[101] <E _[010] =E _[100] <E _[011] <E _[001], and the order of the Young’s modulus of Ge₂ N₂ (GeH₂ ): E _[110] <E _[111] <E _[101] <E _[120] <E _[100] <E _[010] <E _[011] <E _[001] .     

Electronic,optical, and mechanical properties of BN,AlN, and InN with zinc-blende structure under pressure

In this work, the electronic, optical, and mechanical properties of BN, AlN, and InN under the action of pressure are calculated. For each of these compounds, the energy band structure, band gaps(E~L_g, E~Γ_g, E~X_g), refractive index(n),dielectric constants(ε_∞, ε_0), elastic constants(C_11, C_12, C_44), and relevant parameters such as bulk(B_u), shear(S_h), and Young's(Y_0) moduli are studied, and other important parameters such as bond-stretching(α), bond-bending(β) force constant, internal-strain parameter(ζ), effective charges(e~*_T, Z~*), anisotropy factor(I_s), Poisson's ratio(P_o), Cauchy ratio(C_a), the ductility index(μ_D), and linear compressibility(C0_) are also calculated. The effects of pressure on all studied properties are investigated. Our results are in good agreement with the available experimental and theoretical data for BN,AlN, and InN.     

Structural,elastic, and vibrational properties of phase H:A first-principles simulation

Phase H(MgSiO_4H_2), one of the dense hydrous magnesium silicates(DHMSs), is supposed to be vital to transporting water into the lower mantle. Here the crystal structure, elasticity and Raman vibrational properties of the two possible structures of phase H with Pm and P2/m symmetry under high pressures are evaluated by first-principles simulations. The cell parameters, elastic and Raman vibrational properties of the Pm symmetry become the same as the P2/m symmetry at～ 30 GPa. The symmetrization of hydrogen bonds of the Pm symmetry at ～ 30 GPa results in this structural transformation from Pm to P2/m. Seismic wave velocities of phase H are calculated in a range from 0 GPa to 100 GPa and the results testify the existence and stability of phase H in the lower mantle. The azimuthal anisotropies for phase H are A_(P0)= 14.7%,A_(S0)= 21.2%(P2/m symmetry) and A_(P0)= 16.4%, A_(S0)= 27.1%(Pm symmetry) at 0 GPa, and increase to A_(P30)= 17.9%,A_(S30)= 40.0%(P2/m symmetry) and A_(P30)= 19.2%, A_(S30)= 37.8%(Pm symmetry) at 30 GPa. The maximum V P direction for phase H is [101] and the minimum direction is [110]. The anisotropic results of seismic wave velocities imply that phase H might be a source of seismic anisotropy in the lower mantle. Furthermore, Raman vibrational modes are analyzed to figure out the effect of symmetrization of hydrogen bonds on Raman vibrational pattern and the dependence of Raman spectrum on pressure. Our results may lead to an in-depth understanding of the stability of phase H in the mantle.     

NbSi2奇异高压相及其热力学性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO, 并结合第一性原理的VASP程序, 在175 GPa发现NbSi₂的奇异立方高压相. 在此结构中, Nb原子形成金刚石结构, 而Si原子则形成正四面体镶嵌在金刚石结构中. 声子谱计算结果表明该结构是动力学稳定的. 电子结构分析表明, 六角相和立方相NbSi₂均为金属, 对金属性贡献较大的是Nb原子, 而且Nb和Si原子之间存在明显的p-d杂化现象, 电荷更多地聚集在Si四面体中. 利用“应力应变”方法, 计算了NbSi₂的弹性常数, 分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论. 根据剪切模量和体积模量的比值分析了NbSi₂两种相结构的脆性和延展性, 发现压力会导致六角相NbSi₂的延展性增加, 但对立方相结构的延展性影响较小; 采用经验算法计算了NbSi₂两种相结构硬度变化情况, 结合这一比值进行了详细的分析. 弹性各向异性计算结果表明, 随着压力增加, 六角结构的各向异性增强, 而立方结构的各向异性减小.     

