Phonon spectra and thermodynamic properties of rare-earth hexaborides

The phonon spectra g(ν) of rare-earth hexaborides MeB₃ are calculated both in the approximation MeB* + B (B* = B₆) without regard for bonding between metal and boron atoms and in the approximation making allowance for this bonding. The temperature dependences of the heat capacity are calculated from the dependences g(ν), and the results obtained are compared with the experimental data in the range 5–300 K. It is found that stretching vibrations in the metal and boron sublattices and between these sublattices variously affect the thermodynamic functions of hexaborides at low and high temperatures.     

Structural, elastic, and electronic properties of icosahedral boron subcarbides (B12C3, B13C2), subnitride B12N2, and suboxide B12O2 from data of SCC-DFTB calculations

The structural, elastic, and electronic properties of a series of icosahedral phases, such as boron subcarbides B₁₂C₃ and B₁₃C₂, subnitride B₁₂N₂, and suboxide B₁₂O₂, have been studied in the framework of the SCC-DFTB method. It has been found that the B₁₂C₂ and B₁₃C₂ phases manifest metal-like properties, while B₁₂C₃ and B₁₂O₂ are semiconductors. The estimates have shown that the insertion of 2p atoms (C, N, or O) into intericosahedral pores of elemental boron can cause both a decrease in its elastic modulus (an increase in the compressibility of B₁₂N₂) and a sharp increase in the modulus B (in subcarbides B₁₂C₃ and B₁₂BCC). On the other hand, the insertion of 2p atoms into α-B₁₂ will favor an increase in its hardness (suboxide B₁₂O₂ will have a maximum hardness).     

Amorphization and a Polymorphic Transformation of Boron Stimulated by High Dynamic Pressures

Structural transformations of polycrystalline boron have been studied under megabar multiple-shock compression. Shock-wave experiments on the compression and subsequent recovery of polycrystalline samples of β-rhombohedral boron β-B₁₀₆ have been performed. Thermodynamic states of boron under the conditions of the performed experiments have been calculated. According to these calculations, the maximum pressure in the samples is 115 GPa. X-ray diffraction analysis of the stored boron samples has been performed. It has been shown that the dynamic compression of polycrystalline β-B₁₀₆ to 115 GPa results in its partial amorphization and transformation to the tetragonal modification of boron T-B₁₉₂.     

Raman spectroscopy of the BC3 phase obtained under high pressure and high temperature

A direct transformation of the g‐BC₃ phase to a new diamond‐like d‐BC₃ phase was observed in a diamond‐anvil cell (DAC) at high temperature, 2033 ± 241 K, and high pressure, 50 GPa. Analysis of the peak positions of the d‐BC₃, B₄C, α‐boron, and the boron‐doped diamond leads to the conclusion that the positions of the peaks of the d‐BC₃ are more similar to the peak pattern of the boron‐doped diamond rather than that of boron carbide, α‐boron. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanical properties and hardness of new carbon-rich superhard C11N4 from first-principles investigations

The structural, mechanical properties and hardness of the new carbon-rich material C₁₁N₄ are studied by first-principles total energy calculations based on the density-functional theory. We use the empirical equations of state (EOS) to investigate the lattice properties and bulk modulus. It is found that the calculated lattice constants and bulk modulus are in good agreement with previous calculations. And the full set elastic constants are calculated using the stress-strain method. The Voigt-Reuss-Hill approximation is used to evaluate the mechanical moduli. The elastic constants show that the two phases of C₁₁N₄ are mechanically stable. The tetragonal-C₁₁N₄ (α-C₁₁N₄) exhibits larger mechanical moduli than the orthorhombic-C₁₁N₄ (β-C₁₁N₄). The mechanical anisotropy is calculated of several different anisotropic indexes and factors, such as universal anisotropic index (A^U), the percent anisotropy (A_G and A_B) and shear anisotropic factors (A₁, A₂ and A₃). Furthermore, the hardness of α-C₁₁N₄ and β-C₁₁N₄ are evaluated according to the intrinsic hardness calculation theory. α-C₁₁N₄ is predicted to be a superhard material with the Vickers hardness of 67.17 GPa, which is slightly higher than that of the cubic boron nitride. And the β-C₁₁N₄ is also a superhard material with the calculated Vickers hardness of 45.63 GPa. C₁₁N₄ can be considered as candidate superhard compounds.     

