Structural and magnetic properties of CeZnAl<Subscript>3</Subscript> single crystals

We have synthesized single crystals of CeZnAl₃, which is a new member of family of Ce-based intermetallics CeTX₃ (T=transition metal, X=Si, Ge, Al), crystallizing in the non-centrosymmetric tetragonal BaNiSn₃-type structure. Magnetization, specific heat and resistivity measurements all show that CeZnAl₃ orders magnetically below around 4.4 K. Furthermore, magnetization measurements exhibit a hysteresis loop at low temperatures and fields, indicating the presence of a ferromagnetic component in the magnetic state. This points to a different nature of the magnetism in CeZnAl₃ compared to the other isostructural CeTAl₃ compounds. Resistivity measurements under pressures up to 1.8 GPa show a moderate suppression of the ordering temperature with pressure, suggesting that measurements to higher pressures are required to look for quantum critical behavior.     

Reappearance of ferroelectric soft modes in the paraelectric phase of Pb(1-x)Ca(x)TiO3 at high pressures: Raman and x-ray diffraction studies

High pressure Raman spectroscopy and x-ray diffraction measurements have been carried out on Pb(1-x)Ca(x)TiO(3) (x = 0.10 and 0.30). Using high pressure Raman spectroscopic data, it is observed that the phonon instability responsible for the ferroelectric phase reappears in the paraelectric phase after a critical pressure. The observed critical pressures in the Ca(2+) doped PbTiO(3) system are much lower than the unique pressures suggested for PbTiO(3) based materials. A suitable explanation is given to explain this lowering of critical pressure. It is also shown that the ferroelectric phase which stabilizes in the paraelectric phase has a tetragonal symmetry with space group I4cm.     

Decomposition of NO on clean Pt near atmospheric pressures: II. Adsorbate coverages

Temperature programmed desorption (TPD) and temperature programmed adsorption (TPA) have been used to characterize adsorbate coverages during and after NO decomposition on polycrystalline Pt foils at pressures between 10^?4 and 30 Torr. The densities and stoichiometries of tightly bound species were determined after reaction by TPD of NO, N₂, and O₂ following cooling and pumpdown to <10^?8 Torr. For characterization during reaction at pressures up to 10^?3 Torr the ribbon was flashed inside a 35 cm³ reaction cell, and desorption and adsorption spectra of all species were recorded. Using digital acquisition of pressures versus time, peaks as small as 10^?3 of the background pressure could be analyzed. By flashing to different fixed temperatures, adsorption isobars during reaction were determined. These measurements show that there is a tightly bound stoichiometric layer of N and O (perhaps undissociated) and that the reactive state is weakly bound and appears to be strongly inhibited by molecular oxygen. This model also agrees with reaction rate measurements at these pressures.     

Modeling of high-pressure 12-μm NH3 lasers

Experimental measurements of small-signal gain in an optically-pumped NH₃ amplifier are carried out at pressures ranging from 40 Torr to 760 Torr, and the results are used to validate a rate-equation model describing the amplifier dynamics. The gain measurements show that dilute mixtures of <0.5% NH₃ in N₂ are reqired to minimize the problems of gas heating due to pump absorption. The model is used to extrapolate the results to gas pressures of several atmospheres, and to demonstrate the potential for highpressure operation of optically-pumped NH₃ lasers. For a pump intensity of 100 MW/cm², calculations indicate that operation of an NH₃–N₂ laser is feasible up to a pressure of 10 atm, which would provide a maximum continuous tuning range of 4 cm^–1. High-resolution spectroscopy reveals that gain on a few NH₃ transitions is eliminated at high pressures due to the presence of overlapping absorptions in other NH₃ bands.     

Pressure Effects in Phonon Modes and Structure of A II B 2 III C 4 IV Compounds and Combinations

A II B 2 III C 4 VI compounds have been prepared with A II =Zn, Cd, Mn, B III =In, and C VI =S, Se. These atomic substitutions induce modifications in the structure and frequency for some of the phonons. These compounds have been studied by Raman spectroscopy at ambient conditions and under hydrostatic pressures. All phonons appear in the spectral region below 400 cm m 1 . High-pressure measurements have shown the existence of soft modes. New modes seem to appear at high pressures due to the phonon splitting. All low frequency phonons of the Zn 0.6 Mn 0.4 In 2 S 4 compound show a sudden modification in the frequency dependence on pressure above ～2 GPa. Based on the Raman results we propose an assignment for the modes and try to extract information about the structure and the possible phase transitions.     

Neutron diffraction and electrical transport studies on the incommensurate magnetic phase transition in holmium at high pressures

Neutron diffraction and electrical transport measurements have been made on the heavy rare earth metal holmium at high pressures and low temperatures in order to elucidate its transition from a paramagnetic (PM) to a helical antiferromagnetic (AFM) ordered phase as a function of pressure. The electrical resistance measurements show a change in the resistance slope as the temperature is lowered through the antiferromagnetic Néel temperature. The temperature of this antiferromagnetic transition decreases from approximately 122 K at ambient pressure at a rate of -4.9 K GPa(-1) up to a pressure of 9 GPa, whereupon the PM-to-AFM transition vanishes for higher pressures. Neutron diffraction measurements as a function of pressure at 89 and 110 K confirm the incommensurate nature of the phase transition associated with the antiferromagnetic ordering of the magnetic moments in a helical arrangement and that the ordering occurs at similar pressures as determined from the resistance results for these temperatures.     

