Elastic Anomalies at Charge Density Wave Transitions in TaS3 and NbSe3

Abstract

The temperature dependence of the Young's modulus and internal friction of TaS₃ (orthorhombic) and NbSe₃ have been measured by the vibrating reed technique. There is a large minimum 2%, in the modulus of TaS₃ at the commensurate Peierls distortion (T_c = 222K) and the internal friction increases below T_c(ΔQ^?1 = 5×10^?5), although no critical relaxation effects are observed. On the other hand, a very small minimum, ΔE/E ~ 0.06%, is observed at the upper incommensurate transition in NbSe₃, and a small broad increase (0.03%) at the lower transition. The measurements allow estimates to be made of the stress dependence of the transition temperature and of the electron-phonon coupling constant.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Structural and Magnetic Characterisation of Ce@C82

Abstract

We report the structural and magnetic properties of the endohedral metallofullerene Ce@C₈₂. A hexagonal close packing phase [P6₃/mmc [a=11.1544Å, c=18.2256Å] is formed exclusively after vacuum annealing of the solvent precipitated compound. In contrast, sublimed Ce@C₈₂ was found to be dominantly face-centred cubic close packed [Fm-3m; a=15.766Å]. X-ray powder profile calculations revealed that the endohedral cerium atom lies close to 1.8Å from the C₈₂ cage centre in both phases. Hexagonal Ce@C₈₂ has been investigated by magnetic susceptibility measurements. Paramagnetic behaviour is maintained down to 2K attributable to Ce³⁺ ions. Towards lower temperatures, the observed paramagnetic moment falls from the free ion Ce³⁺(μ_eff =2.54μB) value, monotonically approaching 1μB at 2K.     

Magnetic Susceptibility in 2D Superconductor NaxHfNCl System

Abstract

Superconducting critical temperature, T_c and the shielding volume fraction, SVF, of layered nitride superconductor Na_xHfNCl have been studied as a function of x, i.e. Na concentration. Although T_c decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K² by its difference of magnetic susceptibility between HfNCl and Na_0.5HfNCl. The γ value is relatively small compared with the high T_c value, revealing double honeycomb lattice system as new potential higher T_c superconductor series by intercalation.     

NMR Studies of (TMTSF)2PF6

Abstract

Pulsed NMR determinations of the relaxation times T₂, T₂* and T₁, as a function of temperature and field, for the methyl group protons in (TMTSF)₂PF₆ are reported. Below the metal-insulator transition (T_MI)T₂* shortens while T₂ increases, indicating a line which is inhomogeneously broadened due to the onset of a SDW. The SDW also contributes to the spin-lattice relaxation rate, T₁ ^?1, in the neighborhood of T_MI. A frequency dependent maximum in T₁ ^?1 is observed near 20K and is attributed to methyl group rotation. A frequency independent maximum at 58K suggests a structural phase transition involving rearrangement of the methyl and PF₆ groups.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Spectres de vibration du Cristal de Biphényle à Basse Température

Abstract

Low-temperature infrared and Raman spectra of Crystalline biphenyl have been investigated in the 3100–25 cm^?1 range, and those of biphenyl-D₁₀ between 200 and 25 cm^?1. The infrared dichroism of an Oriented crystal at 77°K has been measured in the 3100–400 cm^?1 region. The assignment for the internal modes V ₅(B_2u), v ₆ (B_2u), v ₁(B_1u), v ₁₀(B_1g), v ₂(B_2g) et v ₃ (B_3g) is given.

The band splitting is analized and hte components due ot the correlation effect in the fundamentals are separated from the components due to combinations. Isotopic shifts are used to assign the nine external vibrations as well as the torsional mode. The temperature effect on the frequencies occuring below 500 cm^?1 is discussed.     

Magnetic and Electric Birefringence in the Isotropic Phase of a Nematic of Large Positive Dielectric Anisotropy

Abstract

Magnetic (Δn_H ) and electric birefringence (Δn_E ) in the isotropic phase of strongly positive (Δ 8)trans-p-n-octyloxy α-methyl-p'-cyanophenyl cinnamate (8 OMCPC) have been measured. It is established that they both exhibit a (T - T*)^?1 dependence, T _NI - T* being 1.4 K. Also, the induced birefringence is found to be proportional to the square of the applied field, magnetic or electric.     

