"点石成金?"--碳结构超高压物理力学

文章介绍了石墨和碳纳米管在超高压和／或纳米压痕下的层间sp^2-sp^3键转化、软硬相转换、纳米硬度等研究进展．通过量子力学和分子动力学建模分析研究，发现超高压下石墨和碳纳米管存在软相向硬相转变的双相机制，给出了超高压获取石墨和碳纳米管超硬相的条件．理论计算与实验结果吻合很好，并能合理地解释有关石墨和碳纳米管超高压实验中看似矛盾的各种实验现象．提出了碳结构超高压物理力学概念，可为超高压碳相关物质相变、物化性质调控提供理论方法．     

在熔融金属溶液中石墨在金刚石稳定区结晶的可能性

本文给出了在超高压高温条件下,碳的亚稳相石墨从熔融金属溶液中结晶的实验现象,并以此为例分析了亚稳相从溶液结晶的热力学可能性。提出了促使亚稳相结晶的过饱和的概念,指出:溶液体系中的温度差是亚稳相结晶的推动力。重点给出在一个结晶条件下亚稳相石墨可能结晶的临界温度差的计算。 关键词：     

o-C_8碳:新的超硬碳同素异形体（英文）

通过粒子群优化算法和密度泛函计算,证明了空间群为PMM A的正交晶系的碳同素异形体o-C_8是稳定的超硬相.声子谱计算表明,o-C_8碳相是动力学稳定的;体积压缩计算表明,它是体模量为298.6 GPa的高度不可压缩材料.o-C_8相是一种新型的密度为2.993 g/cm~3、维氏硬度为67.0 GPa的低密度超硬材料.     

SmN晶体的电子结构和高压相变的第一性原理

利用基于密度泛函理论的第一性原理,研究了SmN晶体的电子结构和高压相变. SmN晶体的电子结构具有半金属特征,多数自旋电子显示金属导电性,少数自旋电子显示半导体导电性. 高压相变的结果显示,SmN晶体经历从NaCl型(B1)到CsCl型(B2)结构转变的压致结构相变,相变压力117 GPa. 弹性系数的结果显示,在环境压力下SmN晶体的弹性系数满足玻恩稳定条件,标志着B1相是力学稳定结构. 声子谱结果显示,在环境压力下B1相是热力学稳定结构,与弹性系数的计算结果一致.     

过渡金属单硼化物TMB的第一性原理研究

基于密度泛函理论平面赝势波法的第一性原理计算,研究过渡金属单硼化物TMB（以3d系列中的TiB、VB和CrB;4d系列中的ZrB、NbB和MoB;5d系列中的HfB、TaB和WB为例）的热力学稳定性、力学性质和微观机制.发现过渡金属单硼化物的热力学稳定与硬度异常的规律.当价电子浓度为8 e·（f.u.）^-1时,热力学最稳定,且硬度最高.计算TMB的电子结构,发现TMB的价电子浓度为8 e·（f.u.）^-1时,pd共价键合,有效阻碍了金属双层之间的位错滑动,防止剪切变形,致使其具有高硬度.     

α-碳锗炔稳定性及性质模拟

采用以量子力学为基础的半经验计算方法—–自洽和环境依赖的原子轨道线性理论,预测了类α-石墨炔的碳锗炔结构.研究了α-碳锗炔的稳定结构、电子结构以及热力学稳定性,得到其最稳定的构型是Ge原子在六元环的六个顶角处,晶格常数为8.686?的六角原胞构成的单层平面蜂窝状结构.该结构是带隙为1.078 e V的半导体.α-碳锗炔在很高的温度下都可以保持稳定,直到2280 K时其长程有序态才被破坏,当体系低于此温度时,可以通过降温使其恢复到零温时的稳定平面结构.     

第一性原理计算Al元素含量对高熵合金AlxCoCrCuFeNi的影响

采用第一性原理密度泛函理论,结合平面波赝贽和广义梯度近似(GGA),用虚拟晶体近似(VCA)方法建模,计算了高熵合金Alx CoCrCuFeNi的结构性能、弹性性能及生成热.计算结果表明,高熵合金Alx CoCrCuFeNi的密度随Al元素摩尔含量的增大而减小,晶格常数在Al元素摩尔含量为1时最小.Al元素摩尔含量为2时,高熵合金Alx CoCrCuFeNi符合力学稳定性判据.生成热随Al元素摩尔含量增大而增大,皆为负值的生成热表明高熵合金在热力学条件下稳定.     

o-C8碳：新的超硬碳同素异形体

通过粒子群优化算法和密度泛函计算,证明了空间群为PMMA的正交晶系的碳同素异形体o-C₈是稳定的超硬相. 声子谱计算表明,o-C₈碳相是动力学稳定的;体积压缩计算表明,它是体模量为298.6 GPa的高度不可压缩材料. o-C₈相是一种新型的密度为2.993 g/cm³、维氏硬度为67.0 GPa的低密度超硬材料.     