3-pentanone fluorescence yield measurements and modeling at elevated temperatures and pressures

Measurements of 3-pentanone fluorescence quantum yield (FQY) over a wide range of temperatures and pressures in air and nitrogen bath gases are reported and a comprehensive FQY model in support of quantitative planar laser-induced fluorescence diagnostics at elevated pressures and temperatures is presented. Measurements were made of the FQY for 20 mbar of 3-pentanone in nitrogen and air for pressures between 1 and 25 bar in a high-pressure and high-temperature cell for excitation wavelengths of 248, 266, 277, and 308 nm. The measurements were performed in nitrogen from 298 to 745 K and in air from 298 to 567 K. The 3-pentanone FQY data were used to optimize FQY model parameters, including the oxygen and nitrogen quenching rates and vibrational relaxation cascade parameters for nitrogen and oxygen. This work introduces vibrational energy dependence for cascade parameters, as well as a nitrogen quenching rate. The new 3-pentanone FQY model agrees with the measurements within 10%, as well as with fluorescence signal measurements from optical internal combustion engines at pressures and temperatures up to 28 bar and 1100 K.     

High temperature transport processes in lithium niobate

The electrical conductivity of single crystal lithium niobate (LiNbO₃) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to P_o2^−1/4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons.

Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO₃ to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures.

Thermoelectric measurements indicate that LiNbO₃ at low oxygen partial pressures is n-type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO₂a is 4 × 10¹⁷cm³ with a mobility of 1.7 cm²V sec.

The diffusion of oxygen in LiNbO₃ was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O¹⁸/O¹⁶ isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10⁻⁶ exp (−29.4 kcal mole⁻¹/RT)cm²sec. Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li₂O activity indicate that the ionic conduction is caused by transport of lithium ions.     

Magnetoresistance and magnetic properties of Fe3O4 nanoparticle compacts

In this paper,we report on the magnetic properties of Fe3O4 nanoparticles with different grain sizes under different pressures.In all the samples,the saturated magnetization Ms shows a linear decrease with increasing pressure.The thickness of the magnetic dead layer on the nanoparticle surface nuder different pressures was roughly estimated,which also increases with increasing pressure.The transport measurements of the nanoparticle Fe3O4 compacts show that the low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range;however,the high-field MR value is dependent on grain size,especially at low temperatures.These experimental results can be attributed to the different surface states of the nanoparticles.     

Exploring the compression behavior of HP-BiNbO_4 under high pressure

In the present work, a third form, the so-called HP-BiNbO_4 synthesized at high pressure and high temperature is investigated with the in-situ angle-dispersive x-ray diffraction(ADXRD) measurements under high pressure. We explore the compression behavior and phase stability of HP-BiNbO_4. The structure of HP-BiNbO_4 is first determined. The x-ray diffraction data reveal that the structure HP-BiNbO_4 is stable under pressures up to 24.1 GPa. The ADXRD data yield a bulk modulus K_o = 185(7) GPa with a pressure derivative K_o'= 2.9(0.8). Furthermore, the data are compared with those of other ABO_4 compounds. The results show that the bulk modulus of HP-BiNbO_4(about 185 GPa) is slightly higher than that of tetragonal BiVO_4 and significantly greater than those of the tungstates and molybdates.     

Collapse of 5 f -Electron Ferromagnetism in UPtAl Under High Pressures

UPtAl exhibits a ferromagnetic ordering of U magnetic moments at temperatures below T C =42.5 K. The magnetic-ordering transition is accompanied by an anomaly in the temperature dependence of electrical resistivity. This allows us to determine the value of Curie temperature from 𝜌 vs. T data that can be measured at very high pressures, at which reliable magnetization measurements are difficult. We report on resistivity measurements performed on an UPtAl single crystal under hydrostatic pressures p h 8 GPa. It was observed that the initial increase of T C with p becomes gradually reduced for p >2 GPa until the maximum T C value of , 48 K is reached between 4 and 6 GPa that is followed by a progressively developing downturn of the T C ( p ) curve. The latter result is attributed to the approaching collapse of the U 5 f -moment ferromagnetism. Low-temperature resistivity data point to a rapidly reduced magnetic anisotropy at highest pressures.     

Production of the excited Ne(2p 5 3p) atoms by dissociative recombination in the afterglow of high frequency neon discharge

Experimental results are presented of the measurements of the population of the Ne(2p ⁵ 3p) atoms from the time dependences of the relative intensities of the spectral lines corresponding to 3s -3p transitions in the afterglow period of a high frequency neon glow discharge at pressures from 266 Pa to 4 kPa. The partial recombination coefficients were determined from these measurements as a function of gas pressure and they are compared with measurements of other authors. It is shown that at low gas pressures the Ne(2p ⁵ 3p) atoms are also populated only in the dissociative recombination in spite of the fact that the dominant loss process of charged particles is the ambipolar diffusion.     

