Thermodynamic properties of MgO under high pressure from first-principles calculations

The equations of state (EOS) and other thermodynamic properties of the rocksalt (RS) structure MgO are investigated by ab initio plane-wave pseudopotential density functional theory method. The obtained results are consistent with the experimental data and those calculated by others. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V/V₀ on pressure P, cell volume V on temperature T, and Debye temperature Θ and specific heat C_V on pressure P are successfully obtained. The variation of the thermal expansion with temperature and pressure is investigated, which shows the temperature has hardly any effect on the thermal expansion at higher pressure.     

An isothermal equation of state for solids

An isothermal equation of state (EOS) for solids, recently suggested by the authors in the realistic form, V/V₀=f(P), with relative volume as the dependent and the pressure as the independent variable, was shown to have an advantage for some close-packed materials in that it allows B′_∞=(∂B_s/∂P)_s(P→∞) to be fitted, and this is where the usual standard equations fail. In the present study, our EOS is applied to a number of inorganic as well as organic solids, including alloys, glasses, rubbers and plastics; varying widely in their bonding and structural characteristics, as well as in their bulk modulus values. A very good agreement is observed between the data and fits. The results obtained are compared with those from two well-known equations, expressible in the realistic form, proposed by Murnaghan and Luban. Further, the results are also compared with those from the widely used two- and three-parameter EOSs, expressible in the unrealistic form only, P=f(V/V₀), proposed by Birch—and also with those from the EOS model of Keane in which B′_∞ is explicitly expressed as an equation of state parameter. The results obtained from our model compare well to these EOSs. Our EOS, in general, yields the smallest mean-squared deviations between data and fits. The values of B′_∞calculated from our EOS are compared with those from Keane's model. Further, we have studied the variation of B′_∞with temperature using the experimental isotherms of Mo and W at 10 different temperatures ranging from 100 to 1000 K, and observed that the values of B′_∞ yielded by our model and that of Keane vary, as expected, within a narrow range. Furthermore, our EOS is applied to study the stability of the fit parameters with variation in the pressure ranges with reference to the isothermal compression data on Mo and W—and also to study the variation of isothermal bulk modulus with pressure, with reference to the ultrasonic data on NaCl and noted a very good agreement with experiment. In addition, our model is applied, with B₀ and B′₀ constrained to the theoretical values, to the five theoretical isotherms of MgO at 300, 500, 1000, 1500 and 2000 K obtained on the basis of a first principles approach—a good agreement is observed with the predictions, and the values of B′_∞ inferred at different temperatures tend to converge to a constant value.     

Pressure–temperature studies on conductivity of TlX(X=Cl,Br,I) compounds:: I: phase diagram. II: ionic mobility

We have measured the conductivity σ of TlX(X=Cl, Br, I) compounds up to 5.3 GPa and between 300–823 K. The σ–T dependence for all compounds can be divided into three distinct regions: (i) low temperature (LT), <400 K, with unusual negative σ–T dependence, (ii) intermediate temperature (IT), 400<650 K, with positive σ–T dependence and (iii) high temperature (HT), T>650 K, with positive σ–T dependence. The σ–T isobars were used to construct the T–P solid phase diagram for each compound. The LT region data indicate a new meta-stable phase in the 1.0–3.5 GPa range. The LT→IT transition is characterized by an inverse σ–T dependence followed by normal Arrhenius behavior up to and including the HT region. The extrapolation to 1 atm of the P-dependent boundary between IT and HT regions above 3 GPa for each compound in the P–T plot yields a value close to its respective normal (1 atm) T_melt suggesting a solid order–disorder transition type paralleling -AgI behavior. The abrupt drop in conductivity in the LT region for P between 2.5–4.1 GPa of all compounds is at variance with the Arrhenius behavior observed for unperturbed ion migration implying the appearance of a second factor overriding the Arrhenius temperature dependence. Normal Arrhenius σ–T dependence prevails in both IT and HT regions with Q_c values of 85–100 kJ mol⁻¹ and 50–75 kJ mol⁻¹, respectively. The higher conductivities at 0.4 GPa for TlBr and TlI relative to their 1 atm data and the increasing σ with P are in strong contrast to the normal σ-P behavior of TlCl. The dependence of activation volume ΔV^‡ on T for TlCl, i.e. ΔV^‡>0, shows abnormally high values with a maximum at 500 K for P<3.0 GPa but reasonable ΔV^‡ values appear above 3.0 GPa. The ΔV^‡–T dependence for both TlBr and TlI with ΔV^‡<0 is incompatible with an ion transport mechanism suggesting an electronic conduction process and implying an ionic–metallic transition at higher pressures. These contrasting conductivity features are discussed and interpreted in terms of electronegativity differences and bonding character rather than structure.     

