Ammonium ion dynamics in NH4I at high pressure

Measurements of spin-lattice relaxation time T ₁ have been performed for polycrystalline ammonium iodide NH₄I as a function of hydrostatic pressure and temperature. Activation parameters were obtained for different models of ammonium ion reorientations. It was found that the dominant mechanism for ammonium reorientational motion in the ordered phases III and IV was the reorientation around C₃ axes. Reorientation activation volume and libration frequency of ammonium ions as a function of pressure were calculated for different phases.     

Structural study of ND4Br at high pressure

Abstract

A structure of ND₄Br has been studied at pressures up to 9 GPa by means of time-of-flight neutron diffraction. A phase transition to the high pressure phase V was observed at P=8·2(5)GPa. It was found that the phase V has a tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm which is in strong resemblance with low temperature modification ND₄Br(III). Deuterium positional parameter as a function of pressure was obtained.     

Ammonium ion reorientation in NH4ReO4 by NMR

Second moments of the proton absorption lineshape and spin-lattice relaxation times have been measured for NH₄ReO₄ in the temperature range 77 to 430 K. The results indicate a large degree of freedom for the NH₄⁺ ionic reorientation over this range. The relaxation time results point to the existence of a low-temperature 28° reorientation mode of the ion changing to a general reorientation at higher temperatures. The second-moment results suggest that a transformation from general reorientation to isotropic tumbling of the NH₄⁺ ions is accompanied by a rearrangement of the lattice. A model is proposed whereby this lattice rearrangement is transmitted via the oxygen framework to the rhenium nucleus and thereby produces the anomalous temperature dependence of the ¹⁸⁷Re NQR frequency observed in this material.     

Decomposition of binary sulphate KHSO4 at high pressure

Abstract

Pressure induced decomposition (PID) is known to occur only at elevated temperatures. Here we report its first occurrence at ambient temperature in a binary sulphate, KHSO₄ at 4kbar. Raman spectroscopy is used for identifying the decomposition products as K₃H(SO_{2 4}) and H₂SO₄ from their characteristic spectra. One of the decomposition products being a liquid is argued to be the reason for the occurrence of the phenomenon at ambient temperature. Other possible decomposition routes are also examined.     

Structure and dynamics of liquid formamide at high pressure and high temperature: comparison of X-ray diffraction and molecular dynamics results

New molecular dynamics simulations with optimised potentials for liquid simulation are presented for liquid formamide at high pressures and high temperatures. The structural results are compared to those found by X-ray diffraction measurements, and the H-bonding structure is analysed in detail. While it is ambiguous from purely experimental data, the simulation results support the idea that pressure increase can enhance ring dimer formation at the expense of linear chain structure, but increasing temperature results in an opposite effect. Dynamical results (reorientational correlation times and lifetimes of hydrogen bonds) are in agreement with these findings.     

Splitting of libration mode frequencies in the vicinity of orientation phase transition in NH4Br

Abstract

Vibrational spectra of NH₄Br at high pressures up to 4·5GPa have been studied by means of incoherent inelastic neutron scattering using sapphire anvil high pressure cell technique. Libration mode splitting was investigated in the vicinity of the orientation phase transition (P_tr = 2·7 GPa) and this effect disappears if pressure is less or higher than P_tr. This effect is explained in terms of two-well asymmetric potential.     

Single crystal EXAFS at high pressure

Abstract

We present a new technique for structure characterization under high pressure conditions. The use of an undulator beam of the third-generation ESRF source of synchrotron radiation has enabled the first single crystal EXAFS experiments at high pressure using a diamond anvil cell as pressure generator. Taking advantage of the linear polarization of X-rays the technique becomes an orientation-selective probe of the local structure of materials. We describe the principle of the technique and some applications.     

Chemical kinetics at high pressure

Abstract

Numerous studies have demonstrated the importance to include pressure as a kinetic parameter in the elucidation of inorganic reaction mechanisms. These studies have specially led to a better understanding and a systematic classification of solvent exchange and ligand substitution reactions of octahedral complexes of transition metal elements. The mechanistic picture for substution reactions on square planar complexes is well established and involves a concurrent bimolecular attack by solvent and the nucleophile on the substrate with a considerable discrimination among different entering groups. The search for factor promoting the conversion of the normal associative mode of activation into a dissociative process has then attracted much attention. Two attempts to induce dissociation, as studied by high-pressure NMR, are presented: one is to prevent the formation by means of sterically hindered ligands, the other one is to promote bond weakening at the leaving group.     