Solvothermal Syntheses and Crystal Structures of Rare Earth Thioantimonates [Y(en)<Subscript>4</Subscript>]SbS<Subscript>4</Subscript>·0.5en and [Tm(en)<Subscript>4</Subscript>]SbS<Subscript>4</Subscript>·0.5en

Abstract  

Two rare earth metal thioantimonates [RE(en)₄]SbS₄·0.5en (RE = Y(1), Tm(2); en = ethylenediamine) were synthesized under mild solvothermal conditions. Compounds 1 and 2 are isostructural, and both crystallize in monoclinic space group P2₁/n. Crystallographic data for 1: a = 11.0894(19), b = 12.905(2), c = 16.000(3) ?, β = 91.792(4), V = 2288.7(7) ?³, Z = 4. For 2: a = 11.0870(14), b = 12.8977(16), c = 15.986(2) ?, β = 91.879(3), V = 2284.7(5) ?³, Z = 4. The four-en coordinated rare earth complex cation [RE(en)₄]³⁺ formed in situ balances the charge of the [SbS₄]³⁻ anion in the crystal structure. The RE³⁺ ion is in an eight-coordinated environment involving eight N atoms of four en ligands forming a bicapped trigonal prism. Hydrogen bonds link [RE(en)₄]³⁺, [SbS₄]³⁻ and en species into a three-dimensional structure. The structure determination of 1 and 2 implies that the ionic radii of rare earth metal ions play an important role on the structures of the rare earth metal thioantimonates.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

A new equation for the temperature dependence of density of saturated aqueous solutions of electrolytes

A new empirical three-parameter equation d = d₀ exp[β′(T – T₀)²] (where β′, d₀, and T₀ are constants for a salt) has been proposed to describe the dependence of density d (g/ml³) of saturated aqueous solutions of electrolytes on temperature T (K) in the whole range of temperature in which solubility increases or decreases with an increase in temperature.     

Synthesis,X-ray Crystal Structure and Magnetic Study of a μ<Subscript>1,5</Subscript>-dca Bridged Dimeric Copper(II) Complex

Abstract  

The reaction of aqueous solution of copper(II) nitrate trihydrate with methanolic solution of the ligand HL and sodium dicyanamide in aqueous medium results in the formation of a dimeric dicyanamide complex of Cu(II), [Cu₂(L)₂(μ_1,5-dca)₂]·0.5 H₂O (1) [where L = 1-(N-salicylideneamine)-2-(N-ethyl)-aminoethane]. The single crystal X-ray structure reveals that the asymmetric unit of complex 1 consists of two dinuclear units. The complex crystallizes in the monoclinic space group P2₁ with cell parameters, a = 9.8828(19) ?, b = 19.018(4) ?, c = 14.851(3) ?, (°) = 92.979(6) and Z = 4. χ_M T, stays in the 0.94–0.91 cm³ mol⁻¹ K range between 300 and 2 K, which is slightly higher than the spin-only value (χ_M T = 0.75 cm³ mol⁻¹ K) for two uncoupled copper(II) (S = ?) ions assuming g = 2.0, thus indicating that the complex 1 behaves like a simple paramagnet.     

Proton Spin-Lattice Relaxation Dispersion Study of the Ferroelectric Smectic Phases in Dobambc

The frequency dispersion of the proton spin-lattice relaxation rates T^?1 ₁ in the ferroelectric smectic phases of DOBAMBC has been determined in the MHz region and analyzed in terms of the order fluctuations, self-diffusion, and rotational contributions to T^?1 ₁. In the smectic C* phase the main rate determining contribution are order fluctuations with a T^?1 ₁ ∝ ν^?1/2 _L frequency dependence, whereas in the smectic H* phase the translational diffusion, which is slower than in the smectic H* phase, determines the dispersion of T^?1 ₁.     

Unusual Glassy Smectic-G Liquid Crystal: Heat Capacity of N-p-n-Pentyloxybenzylidene-p'-n-butylaniline between 11 and 393 K

