Mechanical stability and thermodynamic properties of OsC from first-principles calculations

The elastic and thermodynamic characteristics of OsC crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Compared with WC-type OsC, NaCl-type OsC is not only energy unfavorable but also mechanics unstable. The five independent elastic constants (C_ij), bulk modulus (B₀), the dependence of bulk modulus on temperature and pressure as well as the thermal expansion coefficient (α_V) at various temperatures for WC-type OsC are discussed. According to our calculations, WC-type OsC should be an ultra-incompressible material with high bulk modulus about 381 GPa. In addition, the bulk modulus will increase with increasing pressure while decrease with increasing temperature. The researches on the thermal expansion coefficient indicate that there will be a knee point during the process of thermal expansion coefficient variation versus increasing temperature. Our results may provide useful information for theoretical and experimental investigation of OsC.     

Study of anharmonic elastic properties of mixed diatomic crystals AgCl−AgBr and NaCl−NaBr

Summary An interionic-force model has been employed to analyse the anharmonic elastic properties of mixed-diatomic-crystal systems with varying compositions. The interaction system consists of long-range Coulomb and three-body interactions within the framework of Hafemeister-Zarht (HZ) short-range repulsion, effective up to second-neighbour ions. This interionic potential has been used for predicting the third-order elastic constants, the first pressure derivative of second-order elastic constants and the Anderson-Grüneisen (AG) parameter in NaCl−NaBr and AgCl−AgBr mixed crystals. The values of the hardness parameter between nearest neighbours as well as between second nearest neighbours have been evaluated by using the overlap integral method. The results obtained in the present study are generally in good agreement with the available experimental data as compared to those obtained by earlier investigations. The experimental values of the Anderson-Grüneisen parameter have been calculated by using Chang’s relation.

---

Riassunto Un modello di forza interionica è stato usato per analizzare le proprietà anarmoniche elastiche di sistemi di cristalli diatomici misti con composizioni variabili. Il sistema d’interazione consiste in interazioni di Coulomb a largo raggio e a tre corpi nell’àmbito della repulsione a corto raggio di Hagemeister-Zorht (HZ), efficaci fino a ioni secondi vicini. Questo potenziale interionico è stato usato per prevedere costanti elastiche di terz’ordine, la prima derivata di pressione delle costanti elastiche di second’ordine e il parametro di Anderson-Grüneisen (AG) in cristalli misti NaCl−NaBr e AgCl−AgBr. I valori del parametro di durezza tra vicini prossimi e tra secondi vicini prossimi sono stati calcolati usando il metodo dell’integrale di sovrapprosizione. I risultati ottenuti in questo studio sono generalmente in buon accordo con i dati sperimentali disponibili se confrontati con quelli ottenuti da precedenti ricerche. I valori sperimentali del parametro di Anderson-Grüneisen sono stati calcolati usando la relazione di Chang.

---

Резюме Используется модель междуионной силы для анализа ангармонических упругих свойств смешанных двухатомных кристаллических систем с изменяющимся составом. Взаимодействие системы состоит из длиннодействующего кулоновского взаимодействия и взаимодействий трех тел в рамках короткодействующего отталкивания Хафемайстера-Зарта, с учетом взаимодействий с ионами, следующими за соседями. Предложенный междуионный потенциал используется для предсказания упругих постоянных третьего порядка, первой производной упругих постоянных второго порядка по давлению и параметра Андерсона-Грюнайзена в смешанных кристаллах NaCl−NaBr и AgCl−AgBr. Оцениваются параметры жесткости между соседними ионами, а также между ионами, следующими за соседними. Полученные результаты хорошо согласуются с имеущимися экспериментальными данными. Используя соотношение Чанга, вычисляются экспериментальные значения параметра Андерсона-Грюнайзена.

     

Pressure derivatives of isothermal bulk modulus of rare gas solids

In the present study we have derived the expression for the bulk modulus using the Hildebrand approximation and the lattice potential energy for rare gas solids (RGS). The derived relation is further used to derive the expression for first and second order pressure derivatives of the bulk modulus for RGS. The derived relations are also used to compute the values of these constants in the case of Ne, Ar, Kr and Xe which are compared with the available values.     

Interionic forces in AgF,AgCl and AgBr crystals

An interionic force model has been used in which the short range nearest neighbour and the next nearest neighbour repulsive interactions and the van der Waals’ interactions are balanced by the long range electrostatic forces. The nearest neighbour and the next nearest neighbour interactions are derived from the overlap integrals for outer shell electrons. The van der Waals’ interactions are estimated from the Slater-Kirkwood variational method. The cohesive energy, the bulk modulus and its pressure dependence for AgF, AgCl and AgBr crystals have been calculated and compared with experimental data.     