PuC2 气态分子的结构与热力学稳定性研究

在相对论有效原子实势近似下 ,用密度泛函B3LYP方法 ,求得PuC2 气态分子的结构与不同温度下的热力学函数 .根据热力学原理 ,计算得到PuC2 气态分子在不同温度下的标准生成自由能变均为较大正值 ,据此说明 ,PuC2 气态分子不具有热力学稳定性 .     

高压下钨弹性和热力学性质的第一性原理研究

本文利用密度泛函理论研究了高压下bcc结构钨的弹性和热力学性质,计算得到钨的晶格常数、体弹模量以及其对压强的一阶偏导与实验值符合较好;在常压下弹性常数计算值与实验值符合较好的基础上,预测了其高压数据.针对钨的固相结构稳定性问题,根据力学稳定判断标准得到0~600 GPa范围内bcc结构是力学稳定的.此外,通过体模量和剪切模量的计算得到bcc结构钨在压力低于600 GPa时的力学性能表现为韧性.最后,基于准简谐德拜模型,成功预测了钨的热膨胀系数、等压热容、等容热容和熵随着压强和温度的变化关系,为钨及其合金的进一步设计及应用提供参考.     

<Emphasis Type="Italic">Ab initio</Emphasis> calculations of the equation of state and elastic constants of aluminum in the region of negative pressures

The results of ab initio calculations are presented for the specific energy, pressure, and elastic constants of an aluminum fcc single crystal with subnormal densities at T=0. Kinks in the elastic constant vs. density curves are revealed which are caused by the electronic topological transitions. An analysis of the mechanical stability of aluminum fcc crystal at negative pressures suggests that the polymorphic transition to a noncubic structure is possible. A method is suggested for taking into account the thermal nuclear excitation and the influence of zero-point vibrations on the pressure dependence of crystal density.     

压力对有机半导体均苯四甲酸晶体结构转变和电子性质的影响

研究高压条件下均苯四甲酸(C₁₀H₆O₈)材料的结构和性质对探索有机半导体材料的应用有积极意义.基于密度泛函理论的第一性原理赝势平面波方法,开展了0-300 GPa压强下C₁₀H₆O₈晶体的结构、电子和光学性质的研究.晶格常数在压强20 GPa和150 GPa下出现了明显跳变,且原子之间随着压强变化反复地出现成键/断键现象,表明压强可诱导晶体结构变化.电子结构的性质表明,0 GPa的C₁₀H₆O₈晶体是带隙为3.1 eV的直接带隙半导体,而压强增加到150 GPa时,带隙突变为0 eV,表明了晶体由半导体转变为导体.当压强为160 GPa时,晶体又变成了能隙约为1eV的间接带隙半导体,这可能是费米能级附近仅受O-2p轨道电子影响所导致.通过对C₁₀H₆O₈晶体介电函数的分析,再次验证了晶体在150 GPa时发生了结构相变.同时...     

钙钛矿结构ZrBeO3稳定性的第一性原理研究

基于密度泛函理论构建了钙钛矿结构ZrBeO3晶体模型,计算了该晶体模型结合能,表明了该构型热力学稳定性;计算出该结构在不同压力下的弹性常数,并据此计算了ZrBeO3的体积模量、剪切模量、杨氏模量、泊松比、BH/GH(体模量/剪切模量)等参数,结果表明该材料具有机械稳定性,随着等静压力增加,材料由脆性向韧性转变;计算了零压下ZrBeO3的硬度,为34.5 GPa,表明该结构晶体应为超硬材料;计算了ZrBeO3的声子能谱,结果表明ZrBeO3在低温零压下热动力学不稳定,为此分析比较了不同压力下的声子能谱、不同原子轨道及化学键布居值,研究表明随着压力增加,Be原子sp杂化后形成的Be-O共价键成分增强、Zr-O键离子键成分增强,晶格动力学趋于稳定。     

High-pressure X-ray diffraction study of 2-methyl 1,3-cyclopentanedione crystals

Abstract

2-Methyl-1, 3-cyclopentanedione (C₆H₈O₂-enol form) crystals have been studied at high pressures to 3.01(5) GPa by single crystal X-ray diffraction. The pressure dependence of the unit-cell dimensions is strongly anisotropic and non-linear, the largest relative changes are observed for & perpendicular to the planes comprising the planar chains of the hydrogen-bonded molecules. The crystal structure has been determined at 1.50, 2.40 and 3.01 GPa. The structure is very stable, the main structural change with pressure is the compression of intermolecular distances including the O … O hydrogen bond—but the position of the molecule in the unit cell remains unchanged. Some systematic changes of the bond lengths (in the limit of statistical significance) can be noted. The structure has been compared with the structures of 1,3-cyclopentanedione and 1,3-cyclohexanedione (enol forms), both undergoing pressure induced phase transitions. The exceptional stability of the 2-methyl- 1,3-cyclopentanedione crystal can be explained on the basis of its structure: energetically favourable orientation of the dipole moments of the molecules of close neighbouring chains. The R-factor/number of unique reflections for the structure determinations at 1.50, 2.40 and 3.01 GPa are 0.103/153,0.093/142 and 0.097/139, respectively. Over 3.01 GPa the unit cell is compressed to less than 84% of its ambient-pressure volume. The root-mean-square displacements of the atoms decreased on average by the factor of 0.75 at 1.50 GPa and 0.74 at 2.40 and 3.01 GPa.     

