熔融Rb2ZnCl4的分子动力学模拟研究

采用在RbCl模拟和ZnCl₂模拟中选用的两体有效势，进行了晶态和熔融态Rb₂ZnCl₄的分子动力学模拟.模拟给出了Rb₂ZnCl₄ 6种径向分布函数，Zn-Cl和Rb-Cl间径向分布函数与新近的广延X射线吸收精细结构实验结果很好相符.在模拟产生的瞬态构型基础上进行了键序参数分析，研究了晶态和熔融态中的局部结构.模拟结果表明，Zn原子与其近邻Cl原子构成相当稳定的正四面体结构，并且熔融Rb_关键词：     

Evidence for a new phase transition in Rb2ZnCl4 by Raman scattering

The Raman spectra of an oriented single crystal of Rb₂ZnCl₄ give evidence for the existence of a phse mode at low temperature, as predicted by Wada et al. A new soft mode whose frequency decreases as the temperature T₃?72 K is approached from below, was observed in the a(cc)b scattering orientation, strongly supporting the onset of a new phase transition.     

杂质对K2ZnCl4晶体无公度结构相变的影响

对K₂ZnCl₄铁电晶体在无公度-公度转变的相变点附近的孤子行为进行了研究。发现在相变点附近杂质使介电函数明显偏离居里-外斯定律,并导致介电峰值的明显下降;同时还发现杂质对升温相变点T_c^h较之对降温相变点T_c^c有更大的影响,这与Rb₂ZnCl₄等晶体中的情况恰好相反。采用朗道自由能理论可较好地描述杂质在相变点附近对介电函数的影响。分 关键词：     

Studies of the electrical properties of nearly perfect K2ZnCl4 single crystals as measured during commensurate incommensurate-paraelectric phase transitions

Electrical conductivity and dielectric measurements were carried in the temperature range covering the commensurate ferroelectric-incommensurate-paraelectric normal phases (300-600 K) for the three main crystallographic axes of K₂ZnCl₄ single crystals. The values of activation energies in the three phases were calculated and discussed. A thermal hysteresis of about 12 K is observed which deduce the presence of first order transition for the lock-in ferroelectric transition at T_c=404 K. Conductivity anomalies were observed in both ferroelectric and paraelectric phases. The conduction mechanism was discussed. The suggested occurrence of discommensuration in K₂ZnCl₄ crystals upon the lock-in transition in contrast with conductivity and dielectric results explains the anomalous behavior for the b-axis measurements. The orientation of these discommensuration was discussed on a view of projection in the three standard crystallographic directions.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

Harmonic analysis of the repolarization processes in a Rb<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystal near the temperature of domain structure freezing (<Emphasis Type="Italic">T</Emphasis>* ≈ 150 K)

Contributions from the repolarization of rubidium tetrachlorozincate Rb₂ZnCl₄ samples near the domain structure freezing temperature (T* ≈ 150K) and the resulting conductivity to amplitudes of the harmonic components of the Sawyer-Tower circuit output signal were separated. The nonlinear dependence of sample polarization (ignoring dielectric loss) and the dependence of instantaneous conductivity on the applied harmonic voltage were determined. It was shown that the shape of the potential profile of domain wall motion changes appreciably over a relatively narrow temperature range near T*.     

Thermal unpinning of the modulation wave near TI in incommensurate Rb2ZnCl4 detected by 35Cl NQR

Thermal unpinning and floating of the incommensurate modulation wave produces in Rb₂ZnCl₄ close to the incommensurate-paraelectric transition T_I a partial motional averaging of the splitting between the two edge singularities in the ³⁵Cl nuclear quadrupole resonance spectrum. The mean square phase fluctuations are close to T_I proportional to (T_I ? T)^?2β with β = 0.39 ± 0.04.     

