Ultraviolet photoemission spectroscopy of (TaSe4)2I

The vacuum ultraviolet photoemission spectra of quasi-one-dimensional charge density wave ( CDW ) system, (TaSe₄)₂I, were measured for photon energies between 32 and 100 eV at room temperature ( in the normal phase ) and at about 100 K ( in the CDW phase ). The spectrum of Ta 4f core-levels has shown no additional splitting due to the two different Ta sites. The spectra of the valence and conduction bands have revealed the resonant enhancement for the excitation of the Ta 5p core states, which demonstrates the remarkable hybridization of Ta 5d orbitals with Se 4p orbitals with binding energies smaller than 4 eV. In the CDW phase, the partial cross section decreases for both Ta 5d bands and Se 4p bands with Ta 5d components.     

Charge density wave transport in (TaSe4)2I

We report non linear transport properties below the metal-insulat transition temperature T_° = 263 K in the halogened metal transition tetrachalcogenide (TaSe₄)₂I. These non linear properties are similar to those of NbSe₃ and TaS₃ and indicate that (TaSe₄)₂I is a new compound exhibiting charge density wave transport.     

Defect concentration dependent phase transition in the organic quasi-one-dimensional conductor N-Propyl-Quinolinium (TCNQ)2

Anomalous changes in magnetic and electrical properties indicative of a phase transition at T_c ～ 220 K is found in the quasi one dimensional complex charge transfer salt N-propyl-quinolinium (TCNQ)₂. A series of samples with defects induced by neutron irradiation is investigated. The physical properties above 220 K are little affected by the radiation damage, however the temperature of the anomaly is shifted to lower temperatures. A dose which is estimated to damage of the order of 1 % of the molecules lowers T_c from 220 K to 150 K.     

X-ray diffraction study of the CDW phase in (TaSe4)2I: Determination of the CDW modulation amplitude

An extensive X-ray diffraction study of charge density wave (CDW) phase in (TaSe₄)₂I is reported. We have observed the superstructure satellites at 2q in addition to those at q reported by Fujishita et al. The results imply a sinusoidal lattice modulation with polarization almost perpendicular to q (i.e. transverse) and the existence of CDW domains. At 15 K we have extracted an approximate value for the amplitude of the lattice modulation perpendicular to q to be μ_⊥ ∽ 0.087 Å.     

X-ray diffraction study of a structural phase transition in (NbSe4)3I

The second-order structural phase transition was observed by X-ray diffraction technique in (NbSe₄)₃I. The transition temperature (T_c) was 274.2 K. No evidence of the formation of the charge-density wave was found out. Temperature dependence of the intensity of (0,5,12) reflection obeys the Landau theory of the second kind of phase transition near T_c. Temperature dependence of the integral breadth of (0,0,12) reflection indicates the effect of the soft-phonon mode below T_c. The existence of the structural phase transition, probably not associated with charge-density-wave formation, suggests the unique nature of (NbSe₄)Z₃I in the new series of compounds (MSe₄)_xI (M=Nb, Ta).     

Size dependence of current oscillations in the charge density wave compound (TaSe4)2I

The volume dependence of the current oscillations has been examined in the current carrying charge density wave (CDW) state of (TaSe₄)₂I. The spectral width of the oscillations broadens as the specimen cross section is increased, indicating a nonuniform distribution of CDW velocity in the spicemens. The observed behavior is contrasted with that found in the more isotropic compound NbSe₃.     

Charge density wave transport in a novel inorganic chain compound, (TaSe4)2I

We report the observation of nonlinear conductivity with a well-defined threshold electric field E_T, and frequency dependent ac conductivity, in a novel linear chain compound, (TaSe₄)₂I. The material undergoes a phase transition to a semiconducting phase at T ～ 260 K, and nonlinear and frequency dependent transport is observed below this temperature. We argue that the material is a new example of collective mode transport provided by a Peierls-Fröhlich charge density wave condensate.     

