Infrared study on the formation of charge density waves in (TMTSF) 2X (X = Reo4− and PF6− at atmospheric pressure

The temperature dependence of the infrared absorption spectra (4000–200 cm^?) of (TMTSF) ₂Reo₄ and (TMTSF) ₂PF₆ are investigated with the aim at using the appearance of vibronic absorptions originated by electron-molecular vibration interactions to monitor the formation of charge density waves (CDW) on the donor stacks. The presence of vibronic bands at ≈1400, 436 and 264 cm^?1 in (TMTSF) ₂Re0₄ below 180 K shows that the anion ordering transition in this salt induces the formation of commensurate CDW. The absence of vibronic bands in the PF₆^? salt at 8K indicates that no CDW formation nor an underlying 2K_f lattice distortion takes place at the metal-insulator (MI) phase transition. These results show that, at atmospheric pressure both salts do not undergo a conventional Peierls type MI transition.     

Hall mobility in quasi one-dimensional conductors with charge density or spin density wave ground state

High mobilities found for free carriers below the phase transition in quasi one-dimensional crystals such as TTF-TCNQ and (TMTSF)₂PF₆ indicate that defect scattering is unimportant. We calculate the Hall mobility due to phonon scattering and find good agreement with the measured value of 10⁴cm²/Vsec for (TMTSF)₂PF₆ at 4K.     

Far infrared measurements of (TMTSF)2ClO4

Polarized far infrared reflectance measurements from 20 to 330 cm^?1 have been made on the organic superconductor (TMTSF)₂ClO₄. The feature in the reflectance at 28 cm^?1 has been studied as a function of temperature and applied magnetic field. Our results suggest that this feature is associated with a coupled electron-phonon mode rather than a pseudogap due to fluctuational superconductivity as has been suggested by others. In addition, a Kramers-Kronig analysis of the reflectance indicates that there is a concentration of oscillator strength above 100–150 cm^?1.     

Temperature dependence of the reflectance spectrum of (TMTSF)2ClO4

The near-infrared and visible reflectance spectrum was measured on (TMTSF)₂ClO₄ single crystal at various temperatures in the range of 30K–290K. The reflectance spectrum polarized parallel to the stacking axis was analyzed by Drude model, giving the results that the band width 4t is about 1 eV and m¹_p/m is about 1.3. The band parameters obtained here were almost the same as those of (TMTSF)₂PF₆ reported by Jacobsen et al.     

The electrical resistivity of the charge transfer complex (TMTSF)2PF6

The temperature dependence of the resistivity of (TMTSF)₂PF₆ over the temperature range 25–300 K is in excellent agreement with “libron” theory, if a Bloch (1-phonon) contribution corresponding to λ = 0.16 (a value determined from the Peierls transition temperature) is invoked. This contributes about 10% to the resistivity at 300 K, and about 20% at 40 K.     

Study of alkyl chain length dependent characteristics of imidazolium based ionic liquids [CnMIM]+[TFSA]− by Brillouin and dielectric loss spectroscopy

This study examined the acoustic phonon mode of ionic liquids consisting of 1-alkyl-3-methyl-imidazolium family (C_nMIM) cations with n values ranging from 2 to 10 and bis(trifluoromethylsulfonyl)amide (TFSA) anion in the temperature range from 300 K to 100 K. [C_nMIM]⁺[TFSA]^? showed depolarized (VH) components of Brillouin peaks at temperatures below the glass transition temperature when n is larger than 4. On the other hand, in the case of ionic liquids with different anions, such as [C₄MIM]⁺[BF₄]^?, [C₄MIM]⁺[PF₆]^? and [C₈MIM]⁺[BF₄]^?, the VH component of Brillouin peaks was not observed in the temperature range investigated. The dielectric loss spectra showed that the temperature dependence of alkyl chain domain relaxation of all ionic liquids followed the Arrhenius law and showed an increase in activation energy at the temperature where the VH component of Brillouin peak appeared. These results suggest that the observed depolarized component of Brillouin peak might originate from uniquely induced polarization in the 2nd domain composed of head groups of cations and anions.     

Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)2ClO4

Using various transport and magnetotransport probes, we study the coexistence of spin-density wave and superconductor states in (TMTSF)₂ClO₄ at various degrees of ClO₄ anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor and spin-density wave regions overlap on the temperature—dimerization gap V phase diagram, where V is varied by anion ordering. The effect of anion ordering on (TMTSF)₂ClO₄ properties is thus analogous to that of pressure on (TMTSF)₂X (X = PF₆ or AsF₆), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.     

Ammonium dynamics in the disorder a-phase of K1- x (NH4) x Y (Y=Cl,Br, I). a neutron scattering study

Temperature and concentration effects on the lattice parameters and the amplitude weighted phonon density of states (AWPDS) in the mixed salts of ammonium-potassium halides were investigated by neutron powder diffraction (NPD) and incoherent inelastic scattering (IINS). In the disorder α-phase (NaCl type), ammonium ions perform a fast stochastic reorientation at phonon frequencies rate, down to ca. 80 K. The IINS spectra at 10 K displayed the four distinct ammonium excitations, two (resonant) modes below and two (localised) above the cut-off frequency of the AWPDS of potassium halides. The high frequency localised modes correspond to the translational and librational vibrations of NH₄ ions. These modes are typical for ordered phases of ammonium halides. Ammonium concentration effects on the localised and resonant modes were studied for the K_{1? x} (NH₄) _x I mixed salts and the harmonic excitations of ammonium in the hypothetical low temperature α-phase of NH₄I were approximated to ca. 30, 95, 155 and 250 cm^?1. In the real low temperature ordered γ-phase of NH₄I, translational ammonium vibrations were observed at ca. 140-160 cm^?1 and librational at ca. 300 cm^?1.     

Low frequency phonon spectra of mixed crystal CdxHg1−xTe

Far infrared absorption spectra for the mixed crystal Cd_xHg_1?xTe with x=0.18 to 0.45 in the wavenumber region of 10 to 400cm^?1 and temperature region of 4.5 to 300K are reported. A low frequency absorption band has been found between 20 and 50 cm^?1 for all the samples which have been measured in addition to the two-phonon bands in both sides of the reststrahlen absorption bands of the material. By comparison with the phonon densities of states estimated from the phonon frequencies of CdTe, we have attributed this band to TA band modes induced by the disorder and “impurity” effects.     

The properties of five highly conducting salts: (TMTSF)2X, X = PF6-, AsF6-, SbF6-, BF4- and NO3-, derived from tetramethyltetraselenafulvalene (TMTSF)

The properties of five new highly conducting salts of TMTSF, (TMTSF)₂X, X = PF₆^-, AsF₆^-, SbF₆^-, BF₄^- and NO₃^- are reported. The measurements include d.c. and m.w. conductivity, thermopower, optical reflectivity, magnetic susceptibility and ¹⁹F-NMR. Preliminary structural data indicate uniform donor stacks. Metal to insulator transitions in four materials occur below 20 K, where specific conductivities higher than 10⁵ (Ω-cm)^?1 are observed for two salts (X = PF₆^- and NO₃^-).     

Synthetic metals based on tetramethyltetraselenafulvalene(TMTSF): Varied anisotropy in the low temperature (125K) structures of (TMTSF)2X,X=C104-, PF6-, and AsF6-.

The low temperature (125K) X-ray crystal structures of (TMTSF)₂X, X=C10₄^-, PF₆^-, and AsF₆^- reveal decreases in the intermolecular interand intrastack Se-Se contact distances upon cooling (298K to 125K) which are highly anisotropic and different from one salt to another. The changes in the interstack distances, which are normal to the stacking direction, are approximately twice those involving intrastack Se-Se interactions. These observations establish that the anisotropic structural changes which accompany decreased temperature are common to numerous (TMTSF)₂X radical cation conducting salts.     