Dielectric dispersion in the incommensurate phases of [N(CH3)4]2CoCl4 and [N(CH3)4]2ZnCl4

The complex dielectric permittivity ?(ω) of [N(CH₃)₄]₂CoCl₄ and [N(CH₃)₄]₂ZnCl₄ along the a-axis was measured between 0.35 MHz and 100 MHz. It has been found that for both substances the relaxation frequencies are about 5 MHz at T_c. The dielectric relaxation of both substances could be described by a polydispersive process β = 0.74 in the vicinity of T_c. However, for the temperature region of (T?T_c) > 0.6 for [N(CH₃)₄]₂CoCl₄ the dielectric absorption seems to be rather monodispersive.     

A comparative study of the modulated structures of some A2BX4 compounds

The structures of the modulated phases of K₂SeO₄, Rb₂ZnCl₄, Rb₂ZnCl₄ and Cs₂CdBr₄ are compared. Both commensurate and incommensurate modulations are considered. Clear common features can be derived between the polarization vectors of their primary harmonic modulations. Even distortions with very different modulation wavevectors present a clear correlation.     

Classical behaviour of the soliton density in Rb2ZnCl4 studied by 87Rb NMR

The temperature dependence of the soliton density n_s(T) which has been studied in Rb₂ZnCl₄ by ⁸⁷Rb NMR could be fitted to a classical formula using only one fit parameter. From our data we cannot confirm recent experimental results which yield a critical exponent $\text{1}\text{2}$ for the soliton density. The results are discussed with respect to the free energy where the soliton density acts as the order parameter. Moreover, couplings to other degrees of freedom, e.g. the elastic strains, are considered.     

Infrared dispersion of potassium zinc chloride

The infrared reflection spectra for the three main polarizations of K₂ZnCl₄ in the orthorhombic commensurate ferroelectric phase are reported at room temperature. Phonon frequencies are assigned to internal and external modes by comparison with K₂SeO₄ and Rb₂ZnCl₄ data. The dielectric constant along the ferroelectric axis is essentially due to the weight of the low-frequency heavily-damped mode.     

Crystal growth and related dielectric prperties of two ferroelectric–incommensurate systems: Rb2ZnCl4 and [N(CH3)4]2ZnCl4

We report the results of the dielectric measurements performed on Rb₂ZnCl₄ and [N(CH₃)₄]₂ZnCl₄ crystals grown by various methods. The dielectric constant along the ferroelectric axis shows a large thermal hysteresis: it is connected with the presence of various defects which appear during and after the crystal growth.     

Anomalous behavior of ultrasonic properties near 50 K in Rb0.30MoO3 and Rb0.30(Mo1−xVx)O3

Depending on the temperature, the charge density wave (CDW) nonlinear conductivity of the blue bronzes A_0.30MoO₃ (A=K, Rb) shows two different regimes: a strongly damped motion above ∼50 K and motion with almost no damping below ∼50 K. In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurements on Rb_0.30MoO₃ and Rb_0.30(Mo_1−xV_x)O₃ single crystals between 4 K and 300 K. In Rb_0.30MoO₃, at T∼50 K, upon cooling, a large increase of the sound velocity is observed. The ultrasonic attenuation coefficient shows an increase down to 50 K followed by a plateau. In Rb_0.30(Mo_1−xV_x)O₃ (x=0.4 at%) the anomaly broadens and is shifted towards higher temperatures. The results are discussed in terms of CDW glass.     

Structure of the low-temperature phase of Rb3D(SeO4)2 by single-crystal neutron diffraction

Crystal structure of Rb₃D(SeO₄)₂ has been investigated at 25 K (below the transition temperature T_c=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO₄ groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO₄ groups was identified to be in the state closer to a HSeO₄⁻ ion or to a SeO₄²⁻ ion and the dipole arrangement of SeO₄-D-SeO₄ dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.     

Structural information from EPR powder spectra of Mn(II)-doped Rb2MgCl4, Rb2MgCl3Br and Rb2MgCl2Br2

The structure of the compounds Rb₂MgCl₄, Rb₂MgCl₃Br and Rb₂MgCl₂Br₂ has been determined to be the tetragonal K₂NiF₄ structure. EPR spectra at X and Q-band frequencies of the polycrystalline samples of the Mn(II)-doped compounds, indicate that the Br^? ions prefer to order at special sites of the structure, as concluded from the fact that only octahedral groups [MgCl₆] and tetragonal groups [MgCl₄Br₂] are found.     