Modification of the charge ordering transition in the quasi-one-dimensional conductor (TMTTF)2SbF6 under pressure

We have measured the temperature dependences of the conductance G and the dielectric permittivity ε′ of the (TMTTF)₂SbF₆ compound under a moderate pressure. The maximum of G(T) associated with the Mott-Hubbard localization disappears under pressure. With increasing pressure the peak in ε′(T), corresponding to the charge ordering (CO) phase transition, shifts to lower temperatures and broadens. At pressures above 0.24 GPa, ε′(T) becomes strongly frequency dependent. These modifications are explained in the frame of the extended Hubbard model and a slowing down behavior.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

正磷酸盐晶体Ba3(PO4)2和Sr3(PO4)2高温拉曼光谱研究

测量了碱土金属正磷酸盐Ba₃(PO₄)₂和Sr₃(PO₄)₂常温及高温拉曼光谱, 对拉曼振动模式进行指认, 并分析了晶体拉曼振动光谱及晶体结构在高温下的变化. 在温度升高的过程中, 拉曼振动频率向低频移动且振动峰宽度展宽, 晶体中的P-O平均键长随温度升高而变长, 但O-P-O的键角并未发生变化. 晶体在900 ℃以下无结构相变发生. 关键词： 3(PO₄)₂和Sr₃(PO₄)₂')" href="#">Ba₃(PO₄)₂和Sr₃(PO₄)₂ 高温拉曼光谱 振动模式 高温结构     

Single crystal magnetic susceptibility of the quasi-one-dimensional organic conductor (fluoranthene) 2PF6

Magnetic susceptibility was measured on a bundle of single crystals of difluoranthene phosphor hexafluoride [(FA)₂^·+PF₆^?] with external field parallel or perpendicular to the molecular stacking direction. Except for lowest temperatures, where the paramagnetic contribution of sample defects is of importance, magnetic susceptibility is dominated by the strongly anisotropic molecular diamagnetism. Spin paramagnetism is separated in the metallic and semiconducting phase and compared with magnetic resonance results and predictions of available models.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

Charge density transport in a novel halogened transition metal tetrachalcogenide (NbSe4)3.33I

We report the observation of non linear electrical conductivity above a sharp threshold field and of a periodic signal in the a.c. voltage for a novel linear chain compound (NbSe₄)_3.33I below a phase transition which occurs at T_c = 285 K. We conclude that this halogened metal transition tetrachalcogenide is a new compound which exhibits Peierls-Fröhlich charge density wave transport.     

Study of structural phase transitions in (NH4)2SnBr6 by Raman scattering

The two successive phase transitions in (NH₄)₂SnBr₆ were examined by low- frequency Raman scattering experiments. Two low frequency modes which were assigned to the rotary modes of SnBr^2?₆ ions showed soft-mode like behavior in the lowest temperature phase. It was proposed that the higher phase transition is structural in nature associated with the soft rotary mode of the anions whereas the lower one is of an order-disorder type with respect to the relative orientations of the ammonium ions.     

High-pressure Raman spectroscopic studies on orthophosphates Ba3(PO4)2 and Sr3(PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of orthophosphates Ba₃(PO₄)₂ and Sr₃(PO₄)₂ were carried out up to 30.7 and 30.1 GPa, respectively. No pressure-induced phase transition was found in the studies. A methanol:ethanol:water (16:3:1) mixture was used as pressure medium in DAC, which is expected to exhibit nearly hydrostatic behavior up to about 14.4 GPa at room temperature. The behaviors of the phosphate modes in Ba₃(PO₄)₂ and Sr₃(PO₄)₂ below 14.4 GPa were quantitatively analyzed. The Raman shift of all modes increased linearly and continuously with pressure in Ba₃(PO₄)₂ and Sr₃(PO₄)₂. The pressure coefficients of the phosphate modes in Ba₃(PO₄)₂ range from 2.8179 to 3.4186 cm⁻¹ GPa⁻¹ for ν₃, 2.9609 cm⁻¹ GPa⁻¹ for ν₁, from 0.9855 to 1.8085 cm⁻¹ GPa⁻¹ for ν₄, and 1.4330 cm⁻¹ GPa⁻¹ for ν₂, and the pressure coefficients of the phosphate modes in Sr₃(PO₄)₂ range from 3.4247 to 4.3765 cm⁻¹ GPa⁻¹ for ν₃, 3.7808 cm⁻¹ GPa⁻¹ for ν₁, from 1.1005 to 1.9244 cm⁻¹ GPa⁻¹ for ν₄, and 1.5647 cm⁻¹ GPa⁻¹ for ν₂.     