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH₄Cl at pressures of up to 2.6 GPa and of NH₄Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.     

Spectroscopy of II–VI nanocrystals at high pressure and high temperature

We review experimental measurements of CdS, CdSe, and CdS_xSe_{1 − x} nanocrystals which address the properties and phase stability of both powder and embedded nanoparticles under extreme conditions. We address the high pressure phase transition of wurtzite or zincblende phase to the rock salt structure; the high temperature solid to liquid phase transition; and the homogeneous nucleation of nanoparticles in glass from dissolved reactants. We also review the use of high pressure optical measurements to study electronic states.     

Investigation of Nd2CuO4 crystal structure at high pressure by neutron diffraction

Abstract

The crystal structure of Nd₂CuO₄ has been studied by neutron diffraction at pressure up to 5 Gpa. The volume compressibility value was determined as 5·6·10^?3/Gpa. The decrease of positional parameter of neodymium at high pressure has been observed. This structural change is explained by pressure induced neodymium ions charge increase.     

Crystal structures of silicon-rich lithium silicides at high pressure

Silicon (Si) is one of the most essential elements, as it is indispensable for modern electronic technology. The standard Si structure at ambient conditions is the cubic diamond structure, and it has an indirect band gap, which prevents it from being considered as a next-generation platform for semiconductor technologies. Therefore, the search for new allotropes of silicon has attracted great attention. Herein, first principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium containing compounds, LiSi_x ($x=4\text{–}8$). The LiSi₄ stoichiometry was predicted to be stable, and it was found to assume one of the following space groups, P4/mnc, Cmmm, and C2/m within the pressure range of 0 to 50 GPa. By removing the Li atoms from these compounds, three silicon allotropes were obtained that were metastable at ambient pressures. Our work illustrates how novel silicon allotropes can be predicted using the CALYPSO method.     

Brillouin scattering of vitreous silica under high pressure

The pressure dependence of Brillouin spectra of vitreous silica exhibits a maximum in the hypersonic absorption and a minimum in the sound velocity at about 2 GPa almost independent of temperature. The results are discussed on the basis of relaxing defects residing in double-well potentials with a Gaussian distribution of barrier heights and a Lorentzian distribution in the asymmetry. Our analysis suggests that both the distribution functions are strongly changed at higher pressures. It is striking that a phase boundary between α-quartz and coesite exists just at the pressure where absorption and sound velocity of vitreous silica pass their extremum. This might be a hint that a corresponding structural change also occurs in the glass.     

季铵盐型双子表面活性剂16-4-16聚集状态的NMR研究

核磁共振弛豫，自扩散以及2D NOESY谱研究结果表明：双子表面活性剂16-4-16溶液在形成胶束的过程中，联结基团及其邻近的碳氢链质子形成胶束的壳层，而距离离子头较远的疏水质子位于胶束的内部. 与对应的单链的表面活性剂CTAB相比，其分子运动更受限制. 2D NOESY谱显示联接基团及临近的碳氢链的质子间有较强的交叉峰，表明形成胶束时，分子在联结基团附近堆积的较为紧密. 由2D NOESY谱计算得到的质子间距与HYPERCHEM模拟值有偏差，表明这些强交叉峰是分子间相互作用的结果，并且对应质子对在双子表面活性剂16-4-16分子中位于邻近的区域. 因此我们推测，双子表面活性剂16-4-16分子在球形胶束中形成特殊的排列方式.     