高温高压下B4C合成金刚石的研究

 压力为5.7~6.0 GPa及1 370~1 500 ℃温度范围内，B₄C在Ni₇₀Mn₂₅Co₅系统中，高压相变为金刚石。合成的金刚石为深黑色，含硼量大于1wt%。粒度为20 μm左右，晶形多为立方体与八面体的各种聚形。反应后的金属触媒中，存在Ni₂B等硼化物；非金属残留物中，存在B₁₃C₂等高硼碳化物。     

高压下TiAl3结构及热动力学性质的第一性原理研究

采用平面波赝势密度泛函理论研究了钛铝系金属间化合物TiAl₃的结构性质, 计算值与实验值及其他理论值相符合. 通过准谐德拜模型研究了TiAl₃的热动力学性质, 计算得到了相对体积(V/V₀)与压强和温度的关系, 以及不同温度和压强下的热膨胀系数和热容. 与TiAl的计算结果进行对比, 发现随着温度的升高, TiAl的热膨胀系数增大的速度高于TiAl₃, 且随着压强的增大温度效应减弱; TiAl₃的热容值近似为TiAl的热容值的2倍. 关键词： 结构性质 热动力学性质 第一性原理 高压     

高温高压下Zr2AlC的结构及热学性质的第一性原理

利用密度泛函理论研究了高温高压下Zr₂AlC的结构和热力学性质,计算得到Zr₂AlC的晶格参数与实验值符合较好．研究了Zr₂AlC的弹性常数、体模量、剪切模量和杨氏模量等力学性质随压力变化的趋势．同时研究了维氏硬度随压力的变化趋势．通过计算得到的杨氏模量预测了Zr₂AlC的弹性各向异性．最后,基于准简谐德拜模型,成功预测了Zr₂AlC的德拜温度、热容、热膨胀系数和Grüneisen参数随着压强和温度的变化关系.     

Theoretical predictions of the structural and thermodynamic properties of MgZn2 Laves phase under high pressure

The structural and thermodynamic properties of MgZn₂ Laves phase under hydrostatic pressure have been investigated by using a first-principles method based on the density functional theory within the generalized gradient approximation. The calculated equilibrium structural parameters are consistent with the previous experimental and theoretical data. Especially, we study the pressure dependence of the elastic constants, polycrystalline elastic moduli, Poisson’s ratio, elastic anisotropy, and theoretical Vickers hardness of MgZn₂. It is found that the pressure plays a significant role in the elastic properties of MgZn₂ due to the variations of inter-atomic distance. In addition, the density of states and Mulliken analysis are performed to reveal the bonding characteristics of MgZn₂. It is observed that the total density of states exhibits a certain offset with the increase of external pressure. Finally, the dependences of thermodynamic properties on pressure and temperature of MgZn₂ Laves phase have been also successfully predicted and analyzed within the quasi-harmonic Debye model for the first time.     

高温高压条件下三氧化二硼相变过程的研究

 详细考察了三氧化二硼在高温高压条件下的相变过程。研究结果表明，在高温高压条件下，三氧化二硼经历了从立方晶系向六方晶系、正交晶系和非晶相的相变。文中还比较详细地讨论了高温、高压两种因素对结构相变的影响，以及静高压熔态淬火方法在非晶态材料制备方面所具有的独到之处。     

In search of new candidates for ultra-hard materials: the ternary BC3N3 stoichiometry

Starting from formerly investigated graphitic like C₃N₄, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC₃N₃ stoichiometry. Investigations of the geometry optimisation and of the electronic properties were carried out using pseudo potential and full potential computations in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They lead to propose a precursor (2D), a β-structure and a new ultra hard rhombohedral compound with a hardness (B₀∼358 GPa) that reaches the range of formerly studied BC₂N structures built from hexagonal and cubic diamond.     

高温高压下立方氮化硼和六方氮化硼的结构、力学、热力学、电学以及光学性质的第一性原理研究

本文采用基于密度泛函理论的第一性原理平面波赝势和局域密度近似方法,优化了立方和六方氮化硼的几何结构,系统地研究了零温高压下立方和六方氮化硼的几何结构、力学、电学以及光学性质.结构与力学性质研究表明:立方氮化硼的结构更加稳定,两种结构的氮化硼均表现出一定的脆性,而六方氮化硼的热稳定性则相对较差;电学性质研究表明:立方氮化硼和六方氮化硼均为间接带隙半导体,且立方氮化硼比六方氮化硼局域性更强;光学性质结果显示:立方氮化硼和六方氮化硼对入射光的通过性都很好,在高能区立方氮化硼对入射光的表现更加敏感.此外,还研究了高温高压下立方氮化硼的热力学性质,并得到其热膨胀系数、热容、德拜温度和格林艾森系数随温度和压力的变化关系.本文的理论研究阐述了高压下立方氮化硼和六方氮化硼的相关性质,为今后的实验研究提供了比较可靠的理论依据.     

Equation of state of boron subarsenide B12As2 to 47 GPa

Compressibility of boron subarsenide B₁₂As₂ has been studied by synchrotron X-ray diffraction up to 47?GPa at room temperature in a diamond anvil cell using Ne pressure transmitting medium. A fit of experimental p–V data by Vinet equation of state yielded the bulk modulus of 150(4) GPa and its first pressure derivative of 6.4(3). No pressure-induced phase transitions have been observed.     