Phase transition,elastic and electronic properties of topological insulator Sb_2Te_3 under pressure: First principle study

The phase transition, elastic and electronic properties of three phases(phase Ⅰ,Ⅱ, and Ⅲ) of Sb_2Te_3 are investigated by using the generalized gradient approximation(GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus,Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4 GPa(Ⅰ→Ⅱ) and 14.1 GPa(Ⅱ→Ⅲ), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. Phases Ⅰ and Ⅲ are brittle, while phaseⅡ is ductile. Of the three phases, phaseⅡ has the most serious degree of elastic anisotropy and phase Ⅲ has the slightest one.Finally, we investigate the partial densities of states(PDOSs) of three phases and find that the three phases possess some covalent features.     

Hf-N体系的晶体结构预测和性质的第一性原理研究

为了寻找具有优异力学性能的新型超高温陶瓷材料, 结合进化算法和第一性原理, 系统研究了Hf-N二元体系所有稳定存在的化合物及其晶体结构. 除了实验已知的岩盐结构的HfN之外, 本文还找到了Hf₆N(R-3), Hf₃N(P6₃22), Hf₃N₂(R-3m), Hf₅N₆(C2/m)和Hf₃N₄(C2/m)五种新结构, 基于准简谐近似原理计算了这些稳定结构的声子谱以验证其动力学稳定性, 常温甚至更高温度下的吉布斯自由能以验证其高温热力学稳定性. 结果表明, 这些结构是动力学稳定的, 且在1500 K以下都是热力学稳定的. 同时, 本文还列出了在搜索过程中出现的空间对称性较高、能量较低的亚稳态结构, 包括Hf₂N(P4₂/mnm), Hf₄N₃(C2/m), Hf₆N₅(C2/m), Hf₄N₅(I4/m), Hf₃N₄(I-43d)和Hf₃N₄(Pnma). 之后计算了上述所有结构的力学性质(弹性常数、体模量、 剪切模量、 杨氏模量、硬度), 随着N 所占比例的增加, 硬度呈现的整体趋势是先增大后下降, 在Hf₅N₆处取得最大值, 为21 GPa. 其中Hf₃N₂和Hf₄N₅也展现出了较高的硬度, 都为19 GPa. 最后, 计算了这些结构的电子态密度和晶体轨道汉密尔顿分布, 从电子结构的角度分析了力学性能的成因. 研究结果显示, 较强的Hf-N共价键和较低的结构空位率是Hf₅N₆具有优异力学性能的主要原因.     

IrB和IrB$lt;sub$gt;2$lt;/sub$gt;力学性质的第一性原理计算

采用平面波赝势密度泛函理论对0–100 GPa静水压下P₁ -IrB（空间群Pnma）和P₅ -IrB₂（空间群Pmmn）结构的平衡态晶格常数、弹性常数等性质进行了研究. 研究结果表明,P₁ -IrB不可压缩性随着压强的增加而增强;P₅ -IrB₂ 结构在0–100 GPa范围内弹性常数、体弹模量、剪切模量均呈现出有规律的变化,当所加压强为50 GPa时,杨氏模量和在b方向的晶格常数发生异常变化. 对零压下P₁ -IrB和P₅ -IrB₂ 的电子结构的研究发现,二者均没有一个明显的带隙,主要原因为Ir原子和B原子间的共价作用. 从P₁ -IrB和P₅ -IrB₂的能带结构和态密度图可以发现这两种结构均有金属性. 关键词： 1 -IrB')" href="#">P₁ -IrB 5-IrB₂')" href="#">P₅-IrB₂ 第一性原理 力学性质     

Site preference of Ti and Nb in L12-ordered Co-Al-W phase and their effect on the properties of the alloy: first-principles study

In this work,the equilibrium structure,electronic and elastic properties of L1₂-ordered Co-Al-W and Co-Al-W-X(X=Ti and Nb)phase were calculated,using first-principles calculations.Among six nonequivalent sites(Al₁,Al₂,Co₃,Co₄,W₅,W₆),Ti and Nb prefer to occupy the W₆site,since the formation enthalpy of the system is lowest when Ti and Nb occupy the W。site.Both Ti and Nb most affect the density of states of Al atoms.Compared with the Al₂ site,which is the sub-preference site of Ti and Nb,the density of states of Al atoms is higher with the addition of Ti and Nb in the W₆site,which means that the latter system is more stable.According to the bulk modulus B,shear modulus G,Young's modulus E,hardness Hv and Poisson's ratioσ,for Co₃(AI,W)alloy,the addition of Ti and Nb in the W₆site decreases its hardness hut increases its duetility.This work cnnfirms that Ti and Nh can stahilize the Cag(Al,W)alloy and have a positive effect in solving the relatively poor ductility of this alloy,which has important implications for the development of cobalt.based alloys.     