二维圆盘颗粒体系声学行为的数值研究

数值测量了卸载过程中二维单分散圆盘颗粒系统的横波、纵波声速、声衰减系数、非线性系数随压强的变化以及声衰减系数随频率的变化.结果表明,二维(2D)圆盘颗粒体系的横波、纵波声速均随压强呈分段幂律标度:当压强P10~(-4)时,横波、纵波声速随压强的增大而减小;当P10~(-4)时,有v_t~P~(0.202),v_l~P~(0.338).进一步得到其剪切模量和体积模量的比值G/B也随压强呈幂律标度,G/B~P~(-0.502),暗示在低压强下,与三维(3D)球形颗粒体系类似,2D圆盘颗粒体系也处于L玻璃态.水平激励和垂直激励下2D圆盘颗粒系统的衰减系数随频率变化也呈现分段行为:当频率f0.05时,衰减系数不随f变化;当f0.05时,横波纵波的衰减系数α~f;当f0.35时,横波衰减系数α_T~f~2,纵波衰减系数α_L~f~(1.5).此外,竖直水平激励下的2D圆盘颗粒系统的非线性系数和衰减系数随压强也呈现与声速类似的分段规律:当P10~(-4)时,横波非线性系数β_T~P~(-0.230),其余都不随压强变化.当P10~(-4)时,两者均随压强增大呈幂律减小:β_T~P~(-0.703),β_L~P~(-0.684),α_T~P~(-0.099),α_L~P~(-0.105).进而得到2D圆盘颗粒系统中散射相关的特征长度?~*随压强呈幂律标度,当P10~(-4)时,?~*~P~(-0.595);当P10~(-4)时,?~*~P~(0.236).     

Diffusion of Na in sodium borate glasses

Diffusion of ²²Na in ion-conducting xNa₂O·(1−x)B₂O₃ glasses (x=0.2 and 0.3) was investigated under standard conditions and under high pressure employing the radiotracer sectioning method. A slight decrease of the diffusion coefficients with increasing time of diffusion annealing is observed and attributed to a structural relaxation of the glass network. The temperature dependence of diffusion was measured below and also in a small temperature range above the glass transition temperature (T_G). Below T_G the diffusion coefficients obey Arrhenius relations. Diffusion in the glass with x=0.3 is fast and associated with an activation enthalpy of 71 kJ mol⁻¹. The diffusion coefficients of the glass with x=0.2 are significantly smaller with an activation enthalpy of 112 kJ mol⁻¹. The activation volume ΔV of Na-diffusion in 0.3Na₂O·0.7B₂O₃ was determined from high pressure diffusion studies at 623 K. A value of 0.52 molar volume of the glass matrix was obtained. According to our knowledge this is the first measurement of an activation volume of diffusion in ion-conducting glasses. The present results will be discussed and compared with dc conductivity data for borate glasses and diffusion results for other oxide glasses reported in the literature.     

P213 BN: a novel large-cell boron nitride polymorph

A new boron nitride polymorph, P2₁3 BN (space group: P2₁3), is investigated by first-principles calculations, including its structural properties, stability, elastic properties, anisotropy and electronic properties. It is found that the new boron nitride polymorph P2₁3 BN is mechanically, dynamically and thermodynamically stable. The bulk modulus (B), shear modulus (G) and Young's modulus of P2₁3 BN are 91 GPa, 41 GPa and 107 GPa, respectively, all of which are larger than that of Y carbon and TY carbon. By comparing with c-BN, the Young's modulus, shear modulus and Poisson's ratio of P2₁3 BN show tiny anisotropy in the (001), (010), (100) and (111) planes. At the same time, in contrast with most boron nitride polymorphs, P2₁3 BN is a semiconductor material with a smaller band gap of 1.826 eV. The Debye temperature and the anisotropic sound velocities of P2₁3 BN are also investigated in this work.     