Influence of temperature on thermal properties of (Sm,Eu) VO3

The bulk modulus and thermal properties of orthovanadates SmVO₃ and EuVO₃ in the temperature range 5 K≤T≤300 K have been investigated using the Modified Rigid Ion Model (MRIM) by incorporating the effect of lattice distortions. The results on the specific heat, cohesive energy, molecular force constant, the reststrahlen frequency and the Gruneisen parameter following the temperature driven structural phase transitions are presented. Our results are in fair agreement with the available experimental data. The specific heat results can further be improved by including the spin and orbital ordering contributions to the specific heat.     

Mechanical and thermodynamic properties of infrared transparent low melting chalcogenide glass

The properties of infrared (IR) transparent chalcogenide glass Se₅₇I₂₀As₁₈Te₃Sb₂ have been studied for the first time by dynamic mechanical analysis (DMA). This glass demonstrates both high transparency in a broad IR band (1–12 μm) and a particularly low softening point (near 50 °C). It is known as one of the best adhesives intended for the binding of IR optics elements. The DMA method gives valuable information about the complex Young’s modulus of the glass in its softening region. In parallel with the study of mechanical properties of the glass the character of its glass transition process has been investigated. Our results of DMA were then compared with our thermodynamic data obtained by scanning calorimetry. The peak of mechanical losses in DMA appears to be situated considerably higher than the calorimetrically determined glass transition temperature.     

Pressure dependence of melting temperature and shear modulus of hcp-iron

The pressure effects on melting temperature and shear modulus of hcp-iron have been studied based on the semi-empirical approach in the Debye model. The recent well-established pressure-dependent Grüneisen parameter has been applied to derive the analytical expressions of the Debye frequency, the Debye temperature, melting temperature and shear modulus which are of importance to geophysical implications. Numerical calculations have been performed for hcp-iron as functions of pressure up to the pressure of Earth's inner core. Our calculations are compared with those of previous experimental and theoretical data showing the good and reasonable agreements. The present results contribute to the database of high pressure melting, especially Earth's inner core boundary temperature, and could also be used to verify as well as analyze the future high pressure diamond-anvil cell experiments.     

冲击压缩下铝、铜、钨的剪切模量和屈服强度与压力和温度的相关性

对铝、铜和钨在冲击压缩状态下的剪切模量和屈服强度的实验数据进行了综合分析，并与St einberg-Cochran-Guinan(SCG)模型的计算结果进行了比较，结果表明，铝在50 GPa、铜在1 00 GPa、钨在200 GPa冲击压力以内，三种材料的剪切模量和屈服强度随温度和压力的变化 趋势基本相似，即SCG模型的假设Y′_pY₀=G′_pG ₀,Y′_TY₀=G′_TG₀对这三种材料在上述冲击压力范围内近似成立，利用该模型可以较好地描述材料在冲击压缩 下的本构行为. 关键词： 剪切模量 屈服强度 压力 温度     

压力高达27 GPa下天然奥长石的XRD分析

在室温高达27 GPa压力下对天然奥长石(Na_0.86K_0.02Ca_0.12Mg_0.01(Fe_0.01Al_1.12Si_2.87O₈))粉晶进行了原位同步辐射X光衍射(XRD)测量,获得了样品的状态方程。实验数据表明随着压力增大奥长石样品在大约3.5 GPa发生了三斜向单斜的相变(P1→C2)和在大约10.0 GPa发生了单斜对称相变(C2→C2/m)。样品三个相的体模量计算值分别为K₀=73.8 GPa (K′=10.98), K_(C2)=124 GPa (K′=1.05) 和K_(C2/m)=272 GPa (K′=0.625)。样品的元素组成影响其T-O-T 键角的刚度、M-O键的强度和Si-O-Al键角的弯曲,从而导致奥长石样品在高压行为的特殊变化。三斜相的奥长石晶胞压缩性具明显的各向异性。实验结果表明在冷俯冲带奥长石可能是碱金属和碱土金属深循环的载体。     

Mechanical properties of superhard diamondlike BC5

Using first-principles calculations, we systematically studied the mechanical properties and electronic structure of the recently synthesized diamondlike BC₅. Our calculated bulk modulus B, shear modulus G, elastic constant c₄₄, and theoretical hardness H confirm that BC₅ is an ultraincompressible and superhard material. Also, it exhibits mechanical stability and metallic features. Electronic structures show that a strong covalent bond network through sp³ hybridization is the origin of the excellent mechanical properties of BC₅. Our results show that BC₅ has good prospects in electronic application as a superhard material.     

Temperature dependence of thermodynamic properties of a polarizable potential model of water

The temperature dependence of several thermodynamic properties of water is determined at atmospheric pressure by a set of computer simulations with a polarizable potential model. It is found that the maximum of the density occurs at the correct temperature, but the decrease in density with increasing temperature is more rapid than expected for real water. Consequently, the temperature dependence of other quantities, especially that of the thermal expansion coefficient, is reproduced with some inaccuracies. The model, however, turns out to be a definitive improvement over similar non-polarizable ones, for which the density maximum is either missing or found to be shifted considerably with temperature.     