Optical investigations of the metal-insulator transition in a low-dimensional organic conductor (EDT-TTF)4[Hg3I8]

The polarized reflectance spectra of single crystals of the low-dimensional organic conductor (EDT-TTF)₄[Hg₃I₈] undergoing a metal-insulator phase transition at a temperature T < 35 K have been presented. The spectral region of the study is 700–6000 cm^?1 (0.087–0.74 eV), and the temperature range is 300–9 K. It has been shown that the reflectance spectra are determined by a system of quasi-free electrons of the upper half-occupied molecular π-orbitals, which form a half-filled metallic band in the crystals. A high anisotropy of the spectra and their temperature dependences have been found. For two polarizations, the quantitative analysis of the spectra at 100 and 25 K has been performed in the framework of the phenomenological Drude model, the effective mass and the width of the initial metallic π-electron band have been deter-mined, and it has been found that the conducting system in the crystals has a quasi-one-dimensional character. As temperature decreases, the spectra demonstrate substantial changes indicating the formation of the energy gap (or pseudogap) in the spectrum of electronic states in the range of ～1500–2500 cm^?1. In the low-frequency region (700–1600 cm^?1), a vibrational structure has been observed, and the most intense feature of the structure (ω = 1340 cm^?1) is caused by the interaction of electrons with intramolecular vibrations of the C=C bonds of the EDT-TTF molecule. For temperatures of 15 and 9 K, the analysis of the spectra has been performed in the framework of the theoretical “phase phonon” model taking into account the interaction of electrons with the intramolecular vibrations. It has been concluded that the metal-insulator transition observed in the reflectance spectra of the crystals is similar to the Peierls dielectric transition that occurs in a system of electrons coupled with the intramolecular vibrations of the molecules forming the crystal.     

Electron diffraction and infrared study of the semimetallic layered compound Ti1−xVxSe2

The crystals of the system Ti_1?xV_xSe₂ (C ≤ x ≤ 0.05) undergo a second order structural phase transition. Electron diffraction studies show that the transition temperature decreases with progressive V-doping. The phase transition considerably affects the infrared reflectivity, measured at 300 and 77 K in the spectral range 40 cm^?1 to 4000 cm^?1.The presence of free carriers and the existence of optical infrared active E_u phonon modes $\text{(}\text{E}\text{⊥}\text{c}\text{)}$ confer their characteristic appearance to the spectra. At room temperature one phonon structure is measured at 143 cm^?1. At 77 K a new series of phonon peaks appears up to a V-concentration of 5 % as a direct consequence of superlattice formation.At room temperature the plasma-edge shifts towards higher frequencies as the vanadium concentration increases. This effect is caused by a large increase of $\text{N}\text{m}{}^1$, associated with the mixing of impurities. It is indicative of the small density of states at the Fermi level in semimetallic TiSe₂. Our results suggest a phase transition driven by lattice dynamical effects.     

Lattice scattering mobility at low temperatures in quasi one-dimensional metals

We show that for a quasi 1-d metal the resistivity ? varies with temperature T as 1/sinh (? ω /k_BT), ?ω being the energy of the (external) 2k_F phonon involved in the scattering process. The theory fits the observed ? vs. T of (TMTSF)₂PF₆ and other quasi 1-d metals below 60K, where lattice expansion, internal mode scattering and precursor effects of the Peierls transition can be neglected.     

Infrared dielectric dispersion of ZnGeP2 and CdGeP2

The polarised i.r. reflectivity spectra of single crystals of the non-linear chalcopyrite materials ZnGeP₂ and CdGeP₂ have been measured over the spectral range 40–700 cm^?1. Analysis of the spectra using classical dispersion theory and the Kramers-Krönig technique has yielded, in each material, five modes with frequencies between 80 and 385 cm^?1 with the polarisation perpendicular to the optic axis. Parallel to the optic axis, two modes have been observed with frequencies between 300 and 400 cm^?1. The phonon dispersion curves of the isoelectronic analogue GaP have been used, together with a group theoretical analysis, to assign specific atomic displacements to the observed modes.     