相位角对容性耦合电非对称放电特性的影响

电非对称效应作为一种新兴技术,被广泛用于对离子能量和离子通量的独立调控.此外,在改善等离子体的径向均匀性方面,电非对称效应也发挥了重要作用.本文采用二维流体力学模型,并耦合麦克斯韦方程组,系统地研究了容性耦合氢等离子体中当放电由多谐波叠加驱动时,不同谐波阶数k下的电非对称效应,重点观察了相位角θ_n对自偏压以及等离子体径向均匀性的影响.模拟结果表明：在同一谐波阶数下,自偏压随相位角θ_n的变化趋势不尽相同,且当k增大（k>3）时,自偏压随最高频相位角θ_k的变化范围逐渐减小.此外,通过调节相位角θ_n,可以改变轴向功率密度和径向功率密度的相对关系,进而实现对等离子体径向均匀性的调节.研究结果对于利用电非对称效应优化等离子体工艺过程具有一定的指导意义.     

Reactive sintering for oriented and connected Bi-2223 superconductive oxides from a mixture of delaminated Bi-2212 and fine powdered supplemental ingredients

Single crystals of NdBa₂Cu₃O_7−δ (Nd123) have been successfully grown by the top-seeded solution-growth (TSSG) method in 1%, 21% and 100% oxygen partial pressure atmosphere ((P(O₂) = 0.01 atm, P(O)₂) = 0.21 atm and P(O₂) = 1.00 atm). Ba---Cu---O solvent with a Ba to Cu ratio of 3:5 was used in a Nd₂O₃ crucible. Nd is supplied by the reaction between the molten solvent and the Nd₂O₃ crucible. Compositions of Nd123 single crystals grown in different oxygen partial pressure atmospheres were analyzed by inductivity coupled plasma atomic emission spectrometry (ICP-AES) and confirmed to be Nd:Ba:Cu = 1.01:1.97:3.00 for P(O₂) = 0.01 atm, Nd:Ba:Cu = 1.07:1.95:3.00 for P(O₂) = 0.21 atm and Nd:Ba:Cu = 1.10:1.90:3.00 for P(O₂) = 1.00 atm, respectively. The Nd123 single crystals grown in different oxygen partial pressure atmospheres were annealed in a pure oxygen gas flow, and the temperature dependence of the DC magnetization for these crystals was measured using a superconducting quantum interference device (SQUID) magnetometer. The Nd123 single crystal grown in P(O₂) = 0.01 atm, and annealed at 340°C for 200 h showed a steep superconductive transition at 96 K. On the other hand, the Nd123 crystal grown in P(O₂) = 0.21, 1.00 atm and, annealed at 340°C for 200 h exhibited a broad transition at 92 K for P(O₂) = 0.21 atm and at 88 K for P(O₂) = 1.00 atm, respectively. Therefore for Nd123 single crystal production with high quality superconductive characteristics, a low oxygen partial pressure atmosphere during crystal growth is found to be effective for minimizing the substitution of Nd ions into Ba sites.     

ZrV2结构、弹性和热力学性质的第一性原理计算

采用基于密度泛函理论的第一性原理平面波赝势方法研究ZrV₂的晶体结构和弹性,利用准谐Debye模型计算在不同温度(T=0~1200 K)和不同压强(P=0~20 GPa)下ZrV₂的热力学性质,包括弹性模量与压强,热熔与温度,以及热膨胀系数与温度和压力的关系.结果表明:计算的ZrV₂晶格常数与实验值符合较好,晶体材料的弹性常数随着压力增加而增加;在一定温度下,相对体积、热熔随着压强的增加而减小,德拜温度、弹性模量随着压强的增加而增加,且高压下温度对ZrV₂热膨胀系数的影响小于压强的影响.     

Temperature and pressure dependence of NMR frequencies in ferromagnetic Heusler alloys containing cobalt and manganese

The ⁵⁹Co and ⁵⁵Mn NMR frequencies ν in Co₂MnZ (Z = Si, Ge, Sn and Ga) were measured as a function of temperature T and pressure P by the cw method. The ν/ν_4.2 value decreased faster for ⁵⁹Co than for ⁵⁵Mn with increasing T/T_C especially in Co₂MnGa, for which ∂ν/∂P was positive at 0°C in contrast to the results in other alloys.     