Pressure dependence of elastic properties of wurtzite ZnO crystal

We present the pressure dependence of elastic properties of the wurtzite phase of ZnO undergoing wurtzite to the rocksalt phase transition. A simple Landau theory is developed to describe the structural phase transition between wurtzite to rocksalt phases observed in ZnO. We have defined the necessary order parameter of the wurtzite to the rocksalt phase transition. We present a detailed analysis of the pressure dependence of the elastic and shear constants of the wurtzite phase of ZnO. The theoretical predictions are in the line with experimental results.     

Pressure effects on the thermodynamic and mechanical properties of zinc-blende ZnTe compound

In this paper, the moment method in statistical mechanics has been employed to study the pressure effects on thermodynamic and mechanical properties of zinc-blende zinc telluride using many-body potential. We have derived the analytical expressions of the pressure-dependent lattice parameter, volume compression as well as mean-square displacement of zinc-blende type compound. Numerical calculations performed for ZnTe compound up to 12 GPa are found to be in good and reasonable agreement with available experimental data as well as with previous theoretical studies. These results have been used to evaluate the bulk modulus and its first pressure derivative of ZnTe. The present moment method has taken into account the quantum zero-point vibrations at low temperature and the higher-order anharmonic terms in the atomic displacements. This research shows the advantage of moment method on extensively studying thermo-mechanical properties of materials under high pressures.     

Volume dependence of the Grüneisen γ and other thermodynamic properties of NaCI

The quasi-harmonic approximation is used to calculate the Helmholtz free energy F of NaCl. The interionic forces are represented by a deformation-dipole model, which is parameterized self-consistently. We give a parameterized expression for F which can be used to predict the temperature and volume dependence of the thermodynamic properties. The expression for F is tested by comparing our predicted values for the heat capacity C_p, the thermal expansion α, and the bulk modulus B_s with experimental atmospheric-pressure values and by comparing predicted values for the Grüneisen γ at high pressures with the recent measurements of Boehler, Getting and Kennedy. Excellent agreement is obtained. Predictions for the volume dependence of C_p, α, and B_s and for the isothermal equation of state at room temperature are given. Values for the logarithmic derivative q_T = (?Inγ?? In V)_T and for the Anderson-Grüneisen parameters δ_S and δ_T are also given.     

Van der Waals density functional theory study for bulk solids with BCC,FCC, and diamond structures

Proper inclusion of van der Waals (vdW) interactions in theoretical simulations based on standard density functional theory (DFT) is crucial to describe the physics and chemistry of systems such as organic and layered materials. Many encouraging approaches have been proposed to combine vdW interactions with standard approximate DFT calculations. Despite many vdW studies, there is no consensus on the reliability of vdW methods. To help further development of vdW methods, we have assessed various vdW functionals through the calculation of structural properties at equilibrium, such as lattice constants, bulk moduli, and cohesive energies, for bulk solids, including alkali, alkali-earth, and transition metals, with BCC, FCC, and diamond structures as the ground state structure. These results provide important information for the vdW-related materials research, which is essential for designing and optimizing materials systems for desired physical and chemical properties.     

高压下岩盐矿和单斜结构AgCl的电子行为

 利用基于密度泛函理论的赝势平面波方法，计算了AgCl在高压下的结构行为和电子性质，交换关联函数采用广义梯度近似（GGA）。通过比较焓随压力的变化关系，从理论上确定了AgCl从岩盐矿结构相变到单斜结构的转变压强。预测了这两种结构在布里渊区中的价带顶和导带底的位置，结果表明：盐岩矿和单斜结构的AgCl都是具有间接带隙的半导体。还计算了这两种结构的带隙和电子态密度随压强的变化情况，发现在这两种结构相变之前都不会发生金属化转变。电荷转移研究发现，随着压强的增加，Ag原子和Cl原子之间成键的共价性增强，离子性减弱。     

Dielectric relaxation,modulus behavior and thermodynamic properties in [N(CH3)3H]2ZnCl4

[N(CH₃)₃H]₂ZnCl₄ has been analyzed by X-ray powder diffraction patterns, differential scanning calorimetry and impedance spectroscopy. The [N(CH₃)₃H]₂ZnCl₄ hybrid compound is obtained by slow evaporation at room temperature and found to crystallize in the orthorhombic system with Pnma space group. Five-phase transitions at low temperature were detected by differential scanning calorimetry measurements. The analysis of Nyquist plots has revealed the contribution of three electrically active regions corresponding to the bulk mechanism, distribution of grain boundaries and electrode processes. The dielectric relaxation is described by a non-Debye model. The study of the dielectric constants ?′, ?″ and loss tangent tan (δ) with frequency exhibits a distribution of relaxation times. The complex modulus plots have confirmed the presence of grains and grain boundaries as well as a non-Debye type of relaxation in the material. Thermodynamic parameters such as the free energy for dipole relaxation ΔF, the enthalpy ΔH and the change in entropy ΔS have been determined with the help of the Eyring theory.