On the ordering in [(C2H5)4N]2CuCl4 crystal: an X-ray study and theoretical considerations

The crystal structure [(C₂H₅)₄N]₂CuCl₄ has been determined by X-ray diffraction at 243K. The room temperature phase (phase I) belongs to the space group P4₂/nm [1] whereas the low temperature phase (phase II) is orthorhombic and belongs to the space group Pnna. The phase transition at T_c=258K is of improper ferroelastic type and it is associated with the ordering of the CuCl₄ ²⁻ and a partial ordering of the [(C₂H₅)₄N]⁺ ions which are disordered in the high temperature tetragonal phase. At lower temperature, there occurs another instability which could correspond to a complete ordering in the crystal.     

X-ray diffraction investigation of the thermochromic phase transition in an [NH2(C2H5)2]2CuCl4 crystal

The unit cell parameters of an [NH₂(C₂H₅)₂]₂CuCl₄ crystal are determined using x-ray diffraction analysis, and the thermal expansion coefficients along the principal crystallographic directions are calculated in the temperature range 100–330 K. The behavior of the intensities of the diffraction reflections from the (100), (010), and (001) crystallographic planes is studied in the vicinity of the thermochromic phase transition temperature. The occurrence of a first-order phase transition in the [NH₂(C₂H₅)₂]₂CuCl₄ crystal at T ≈ 324 K is confirmed experimentally.     

Infrared and Dielectric Studies of [(C2H5)4N]2SiF6

The infrared spectrum of [(C₂H₅)₄N]₂SiF₆ was recorded and discussed in relation to its crystal structure. This spectrum indicates that cations and anions are distorted and are not hydrogen bonded. Two structural phase transitions were observed in the tetraethylammonium compound [(C₂H₅)₄N]₂SiF₆ by means of dielectric measurements. High-frequency dielectric dispersion phenomena in this compound were also analysed. The evolution of the dielectric constant with the temperature indicates the presence of two phase transitions, at high temperatures, which are of order-disorder character.     

X-ray scattering study of the phase transitions in crystalline fullerene-biphenyl C 60 [(C 6 H 5 ) 2 ]

The phase diagram of the newly synthesized mixed crystal C₆₀-biphenyl is investigated as a function of temperature by single-crystal X-ray scattering. Diffuse scattering investigations evidencing complex disorder and local order effects are presented. Two phase transitions leading to two different doublings of the high temperature unit cell are observed, at 212 K and 147 K. The first transition is attributed to the ordering of twisted biphenyls, which couples to the orientational ordering of the C₆₀ molecules as the temperature decreases. Full ordering of the C₆₀ molecules is achieved below 100 K only, in the low temperature phase. The rich phase diagram of C₆₀-biphenyl is due to the interplay between fullerene and biphenyl ordering phenomena. Received 31 August 2001 and Received in final form 4 December 2001     

Electrical properties and conduction mechanism of [N(C2H5)4][N(CH3)4]CuCl4 compound

The [N(CH₃)₄][N(C₂H₅)₄]CuCl₄ single crystal has been synthetized in order to determinate the temperatures transition and to study the electrical properties and the conduction mechanism. At room temperature, this compound crystallizes in the tetragonal system with P-42₁m space group. The calorimetric study shows three anomalies at 248, 284 and 326 K. Electrical conduction and dielectrical relaxation mechanisms at various frequencies and temperatures were analyzed by impedance spectroscopy and the equivalent circuit based on the Z-View-software was proposed. The variation of f_p relaxation determinate by the modulus study and σ_dc specific to the AC conductivity as a function of temperature and confirm the all transitions for our sample. The values of the activation energy are determined and compared by those, which are found in the similar compound. Frequencies dependence of alternative current (AC) conductivity is interpreted in terms of Jonscher's law and the conduction mechanisms for each phase are determined with the Elliot's theory.     

Thermochromic Phase Transition in [NH2(C2H5)2]2CuCl4 Crystals

The absorption spectra of [NH₂(C₂H₅)₂]₂CuCl₄ crystals in the visible spectral region in the vicinity of the thermochromic phase transition at T ₁ = 311 K are investigated. It is shown that in these crystalline compounds the phenomenon of thermochromism is primarily associated with the change of the plane-quadratic geometry of the coordination environment of Cu²⁺ to the tetrahedral form. The influence of ionizing irradiation on the phase-transition temperature and on the thermochromic properties of this crystal is studied.     

Microwave permittivity and phase transitions in Cs2HgBr4 and [(CH3)4N]2CuCl4

Phase transitions in the compounds Cs₂HgBr₄ and [(CH₃)₄N]₂CuCl₄ have been observed by the study of complex permittivity at frequencies of 37 GHz. These observations are compared with previous results on these compounds obtained with other techniques such as DTA.     

Dielectric properties of [(CH3)2NH2]3CuCl5 crystal in external electric field

Dielectric properties of [(CH₃)₂NH₂]₃CuCl₅ under the influence of external electric field was studied. At constant values of electric field intensity, the characteristic temperatures T _m and T _c are distinguished, where the maximum value of permittivity and the jump-like decrease of permittivity are observed, respectively. The splitting of the transition is connected with the presence of a defect density wave. Observed in the sinusoidal field, the hysteretic behaviour of permittivity is connected with the slow transition to the ferroelectric phase and the coexistence of the phases in the range (T _m–T _c). The experimentally observed behaviour was explained qualitatively using the Holakovský–Dvo?ák theoretical model.     