Mechanical,elastic, anisotropy,and electronic properties of monoclinic phase of m-Si_xGe_(3-x)N_4

The structural,mechanical,elastic anisotropic,and electronic properties of the monoclinic phase of m-Si_3N_4,mSi_2GeN_4,m-SiGe_2N_4,and m-Ge_3N_4are systematically investigated in this work.The calculated results of lattice parameters,elastic constants and elastic moduli of m-Si_3N_4and m-Ge_3N_4are in good agreement with previous theoretical results.Using the Voigt–Reuss–Hill method,elastic properties such as bulk modulus B and shear modulus G are investigated.The calculated ratio of B/G and Poisson’s ratio v show that only m-SiGe_2N_4should belong to a ductile material in nature.In addition,m-SiGe_2N_4possesses the largest anisotropic shear modulus,Young’s modulus,Poisson’s ratio,and percentage of elastic anisotropies for bulk modulus ABand shear modulus AG,and universal anisotropic index AUamong m-Si_xGe_(3-x)N_4(x=0,1,2,3.)The results of electronic band gap reveal that m-Si_3N_4,m-Si_2GeN_4,m-SiGe_2N_4,and m-Ge_3N_4 are all direct and wide band gap semiconducting materials.     

InOOH压致氢键对称化及其弹性性质

利用第一性原理研究了InOOH在高压下的氢键对称化行为及其对InOOH弹性等性质的影响.结果表明约在18 GPa时InOOH中的氢键发生了对称化转变,导致轴比率b/c对压力的斜率由负值变为正值;压缩弹性常数、非对角弹性常数、体积模量和纵波波速出现异常增加,如体积模量增加了20%—40%.高压下InOOH弹性性质呈现出更加明显的各向异性.常压下InOOH呈现韧性,且伴随着氢键对称化韧性异常增加.对畸变金红石型MOOH(M=Al,In,Ga,Fe,Cr)化合物在高压下的弹性性质转变与氢键性质转变的耦合规律进行了初探.     

FORMATION AND MAGNETIC PROPERTIES OF A NOVEL Gd3(Fe,Ti)29Ny NITRIDE

The novel Gd₃(Fe_0.978Ti_0.022)₂₉ compounds and their nitride have been synthesized. The X-ray diffraction patterns of the Gd₃(Fe,Ti)₂₉N_y and its parent compounds were indexed in the Nd₃(Fe,Ti)₂₉-type structure with a monoclinic symmetry and space group P2₁/c, Both the nitride and the parent compounds exhibit ferrimagnetlc coupling. Nitrogenatio led to an increase in Curie temperature and saturation magnetization. The Curie temperatures are 517K for the parent and 765K for the nitride. The saturation magnetizations σ_s at 4.2K are 101.9A·m²/kg for the parent and 128A·m²/kg for the nitride. Both the nitride and parent exhibit a planar anisotropy. Nitrogenation led to a decrease of the contribution of Fe-sublattice to planar anisotropy. The anisotropy fields at 4.2K are 9.8T for the parent and 6.5 T for the nitride.     