Analysis of frequency-dependent series resistance and interface states of In/SiO2/p-Si (MIS) structures

In this work, the investigation of the interface state density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G/ω−V) characteristics in In/SiO₂/p-Si metal–insulator–semiconductor (MIS) structures with thin interfacial insulator layer have been reported. The thickness of SiO₂ film obtained from the measurement of the oxide capacitance corrected for series resistance in the strong accumulation region is 220 Å. The forward and reverse bias C–V and G/ω−V characteristics of MIS structures have been studied at the frequency range 30 kHz–1 MHz at room temperature. The frequency dispersion in capacitance and conductance can be interpreted in terms of the series resistance (R_s) and interface state density (D_it) values. Both the series resistance R_s and density of interface states D_it are strongly frequency-dependent and decrease with increasing frequency. The distribution profile of R_s–V gives a peak at low frequencies in the depletion region and disappears with increasing frequency. Experimental results show that the interfacial polarization contributes to the improvement of the dielectric properties of In/SiO₂/p-Si MIS structures. The interface state density value of In/SiO₂/p-Si MIS diode calculated at strong accumulation region is 1.11×10¹² eV⁻¹ cm⁻² at 1 MHz. It is found that the calculated value of D_it (≈10¹² eV⁻¹ cm⁻²) is not high enough to pin the Fermi level of the Si substrate disrupting the device operation.     

Effect of oxide thickness on the capacitance and conductance characteristics of MOS structures

In this work, the investigation of the interface states density and series resistance from capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics in Au/SnO₂/n-Si (MOS) structures prepared at various SnO₂ layer thicknesses by spray deposition technique have been reported. It is fabricated five samples depending on deposition time. The thicknesses of SnO₂ films obtained from the measurement of the oxide capacitance in the strong accumulation region for MOS Schottky diodes are 37, 79, 274, 401, and 446 Å, for D1, D2, D3, D4, and D5 samples, respectively. The C–V and G–V measurements of Au/SnO₂/n-Si MOS structures are performed in the voltage range from −6 to +10 V and the frequency range from 500 Hz to 10 MHz at room temperature. It is observed that peaks in the forward C–V characteristics appeared because of the series resistance. It has been seen that the value of the series resistance R_s of samples D1 (47 Ω), D2 (64 Ω), D3 (98 Ω), D4 (151 Ω), and D5 (163 Ω) increases with increasing the oxide layer thickness. The interface state density D_it ranges from 2.40×10¹³ cm⁻² eV⁻¹ for D1 sample to 2.73×10¹² cm⁻² eV⁻¹ for D5 sample and increases with increasing the oxide layer thickness.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

Effects of oxygen flow rate on the properties of HfO2 layers grown by metalorganic molecular beam epitaxy

The investigations on the properties of HfO₂ dielectric layers grown by metalorganic molecular beam epitaxy were performed. Hafnium-tetra-tert-butoxide, Hf(C₄H₉O)₄ was used as a Hf precursor and pure oxygen was introduced to form an oxide layer. The grown film was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and capacitance–voltage (C–V) and current–voltage (I–V) analyses. As an experimental variable, the O₂ flow rate was changed from 2 to 8 sccm while the other experimental conditions were fixed. The XPS spectra of Hf 4f and O 1s shifted to the higher binding energy due to the charge transfer effect and the density of trapped charges in the interfacial layer was increased as the oxygen flow rate increased. The observed microstructure indicated the HfO₂ layer was polycrystalline, and the monoclinic phases are the dominant crystal structure. From the C–V analyses, k = 14–16 and EOT = 44–52 were obtained, and the current densities of (3.2–3.3) × 10⁻³ A/cm² were measured at −1.5 V gate voltage from the I–V analyses.     

First-Principle Calculations for Thermodynamic Properties of LiBC Under High Temperature and High Pressure

The thermodynamic properties of LiBC are investigated by using the full-potential linearlzed muffin-tin orbital method （FP-LMTO） within the frame of density functional theory （DFT） and using the quasi-harmonic Debye model. The dependencies of the normalized lattice parameters a/a0 and c/c0, the ratio （c/a）/2, the normalized primitive volume V/V0 on pressure and temperature are successfully obtained. It is found that the interlayer covalent interactions （Li-B bonds or Li-C bonds） are more sensitive to temperature and pressure than intralayer ones （B-C bonds）, as gives rise to the extreme lattice anisotropy in the bulk hop LiBC.     