Field-induced staggered magnetization and magnetic ordering in Cu2(C5H12N2)2Cl4

We present a 2D NMR investigation of the gapped spin-1/2 compound Cu2(C5H10N2D2)2Cl4. Our measurements reveal the presence of a magnetic field-induced transverse staggered magnetization (TSM) which persists well below and above the field-induced 3D long-range magnetically ordered (FIMO) phase. The symmetry of this TSM is different from that of the TSM induced by the order parameter of the FIMO phase. Its origin, field dependence, and symmetry can be explained by an intradimer Dzyaloshinskii-Moriya interaction, as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict that the transition into the FIMO phase is not in the BEC universality class.     

Time relaxation of permittivity in [(CH3)2NH2]3CuCl5 crystal under external electric field

Continuation of our previous studies connected with dielectric properties of tris-dimethylammoniumpentachlorocuprate crystal [(CH₃)₂NH₂]₃CuCl₅ crystal under the influence of electric field are presented in this article. We studied the time dependence of the permittivity under the influence of electric field at constant temperatures above T _c. In experiment, slow but significant decrease of permittivity was observed, which can be described with two main relaxation processes. The observed dielectric behavior can be related to the coexistence of phases and slow transition to ferroelectric one through long-time metastable states connected with the presence of incommensurate phase in the studied crystal. The coexistence of phases and slow transition was confirmed from the appearance of induced polarization under external electric field, measured as depolarization current.     

2,3-二羟基萘钼多酸有机衍生物的合成及1H NMR和IR谱学研究

本文对二种新合成的2，3－二羟基萘二钼和四钼多酸有机衍生物[n-Bu)4N]2[Mo2O5(OC10H6O)2](Ⅰ）和[n-Bu)4N]2[Mo4O10(OC10H6O)2(OCH3)2](Ⅱ)进行了红外光谱与核磁共振波谱研究，发现[Mo2O5]^2 中钼氧多桥键的红外振动频率较[Mo4O10(OCH3)2]^2 中钼氧多桥键的红外振动频率红移，而在配合物Ⅱ中2，3－二羟基中芳环的^1H化学位移较配合物Ⅰ中向低场移动。同时还发现含二钼配位中心[Mo2O5]^2 的[Mo2O5(OC10H6O)2]^2-与含四钼配位中心[Mo4O10(OCH3)2]^2 的[Mo4O10(OC10H6O)2(OCH3)2]^2-生成条件的差异仅仅只在反应体系的pH值的微小变化，说明钼多酸有机衍生物阴离子是对体系酸碱度极为敏感的物质。     

Pressure-induced weak moment in the two-dimensional antiferromagnet (C2H5NH3)2CuCl4

Abstract

The two-dimensional Heisenberg antiferromagnet (C₂H₅NH₃)₂CuCl₄ has the ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is antiferromagnetic. Neutron scattering experiments under high pressures have been performed on this compound. We confirm that the spin structure changes around 1～2 GPa from the collinear alignment along the a-axis to a spin-canting one. The weak moment due to the canting is parallel to the c-axis. The results indicate that the ferromagnetic intralayer and the antiferromagnetic interlayer exchange interactions are maintained up to 1～2 GPa. Why the weak ferromagnetic moment along the c-axis occurs is due to a lowering of crystal symmetry by pressure.     

适用于全光开关的[(C3H7)4N][Au(C3S5)2]三阶非线性光学性质研究

合成了一种金属有机配合物[(C3H7)4N][Au(C3S5)2].配制了浓度为1×10-3 mol/L的[(C3H7)4N][Au(C3S5)2]/乙腈溶液,并用旋涂法制备了掺杂浓度质量比为1%的[(C3H7)4N][Au(C3S5)2]/PMMA复合薄膜.运用Z扫描方法,分别研究了样品溶液和薄膜在波长为1064nm...     

Ultrasonic study of the structural phase transitions in [N(CH3)4]2CuCl4

The temperature dependence of the six principal stiffness componentsc ₁₁ toc ₆₆ was measured on single-crystal samples by the pulse-echo overlap technique between 120 and 310 K. We tried to interpret the observed anomalies in terms of interactions between order parameter, soft phonons, amplitons and phasons. A new phase transition at 127 K is found.Dedicated to Prof. Dr. H.E. Müser on the occasion of his 60th birthday     

Phase transition and structure of (C3N2H5)2SbF5 single crystal

New crystal of the formula (C₃N₂H₅)₂SbF₅ was obtained and characterized with DSC, DTA, TGA, structural and dielectric studies. DSC and dielectric studies revealed a structural phase transition of the first order at 216 K on cooling and 220 K on heating. The entropy of the transition ΔS equal to 11.5 J/mol·K gives evidence that the phase transition is order-disorder type. X-ray studies showed that transition undergoes from orthorhombic phase I with a space group of Pmmn to monoclinic phase II with a space group P2₁/m. The phase transition is proposed to be ferroelastic type. The molecular mechanism of the phase transition is related to ordering of imidazolium cations in phase II that are disordered in phase I.     

适用于全光开关的[(C3H7)4N][Au(C3S5)2]三阶非线性光学性质研究

合成了一种金属有机配合物[(C3H7)4N][Au(C3S5)2].配制了浓度为1×10－3 mol/L的[(C3H7)4N][Au(C3S5)2]/乙腈溶液,并用旋涂法制备了掺杂浓度质量比为1%的[(C3H7)4N][Au(C3S5)2]/PMMA复合薄膜.运用Z扫描方法,分别研究了样品溶液和薄膜在波长为1 064 nm,脉宽为20 ps条件下的三阶非线性光学性质.研究发现薄膜的三阶非线性极化率χ(3)比溶液高出三个数量级.其中,薄膜的非线性折射率n2为－1.76×10－15 m2/W、三阶非线性极化率χ(3)为9.37×10－10 esu.结果表明,该材料在全光开关方面具有潜在的应用价值.