Size effects on the charge-density-wave pinning in the quasi-one-dimensional conductor K0.30MoO3

We report size effects on the charge-density-wave pinning in the quasi-one-dimensional conductor K_0.30MoO₃ in the temperature range 77 K-160 K. The threshold field is approximately one order of magnitude larger in needle-like samples than in bulk crystals, temperature independent and strongly dependent on the width of the sample. In addition, the nonlinear conductivity shows a maximum near 130 K. The results are discussed in relation with various pinning models and CDW dislocations.     

EPR study of a new low-temperature phase transition in Zn(ClO4)2·6H2O

From a temperature variation EPR study of Mn²⁺ doped single crystals of Zn(ClO₄)₂·6H₂O phase transition has been detected at T₂~290 K. The phase relationships in this crystal are as follows. Phase I transforms atT₂~346K to Phase II, which in turn transforms to Phase III at T₂ ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn2₁ both above and below T₂. The water-perchlorate sublattice symmetry below T₂ is found to be lower than the P6₃mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T₂, is reflected through the observed temperature dependence of the rhombic distortion parameter E. It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.     

Calorimetric and single crystal X-ray study of the phase transition of (PyH)2PdCl4

Polymorphic transition of pyridinium tetrachloropalladate(II) was investigated by heat capacity measurements and by single crystal X-ray structural analysis. A large λ-type anomaly was detected at 240 K in the temperature dependence of the heat capacity. The low-temperature phase (LTP) belongs to the triclinic space group with a=6.856(1), b=7.293(1), c=7.721(1) Å, α=75.180(2)°, β=71.081(2)°, γ=81.109(3)° at 100 K, and the high-temperature phase (HTP) to the same space group with a=7.217(2), b=7.470(2), c=7.880(2) Å, α=73.438(3)°, β=65.195(3)°, γ=82.727(4)° at 293 K. The pyridinium cations are ordered antiferroelectrically in LTP. In HTP, however, an orientational disorder of the cation was observed. The energy difference between potential wells for the reorientation of pyridinium ion in HTP is discussed referring to the results of the present single crystal X-ray and heat capacity as well as the previous ¹H NMR measurements. A five-site disorder model is shown to be consistent with both of the observations of ¹H NMR and X-ray study.     

Electron paramagnetic resonance of Mn IN (NH4)2Co2(SO4)3 and (NH4)2Ni2(SO4)3 single crystals

EPR studies are reported on single crystals of ammonium cobalt sulphate and ammonium nickel sulphate containing Mn²⁺ ions. In each case only one magnetic complex of Mn²⁺ ion is found. The resonance lines in the case of Mn²⁺ doped ammonium nickle sulphate are characterised by a strong angular dependence of line intensities. The resonance lines in both the cases are fitted to a spin-Hamiltonian corresponding to orthorhombic symmetry.     

Observation of a soft phonon in linear chain compound (NbSe4)3I by Raman scattering

The phase transition of the linear chain compound (NbSe₄)₃I was studied by Raman scattering. At 78 K three new peaks were observed at 73 cm^?1, 205 cm^?1 and 261 cm^?1. The totally symmetric Raman peak at 73 cm^?1 shows anomalous temperature dependence. The frequency decreases with increasing temperature, and at high temperatures an anticrossing occurs with another peak observed at about 58 cm^?1. The Raman intensity decreases and the linewidth broadens remarkably as the temperature increases. These properties allow us to assign this peak to a soft phonon. This fact indicates clearly the existence of a structural phase transition of a displacive type below room temperature.