Thermodynamic ground states of platinum metal nitrides

The thermodynamic stabilities of various phases of the nitrides of the platinum-metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir, and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.     

高压下MgN8晶体结构理论模拟与物性研究

基于密度泛函理论第一性原理的方法,使用CALYPSO结构搜索技术结合VASP软件,在0~100 GPa压强范围内对MgN8的晶体结构进行预测,并对预测的结构进行系统研究。结果表明:在常压下,空间群为P4/mbm的α-MgN8晶体结构的焓值最低;当压强达到24.3 GPa和68.3 GPa时发生相变,分别相变成空间群为P4/mnc的β-MgN8相和空间群为Cmcm的γ-MgN8相,两次相变均为对应体积坍塌的一级相变。电子性质计算结果表明,α-MgN8相的导带与价带之间具有3.09 eV的带隙,表明该结构具有非金属性;β相和γ相具有明显的金属特征。Bader电荷转移计算表明,随着压力的增加,Mg原子向N原子转移的电荷逐渐增多。     

The structural, elastic and thermodynamic properties of thorium tetraboride

The structural, elastic and thermodynamic properties of thorium tetraboride (ThB₄) have been investigated by using first-principles plane-wave pseudopotential density functional theory with generalized gradient approximation. The behaviors of structural parameters under 0-70 GPa hydrostatic pressure are studied by means of Broyden, Fletcher, Goldfarb, and Shanno (BFGS) geometry optimization scheme. By using the stress-strain method, single crystal elastic constants are calculated to test the mechanical stability of the crystal structure and to determine mechanical properties such as bulk modulus at each pressure. However, in order to study the thermodynamic properties of ThB₄, the quasi-harmonic Debye model is used. Then, the dependencies of bulk modulus, heat capacities, thermal expansions, Grüneisen parameters and Debye temperatures on the temperature and pressure are obtained in the whole pressure range 0-70 GPa and temperature range 0-1500 K.     

Influence of a lubricant modified with fine-dispersed β-sialon on a steel surface structure under friction loading

Steel sample surfaces subjected to different friction loads with lubricants modified with fine-dispersed β-sialon are studied via electron spectroscopy and x-ray diffraction analysis. It is revealed that friction loading in β-sialon-containing lubricants stimulates the ordering of the crystalline structure of surfaces (a decrease in the relative root-mean-square microstrain and dislocation density). The assumption is made that, in the presence of heavy loads, the friction surface is formed from a complex α-Fe compound with nitrogen and carbon atoms embedded into its crystal lattice and silicon atoms in the positions of iron atoms.     

Physical Properties of C-Si Alloys in C2/m Structure

Using the first principles calculations based on density functional theory, the crystal structure, elastic anisotropy, and electronic properties of carbon, silicon and their alloys(C_(12)Si_4, C_8Si_8, and C_4Si_(12)) in a monoclinic structure(C2/m) are investigated. The calculated results such as lattice parameters, elastic constants, bulk modulus,and shear modulus of C_(16) and Si_(16) in C2/m structure are in good accord with previous work. The elastic constants show that C_(16), Si_(16), and their alloys in C2/m structure are mechanically stable. The calculated results of universal anisotropy index, compression and shear anisotropy percent factors indicate that C-Si alloys present elastic anisotropy,and C_8Si_8 shows a greater anisotropy. The Poisson's ratio and the B/G value show that C_8Si_8 is ductile material and other four C-Si alloys are brittle materials. In addition, Debye temperature and average sound velocity are predicted utilizing elastic modulus and density of C-Si alloys. The band structure and the partial density of states imply that C_(16) and Si_(16) are indirect band gap semiconductors, while C_(12)Si_4, C_8Si_8, and C_4Si_(12) are semi-metallic alloys.     

Compression behavior and spectroscopic properties of insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene from dispersion-corrected density functional theory

Using dispersion corrected density functional theory, we systematically examined the pressure effect on crystal structure, cell volume, and band gap of 1,3,5-triamino-2,4,6-trinitrobenzene(TATB) to understand its extraordinary chemical stability. Analysis of the Mulliken population and the electron density of states implied a possible charge transfer in TATB with increasing pressure. Raman and infrared spectra of TATB under hydrostatic pressure up to 30 GPa were simulated.The observed strong coupling between NH_2 groups and NO_2 groups with increasing pressure, which is considered to have a tendency of energy transfer with these vibrational modes, was analyzed. The pressure-induced frequency shift of selected vibrational modes indicated minor changes of molecular conformation mainly by the rotation of NH_2 groups. Compression behavior and spectroscopic property studies are expected to shed light on the physical and chemical properties of TATB on an atomistic scale.