Surface effects on liquid crystals constrained in nanoscaled pores investigated by field cycling NMR relaxometry and Monte Carlo simulations

Proton relaxation rates of nematic liquid crystals confined in nanoporous cavities were measured in a broad frequency range with the help of field cycling nuclear magnetic resonance relaxometry. The shape of relaxation dispersion curves in confined materials strongly deviates from the behavior in bulk, both above and below the bulk isotropization temperature. A strong increase in relaxation rates, exceeding by two orders of magnitude that of the bulk sample, is observed in the range of a few kilohertz. Relaxation rates in bigger pores decreased. Experimental findings are interpreted in terms of surface-induced orientational order and diffusion between sites with different orientations of local directors. With the aid of Monte Carlo simulations, two processes affecting low-frequency relaxation could be identified: (a) exchange losses of molecules from the surface-ordered phase to the bulk-like phase, and (b) Reorientations Mediated by Translational Displacements, which dominate the long-time scale and account for the recovery of correlation in molecular orientations as molecules probe different surface sites. It is shown that the width of the oriented layer may strongly affect the slope of dispersion curves and that cross-over between plateau and power law dispersion regimes shifts towards lower frequencies for bigger pores.     

二级6—8型大腔体装置的高压发生效率机理研究

利用自行设计与集成的二级6—8型大腔体静高压装置,研究了影响八面体压腔高压发生效率的主要因素及机理,并提出了一种八面体压腔密封的简化力学模型.针对于10/4（八面体传压介质边长为10 mm,二级WC立方体增压块截角边长为4 mm）组装的实验结果发现：预密封边尺寸会显著影响八面体压腔的压力产生效率;在腔体压力为12 GPa左右时,高压发生效率随八面体MgO传压介质初始密度的增加而提高;在15 GPa以上时,影响压力产生效率的主要因素是WC增压立方块本身的强度以及加压过程中所形成密封边的尺寸及材料. 关键词： 6—8型大腔体静高压装置 压力产生效率     

Burning velocity correlation of methane/air turbulent premixed flames at high pressure and high temperature

Turbulent burning velocities for methane/air mixtures at pressures ranging from atmospheric pressure up to 1.0 MPa and mixture temperatures of 300 and 573 K were measured, which covers the typical operating conditions of premixed-type gas-turbine combustors. A bunsen-type flame stabilized in a high-pressure chamber was used, and OH-PLIF visualization was performed with the pressure and mixture temperature being kept constant. In addition to a burner with an outlet diameter of 20 mm for the high-pressure experiments, a large-scale burner with an outlet diameter of 60 mm was used at atmospheric pressure to extend the turbulence Reynolds number based on the Taylor microscale, R_λ, as a common parameter to compare the pressure and temperature effects. It was confirmed that R_λ over 100 could be attained and that u′/S_L could be extended even at atmospheric pressure. Based on the contours of the mean progress variable c = 0.1 determined using OH-PLIF images, turbulent burning velocity was measured. S_T/S_L was also found to be greatly affected by pressure for preheated mixtures at 573 K. The bending tendency of the S_T/S_L curves with u′/S_L was seen regardless of pressure and mixture temperature and the R_λ region where the bending occurs corresponded well to the region where the smallest scale of flame wrinkling measured as a fractal inner-cutoff approaches the characteristic flame instability scale and becomes almost constant. A power law of S_T/S_L with (P/P₀)(u′/S_L) was clearly seen when S_T was determined using c = 0.1 contours, and the exponent was close to 0.4, indicating agreement with the previous results using the mean flame cone method and the significant pressure effects on turbulent burning velocity.     

Behavior of decomposed ammonia borane at high pressure

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 °C under the pressure at ∼0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 h. The hydrogen was sealed in DAC well and cooled down to room temperature. Applying higher pressure up to ∼10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.     

分子串模型中空间弛豫模式的弛豫动力学的蒙特卡罗模拟

基于玻璃化转变的分子串模型的分子串弛豫方程,提出了更为精确的模拟分子串中所有空间弛豫模式(SRM)的蒙特卡罗模拟方案. 模拟得出各个SRM的弛豫时间随温度和分子串长度的变化结果与分子串模型中分子串弛豫方程的预言完全一致,即理论预期和模拟结果相互印证. 应当指出,分子串能否作为液态中集体单元的必要条件是在考虑到分子串之间的不均匀随机相互作用后,分子串的所有SRM的定性特征是不能改变的,这就需要对不同分子串的SRM之间的耦合进行研究. 但是迄今为止,仍未发现相关的严格解,仅有近似的自洽弛豫平均场方法. 由此可知,所提出的模拟方案为研究不同分子串的SRM之间的耦合(包括上述自洽场的可行性)提供了必要的基础. 关键词： 玻璃化转变 弛豫动力学 蒙特卡罗模拟 分子串