Spectral Intensity and Lineshape Measurements in the First Overtone Band of HF Using Tunable Diode Lasers

High-resolution absorption lineshapes for the P(3) and P(6) transitions of the first overtone (v = 2-0) band of HF at 296 K have been measured using a pair of distributed feedback diode lasers operating near 1.31 and 1.34 μm, respectively. Spectral line intensities and self-broadening parameters were determined by fitting the measured spectra with Voigt, Galatry, and Rautian lineshape models. Voigt profiles fit the low-pressure (<10 Torr) spectra of the P(3) transition reasonably well due to the relatively strong collisional broadening effect. Lineshape measurements of the P(6) transition (for pressures ranging from 5 to 60 Torr) show significant variation from the Voigt lineshape model due to velocity-changing collisions that effectively reduce the Doppler component of the spectral line. Lineshape models that include motional (Dicke) narrowing effects, Galatry (soft collision) and Rautian (hard collision) profiles yield significant improvements in the spectral lineshape fits compared with Voigt profiles. The collisional broadening coefficient (gamma) of the P(6) transition obtained from a Voigt fit is approximately 4% lower than those found with either Galatry or Rautian profile fits. The measured intensities and self-broadening coefficients are compared with values in the HITRAN database and previous measurements. Copyright 1999 Academic Press.     

Neutron Diffraction Study of the p - T Phase Diagram for Erbium

Electrical resistivity measurements performed on a single crystal of erbium as a function of temperature and hydrostatic pressure have provided a preliminary p - T phase diagram. The results have been interpreted in terms of a model for the magnetic structures of Er deduced from neutron diffraction studies at ambient pressure. This model predicted the existence of a magnetic structure with a wave vector of Q =2/7 c * at 4.2 K, when the applied pressure is larger than 3 kbar. This paper reports a neutron diffraction study of erbium made in the temperature range of 4 to 100 K, at pressures between 0.5 and 6 kbar. We have observed the predicted suppression of the low temperature conical ferromagnetic phase and the emergence of a new phase with Q =8/33 c *. The neutron diffraction measurements has enabled us to identify the various phases that develop from the cycloidal phases previously observed at atmospheric pressure.     

低温汽液相平衡实验系统的研制与验证

本文介绍了一套汽液相平衡(VLE)实验系统的设计、搭建和验证工作.该实验系统具有测试温区大(140 K＜T＜300 K),压力范围广(O＜P＜7 MPa),精度高等优点,并对C2H6 C3H8的二元混合物以及C3H8的饱和蒸汽压进行了实验,对比参考文献提供的数据,具有很高的精度.     

高气压快放电XeCl准分子激光器的放电特性

基于快放电(毫微秒)XeCl激光器电极两端电压和电流的精确测量和计算机分析,给出高气压(3—8大气压力)激光放电等离子体的动态阻抗特性。在几毫微秒内,阻抗降到欧姆量级。研究了放电峰值电流和平均阻抗与充电电压和气体压力的关系。结果显示,放电等离子体阻抗与传输线阻抗精确匹配并不是十分重要和实际的。 关键词：     

Solidification of 3He in a porous glass

NMR relaxation measurements of ³He in porous high silica glass within a temperature range of 0.5 to 4.2 K at various applied pressures up to 14.6 MPa are reported. The onset of solidification of ³He in the pores was observed at excess pressures ≈ 2.3?4.7 MPa above the bulk melting pressure between 1.3 and 2.1 K. The specific heat of ³He in the porous glass was measured to supplement the NMR measurements and the result confirmed the solidification.     

Structural phase transition of GeAs under pressure

Abstract

We have measured the effect of pressure on structural properties and the optical response of the layered compound GeAs by means of x-ray powder diffraction and optical reflectivity. The low-pressure semiconducting phase is found to be stable up to 13 GPa. At higher pressures the compound crystallizes in the rocksalt structure. Reflectivity measurements show that the structural change is associated with a semiconductor-to metal-transition. Returning to lower pressures from the region of the cubic phase results in the formation of a metastable tetragonal phase.     

Partial magnetic order in the itinerant-electron magnet MnSi

MnSi is an itinerant ferromagnet with a long-wavelength helical modulation of the spin structure. Macroscopic measurements suggest that the ordering temperatureT _c is reduced with increasing pressure fromT _c = 30 K atp = 0 to zero at the critical pressurep _c = 14.6 kbar. Resistivity measurements show that MnSi enters a non-Fermi liquid state atp _c, which remains to be understood. Neutron scattering techniques have been used to investigate the magnetic structure at and abovep _c, i.e. triple-axis spectrometry and small angle neutron scattering. Surprisingly, sizeable quasi-static moments were found to survive to pressures considerably abovep _c. They are, however, organized in a highly unusual way such that the magnetic Bragg reflections are sharp in the longitudinal direction but are very broad in the transverse direction, implying a partial magnetic order that was never seen before.