Antiferromagnetic resonance in Rb2Mn(1−x)CrxCl4: A randomly mixed insulating antiferromagnet and ferromagnet

Electron spin resonance experiments at submillimeter and millimeter wavelengths have been performed on the random mixture, Rb₂Mn_(1-x)Cr_xCl₄ of an insulating antiferromagnet Rb₂MnCl₄ and an insulating ferromagnet Rb₂CrCl₄. From the antiferromagnet resonance in the Mn-rich concentrations, the antiferromagnetic exchange field is shown to become small with Cr concentration. Thus, a competition between the antiferromagnetic and ferromagnetic exchange interactions really exists in this mixed system.A competition of the uniaxial anisotropy of Rb₂MnCl₄ with the planar one of Rb₂CrCl₄ has also been found.     

BiFeO3 ceramic matrix with Bi2O3 or PbO added: Mössbauer, Raman and dielectric spectroscopy studies

In this paper Mössbauer, Raman and dielectric spectroscopy studies of BiFeO₃ (BFO) ceramic matrix with 3 or 10 wt% of Bi₂O₃ or PbO added, obtained through a new procedure based on the solid-state method, are presented. Mössbauer spectroscopy shows the presence of a single magnetically ordered phase with a hyperfine magnetic field of 50 T. Raman spectra of BFO over the frequency range of 100-900 cm⁻¹ have been investigated, at room temperature, under the excitation of 632.8 nm wavelength in order to evaluate the effect of additives on the structure of the ceramic matrix. Detailed studies of the dielectric properties of BiFeO₃ ceramic matrix like capacitance (C), dielectric permittivity (ε) and dielectric loss (tan δ), were investigated in a wide frequency range (1 Hz-1 MHz), and in a temperature range (303-373 K). The complex impedance spectroscopy (CIS) technique, showed that these properties are strongly dependent on frequency, temperature and on the added level of impurity. The temperature coefficient of capacitance (TCC) of the samples was also evaluated. The study of the imaginary impedance (−Z″) and imaginary electric modulus (M″) as functions of frequency and temperature leads to the measurement of the activation energy (E_ac), which is directly linked to the relaxation process associated with the interfacial polarization effect in these samples.     

The thermoelectric power of charge-density-wave conductors Rb0.3MoO3 and Rb0.15K0.15MoO3

The thermoelectric power (TEP) of the quasi-one-dimensional charge-density-wave (CDW) conductors rubidium blue bronze Rb_0.3MoO₃ and its alloy Rb_0.15K_0.15MoO₃ were measured in the temperature range 80-280 K. The result showed a sign change from a small positive value to a great negative value where the Peierls transition temperatures (T_p) are 183 and 180 K for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃, respectively. Above T_p, the TEP for both samples can be described with the empirical relation S=AT+B; while below T_p, the TEP fits well the relation S=AT+B/T based on the experimental data. The Fermi energies ε_F for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃ are estimated to be 1.55 and 0.53 eV, respectively.     

Observation of electromagnetic resonances in crystals of K2Pt (CN)4 Br0.3.3H2O at microwave frequencies

Electromagnetic dielectric resonances in the range 8–11 GHz have been observed in millimeter sized crystals of K₂Pt(CN)₄Br_0.3.3H₂O. This observation results directly from the existence of large dielectric constants; values for the longitudinal and transverse dielectric constants at 4°K are estimated to be ?6 >～ 3000 and ?⊥ ?4.     

N2- and O2-broadening parameters in the ν3 band of 14N216O

Broadening parameters of lines belonging to the ν₃ band of N₂O in collision with N₂ or O₂ molecules have been measured with a vacuum high-resolution grating spectrometer. The measurements were carried out in the R branch, up to J = 73 at 297 K and up to J = 50 at 195 K. The experimental results are compared with values calculated from two semiclassical theories.