Infrared and Raman spectroscopic studies of the charge localization in one‐dimensional organic metals (DMtTTF)2X (X = ReO4, ClO4) with regular organic stacks

A novel selective synthesis of the unsymmetrically substituted tetrathiafulvalene dimethyltrimethylene‐tetrathiafulvalene (DMtTTF) is described together with its electrocrystallization to the known conducting mixed‐valence ClO₄^– and ReO₄^– salts. Infrared (IR) and Raman spectra of the two isostructural quasi‐one‐dimensional cation radical salts (DMtTTF)₂X (X = ReO₄^–, ClO₄^–) are investigated as a function of temperature (T = 5–300 K). At ambient temperature, these salts show metallic‐like properties and below T_ρ = 100–150 K, they undergo a smeared transition to semiconducting state. To study this charge localization, we measured temperature dependence of polarized IR reflectance spectra (700–16 000 cm^–1) and Raman spectra (150–3500 cm^–1, excitation λ = 632.8 nm) of single crystals. For both compounds, the Raman data and especially the bands related to the C=C stretching vibration of the DMtTTF molecule show that the charge distribution on molecules is uniform down to the lowest temperatures. Similarly, IR data confirm that down to the lowest temperatures, there is neither charge ordering nor important modification of the electronic structure. However, the temperature dependence of Raman spectra of both salts reveals a regime change at about 150 K. Additionally, using Density Functional Theory (DFT) methods, the normal vibrational modes of the neutral DMtTTF⁰ and cationic DMtTTF⁺ species and also their theoretical IR and Raman spectra were calculated. The theoretical data were compared with the experimental IR and Raman spectra of neutral DMtTTF⁰ molecule. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Electron and electron-phonon effects in the quasi-two-dimensional molecular conductor θ-(BETS)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl): Optical studies at 300–15 K

This paper reports on a study of the polarized reflectance and optical conductivity spectra of the quasi-two-dimensional molecular conductor θ-(BETS)₄HgBr₄(C₆H₅Cl) within the 700–6500-cm^?1 region at 300–15 K and within the 9000–40 000 cm^?1 region at 300 K performed along two principal directions in the crystal plane parallel to the conducting layers of the BETS molecules. The IR spectra obtained at 300 K follow a close-to-Drude behavior, with strong broad features (1200–1400 cm^?1) due to electron-vibrational (vibronic) coupling (VC) superposed on the high Drude background. As the temperature is lowered in the range 180–80 K, in the spectra there appears a Lorentz term with ω_t=2900 cm^?1, as well as three additional VC-induced bands in the 800–1180-cm^?1 region, which disappear as the temperature is decreased further. The results obtained indicate the existence of unstable structural distortions along the two principal directions in the crystal, which are accompanied by the formation of a commensurate charge-density wave.     

Presence of Jahn-Teller distortions in a novel six-coordinate Ag(II) complex: temperature dependent EPR,optical and magnetic susceptibility measurements

Single-crystal variable temperature EPR, optical and polycrystalline magnetic susceptibility studies have been made on a novel six-coordinate Ag(II) complex. Temperature dependent EPR studies on pure single crystals of this compound reveal that dynamic Jahn-Teller distortion operates above 230 K, between 230 K and 120K static Jahn-Teller distortion sets in and below 110 K there is evidence of exchange interaction. Crystal g values were obtained by least-squares fitting with the data obtained from the orientation dependent EPR spectra of the undiluted single crystal of this complex at 300 K and 77 K. From an optical study the Jahn-Teller stabilization energy is found to be ～2250cm^?1. Comparison of Abs_max values for other silver(II) compounds enables us to conclude that the formal geometry of this complex is a tetragonally distorted octahedral. Infrared spectra of this complex were also recorded over a wide range of temperature. Magnetic susceptibility measurements over a wide range of temperature on the powder sample of this compound reveal that the complex is antiferromagnetically coupled in the temperature range 5–40 K with 2J = 0.906cm^?1, and above 40K it is ferromagnetically coupled with 2J = +7.4cm^?1. The effective magnetic moment (μ_eff) of this complex has been compared with that of a series of other silver(II) complexes available in the literature. Finally, the spectral and magnetic data of this complex have been compared with those of a corresponding isostructural and isomorphous copper(II) complex.