高压下ErNi2B2C弹性性质、电子结构和热力学性质的第一性原理研究

采用密度泛函理论中的赝势平面波方法研究了高压下超导材料 ErNi₂B₂C 的弹性性质、电子结构和热力学性质.分析表明, 弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显. 电子态密度(DOS)的计算结果显示, 在费米能级(E_F)处的 DOS 峰随外界压强的增大显著降低, 由于 ErNi₂B₂C 相对较高的超导温度(T_c)起因于E_F处的 DOS 峰, 因此推测压强增大可能会降低 ErNi₂B₂C 的 T_c.类似的现象在超导材料 MgB₂和 SrAlSi 中已被发现.此外, 基于准谐德拜模型, 对 ErNi₂B₂C 在高温高压下的热力学性质的研究表明, 在一定范围内, 温度和压强将对其热膨胀系数和热容产生明显的影响. 关键词： 高压 弹性性质 电子结构 热力学性质     

Ab initio studies of copper hydrides under high pressure

The crystal structure, electronic structure, and superconductivity of copper hydrides at high pressure have been studied by ab initio calculation. Consistent with experimental report, results show that the predicted stoichiometry Cu₂H with the P-3m1 space group is stable above 16.8 GPa. The stoichiometry of CuH with the Fm-3m space group is predicted to be synthesized above 30 GPa, but it is metastable and dynamical instable up to 120 GPa. The electronic band calculations reveal that Cu₂H is a good metal at a stable pressure range, whereas CuH is an insulator. Moreover, the other hydrogenrich compounds CuH₂ and CuH₃ are thermodynamically and dynamically unstable, respectively. The calculated superconducting transition temperature (T _c) of Cu₂H at 40 GPa is 0.028 K by using the Allen-Dynes modified McMillan equation.     

点缺陷浓度对非化学计量比L12型结构的A13Sc弹性性能的影响

采用密度泛函理论与Wagner-Schottky热力学模型计算了金属间化合物L1₂-A1₃Sc中点缺陷浓度与温度、成分间的关系. 结果表明: 在考察的温度区间(T=300-1200 K), 理想化学计量比L1₂-A1₃Sc中的点缺陷主要为Al空位和Sc空位, 且缺陷浓度较低(在1200 K时仅约为10^-6). 当L1₂-A1₃Sc偏离化学计量比成分时, 富Al成分端的点缺陷主要为Al反位与Sc空位, 且两种缺陷的浓度相当; 富Sc成分端的点缺陷则主要为Sc反位. 利用超胞模型进一步计算了含点缺陷L1₂-A1₃Sc晶体的弹性常数, 并计算预测了点缺陷形式和浓度对其弹性性能的影响. 结果表明: 在理想化学计量比成分附近, 点缺陷的引入均会降低非化学计量比L1₂-Al₃Sc晶体的杨氏、剪切和体积弹性模量, 增加非化学计量比L1₂-Al₃Sc弹性性能的各向异性, 但是对其脆-韧性的影响不大.     

高压下Ti_2AlX(X=C,N)的结构、力学性能及热力学性质

采用基于密度泛函理论的第一性原理方法,计算研究了压力对Ti_2AlC与Ti_2AlN结构、力学性能的影响.研究发现压力的增大会使体系的体积比降低,Ti_2AlC压缩性较Ti_2AlN好.力学性能研究发现,压力的增大使材料抵抗变形能力增强,体系的延展性有了很大的提升,当压力超过40 GPa后,Ti_2AlC与Ti_2AlN从脆性材料转变为延性材料,体模量与剪切模量的比值达到1.75,延展性有了很大的提升.通过准谐德拜模型,分析了压力与温度对Ti_2AlC与Ti_2AlN体模量、热容及热膨胀系数的影响.结果表明,随着温度的升高,Ti_2AlN与Ti_2AlC的体模量下降.定容热容与定压热容的变化趋势相同,但在高温下,定容热容遵循Dulong-Petit极限,温度对热容的影响效果较压力明显.温度与压力对Ti_2AlN与Ti_2AlC线膨胀系数的影响主要发生在低温区域.     

Ab initio calculation of elastic properties of rock-salt and zinc-blend MgS under pressure

The elastic properties of zinc-blend (ZB) and rock-salt (RS) MgS are calculated by ab initio method. The calculation shows that the enthalpy for RS structure and for ZB structure essentially is the same at ambient pressure. The ZB structure becomes unstable above 5 GPa. For these two structures, the pressure dependences of typical elastic properties, i.e. the bulk modulus, the shear modulus, the Young's modulus, the Poisson's ratio, and the anisotropy factor, are presented. The Debye temperature and sound velocity under high pressure have also been calculated. Debye approximation is used to estimate the zero-point vibrational energy.