Bias-dependent timing jitter of 1-GHz sinusoidally gated InGaAs/InP avalanche photodiode

We characterized the dependence of the timing jitter of an InGaAs/InP single-photon avalanche diode on the excess bias voltage(V_(ex)) when operated in 1-GHz sinusoidally gated mode.The single-photon avalanche diode was cooled to-30 degrees Celsius.When the V_(ex) is too low(0.2 V-0.8 V) or too high(3 V-4.2 V),the timing jitter is increased with the V_(ex),particularly at high V_(ex).While at middle V_(ex)(1 V-2.8 V),the timing jitter is reduced.Measurements of the timing jitter of the same avalanche diode with pulsed gating show that this effect is likely related to the increase of both the amplitude of the V_(ex) and the width of the gate-on time.For the 1-GHz sinusoidally gated detector,the best jitter of 93 ps is achieved with a photon detection efficiency of 21.4%and a dark count rate of ~2.08×10~(-5) per gate at the V_(ex) of 2.8 V.To evaluate the whole performance of the detector,we calculated the noise equivalent power(NEP) and the afterpulse probability(P_(ap)).It is found that both NEP and P_(ap) increase quickly when the V_(ex) is above 2.8 V.At ~2.8-V V_(ex),the NEP and P_(ap) are ~2.06×10~(16)W/Hz~(1/2) and 7.11%,respectively.Therefore,the detector should be operated with V_(ex) of 2.8 V to exploit the fast time response,low NEP and low P_(ap).     

X-ray and high resolution selenium-77 solid state NMR spectroscopy as complementary probes to structural studies of organophosphorus diselenides

high resolution solid state NMR spectroscopy was employed to study structural properties of bis(diisopropoxyphosphorothioyl) diselenide 1 and bis(dineopentoxyphosphorothioyl) diselenide 2. The principal elements T_ii of effective dipolar/chemical shift tensor were calculated from spinning sideband intensities employing the WIN-MAS program. The values of anisotropy and asymmetry parameters reflect the distortion of the selenium environment. It was found that the T₃₃ component mostly contributes to changes in the isotropic chemical shifts. CP/MAS experiments were used to decide the assignment of space group by counting the number of crystallographically unique selenium centers in the unit cell. Crystals of diselenide 1 are triclinic, space group P with a=8.485(3) Å, b=8.508(1) Å, c=8.511(2) Å, =98.835(15)°, β=111.653(24)°, γ=93.524(21)°, V=559.5(3) ų, D_c=1.544(2) g/cm³ and Z=1. Refinement using 2222 reflections for 157 variables gives R=0.037. Crystals of diselenide 2 are triclinic, space group P1 with a=9.1418(8) Å, b=9.1465(8) Å, c=9.9200(9) Å, =74.751(8)°, β=74.629(7)°, γ=82.216(7)°, V=769.7(1) ų, D_c=1.365(2) g/cm³ and Z=1. Refinement using 3316 reflections for 297 variables gives R=0.0272.     

Structural and Thermodynamic Properties of Wurtzitic Boron Nitride from First-Principle Calculations

Using ab initio plane-wave pseudopotential density functional theory method, we have studied the structural and thermodynamic properties of the wurtzite boron nitride （w-BN）. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependencies of the normalized lattice parameters a/ao and c/co, the axial ratio c/a, and the normalized primitive cell volume V/Vo on pressure P and temperature T are investigated. Furthermore, the Debye temperature θ, the variation of thermal expansion α, as well as the heat capacity Cv as functions of P and T are studied systematically.     

Thermo acoustic parameters in the nematic and isotropic phases of 5CB and tetraethyl methane in 5CB

The addition of non-rod like solute impurity to a liquid crystalline solvent leads to some exciting results such as depression of nematic–isotropic transition temperature and the formation of two-phase region. In view of this, specific volume and thermal expansion studies in the nematic and isotropic phases of p-n-pentyl-p¹-cyanobiphenyl (5CB) and dilute solution of the quasispherical solute tetraethyl methane were considered. From these results thermo acoustic parameters such as isochoric temperature coefficient of internal pressure (X), isochoric volume expansivity (X¹), the reduced compressibility (β), the reduced volume (V), isothermal microscopic Gruneisen parameter (Γ), Huggins parameter (F), fractional free volume (f) and Sharma parameter (S_o) are estimated both in nematic and isotropic regions. Using the data given by Oweimreen et al. [G.A. Oweimren, A.K. Shihab, K. Halhouli, S.F. Sikander, Mol. Cryst. Liq.Cryst. 138 (1986) 327] variations of these thermo acoustic parameters with temperature and two phase region are critically examined.