LnP5O14铁弹相变的喇曼光谱研究

在200—300℃范围测量了单斜I镧系五磷酸盐LnP₅O₁₄(Ln=La,Ce,Pr、Nd,Sm,Eu,Gd,Tb)晶体的软光学模喇曼谱。不同LnP₅O₁₄的A_g-B_2g和A_g-A_g软模在铁弹相变点都软化到20cm^-1,另一个B_g-B_3g软模则软化到38cm^-1。软模频率对温度的依赖关系表明,这些晶体是很好的平均场理论系统。A_g-B_2g软模与e₅的耦合对铁弹相变起主要作用,此软模的强度变化可由双向线性模耦合模型来解释。这个系统T_c从La到Tb的增高是镧系收缩的结果。 关键词：     

Kinetics of thermally stimulated recombination processes in lithium borate crystals

This paper reports on the results of the experimental and theoretical investigations of thermally stimulated recombination processes in crystals of the lithium borates Li₂B₄O₇ and LiB₃O₅. For both types of crystals, the measurements of thermally stimulated luminescence curves, spectra, and temperature dependences of the intensities of steady-state X-ray luminescence have been performed in a single experimental cycle. In the framework of a unified model for the Li₂B₄O₇ and LiB₃O₅ crystals, the thermally stimulated recombination processes have been calculated and the obtained results have been interpreted talking into account all available experimental data.     

Graphite–boron composite heater in a Kawai-type apparatus: the inhibitory effect of boron oxide and countermeasures

We have investigated the performance of a graphite–boron composite (GBC) with 3?wt % boron as a precursor for a boron-doped diamond heater in a Kawai-type apparatus at 15?GPa. We first tested a machinable cylinder of GBC sintered at 1000°C in Ar/H₂ gas (99:1 molar ratio). Boron oxide (B₂O₃) formed during sintering frequently hindered the GBC heater from stable operation at temperatures higher than 1400°C by producing melt throughout the heater together with oxide and/or silicates. We then rinsed the GBC heater in hydrochloric acid to remove B₂O₃. After rinsing, we succeeded in stably generating temperatures higher than 2000°C. We also improved a molding process of different-sized GBC tubes for convenient use and tested the molded GBC heater. It was free from the B₂O₃ problem. The electromotive force of the W/Re thermocouple was successfully monitored up to 2400°C.     

High-temperature specific heat of Cu5Bi2B4O14

Cu₅Bi₂B₄O₁₄ single crystals are grown by spontaneous crystallization from a melt of a CuO, Bi₂O₃, and B₂O₃ mixture. The specific heat of the crystals in the temperature range 396–633 K is measured by differential scanning calorimetry. Using the experimental data, the thermodynamic properties of the Cu₅Bi₂B₄O₁₄ solid compound are calculated.     

Thermodynamic properties of cubic ZrC under high pressure from first-principles calculations

The elastic and thermodynamic properties of Zirconium carbide (ZrC) are investigated by ab initio plane-wave pseudopotential density function theory method. The obtained lattice constant, elastic constant and bulk modulus B are consistent with the experimental and theoretical data. Through the quasi-harmonic Debye model, the dependences of the normalized volume V/V ₀ and the bulk modulus B on pressure P, as well as the specific heat C _V on the temperature T are obtained successfully. The relationships of the thermal expansion α with temperature and pressure are also investigated, which indicate the temperature hardly has any effect on the thermal expansion α at high pressure. Supported by the National Natural Science Foundation of China (Grant No. 10776022)     

高温高压下β相硼的电导率测量

 硼在高压下具有复杂的结构和多样的物理性质,对其结构和性质的深入研究具有很重要的意义,一直引起理论和实验研究领域的关注。高压下进行电学性质测量是获得物质物理性质的有效手段,利用集成在金刚石对顶砧上的微电路,在高压下和两个不同温度范围内对β相硼进行了电导率测量,分析了导电机制随压力的变化规律。在0～28.1 GPa范围内,β相硼的电导率随着压力的增大是逐渐增大的,卸压后样品的电导率不能回到最初的状态,是一个不可逆的变化过程;由室温到423 K的范围内,β硼的电导率随着温度的不断增加有明显的上升趋势,并且随着压力的升高,电导率变化逐渐加快。此外,对样品在14.5 GPa和18.6 GPa压力下,用溅射到金刚石对顶砧上的氧化铝薄膜做绝热层,对样品进行了激光加热实验,最高温度达到2 224 K,电导率随着温度的上升而增大,结果显示,β相硼的电学特征仍然属于半导体的特征范围内。     

Melting of B12P2 boron subphosphide under pressure

Melting of boron subphosphide (B₁₂P₂) to 26?GPa has been studied by in situ synchrotron X-ray powder diffraction in a laser-heated diamond anvil cell, and by quenching and electrical resistance measurements in a toroid-type high pressure apparatus. B₁₂P₂ melts congruently, and the melting curve has a positive slope of 23(6)?K/GPa. No solid-state phase transition was observed up to the melting in the whole pressure range under study.