Abstract

The heat capacities of the title compound (C₃H₁₁,O—C₆H₄,- CH=N—C₆H₄,—C₄H₉, abbreviation 5O ? 4) with a purity of 99.92 mole percent have been measured with an adiabatic-type calorimeter between 11 and 393 K. The transition temperature and the enthalpy and entropy of phase transition for stable crystal → S_G, S_G → N and N → isotropic liquid were T _c = 299.69 K/ΔH = 22.68 kJ mol^?1/ΔS = 75.70 JK^?1 mol^?1, 325.72/7.11/21.79 and 342.48/1.78/5.22, respectively. The crystal which melts at 285.5 K is a metastable modification. The S_A phase hitherto reported in between S_G and N does not exist. The glassy So state was realized by rapid cooling of the specimen from the So phase. The molar enthalpy of the glassy S_G state at 0 K was by (10.1±0.1) kJ mol^?1 higher than that of the stable crystalline state and the residual entropy of the glassy state was (9.40±0.83) JK^?1 mol^?1. The relaxational heat-capacity anomaly was observed from as low as 100 K and double glass transition phenomenon occurred around 200 K; a quite unusual phenomenon which has never been observed for the glassy states of nematic and cholesteric liquid crystals. The present results give a fair evidence that the unusual glass transition phenomenon previously found for the S_G state of 6O?4 (a homologous compound) is not exceptional at all but common to the smectic glasses; at least common to the glassy S_G states. Two possible origins responsible for the double glass transitions have been discussed.     

Magnetic Resonance on (QP)4(SbF6)3, a Radical Cation Salt of P-Quaterphenylene

Abstract

The unit cell of (QP)₄(SbF₆)₃ shows triads of QP in an arene stack and one QP in the anion sheet. The sum formula is therefore (QP)₃QP(SbF₆)₃. Electron-electron repulsion force le^?/site resulting in a semi-conducting state. This is in agreement with an activated conductivity of max 10^?1 Scm^?1 at 300 K.

Magnitude and activation energy of the paramagnetic susceptibility do not coincide with those of the conductivity; spins and charges are localized and decoupled due to the strong alternation of distances within the cation stack. Spin diffusion along the stacks governs, at least above 180 K, the magnetic properties: narrow ESR-lines and proton spin relaxation via hyperfine interaction with the relaxation dispersion T ^?1 _1H α ω^?1/2 above 200 K.

A lower value for the intrachain exchange rate τ₁ ^?1> 8.10¹¹ sec^?1 and a lower limit of 10³ for the anisotropy can be evaluated.

Keywords: quaterphenylene, molecular crystal, radical cation salt, low dimensionality, magnetic resonance     

Electrical conductivity of pyrolyzed polyacrylonitrile

Abstract

Using ultrapure samples of polyacrylonitrile (PAN) of 485,000 or 150,000 average molecular weight solution cast in dimethylformamide, the dc conductivity ([sgrave]) of pyrolyzed PAN (PANP) films has been studied for pyrolysis temperatures (T_p) of 280-435°C. Conductivity measurements made during pyrolysis indicate the onset of a dramatic increase in [sgrave] for T_p of 390-435°C. Conductivities as high as 5 (ohm-cm)^?1 have been observed for T_p < 435°C. This situation contrasts sharply with previous literature which had indicated that a increased uniformly and monotonically with T_p for 200°C < T_p < 900°C and that values of [sgrave] > 1 (ohm-cm)^?1 were observed only for T_p > 600°C. Our results indicate that the maximum value of [sgrave] obtained is not a strong function of T_p (390°C < T_p < 435°C) or of molecular weight. However, the rate of increase of [sgrave](t) is strongly dependent on T_p in this range. After pyrolysis, repeated heating and cooling below T_p do not alter [sgrave](T). IR spectra show that the sudden increase in [sgrave] is correlated to the formation of conjugated C=C and C=N bonds. The shape of [sgrave](T) suggests that conduction is probably due to hopping.     

Molecular structure and spectra of bis (1,4-dimethylpyridinium)-tetrachlorocuprate(II)

Reaction of 1,4-dimethylpyridinium chlorosulphite with anhydrous copper(II) chloride in an atmosphere of dry nitrogen yields an ionic complex containing discrete [CuCl₄]^2– anions and 1,4-dimethylpyridinium cations. Electronic (solid and in solution), far infrared and¹H NMR spectral data predict a distortedT _d structure which has been confirmed by X-ray crystallography. The crystal structure was solved by the heavy-atom method and refined by full-matrix least squares toR=0.059 for 1685 observed reflections. The [CuCl₄]^2– anions exhibit twofold symmetry and there are no unusual interactions with the cations. Bond lengths and angles place it in the select class of complexes with the least flattenedT _d structure.     

Hydrothermal crystallization and X-ray structure determination of a new decavanadate with mixed cations [Mn(H2O)6]2[N(CH3)4]2[V10O28]·2H2O

A new decavanadate with mixed cations, [Mn(H₂O)₆]₂[N(CH₃)₄]₂[V₁₀O₂₈]·2H₂O (1), was crystallized from a hydrothermal reaction between MnCO₃ and V₂O₅ in the presence of N(CH₃)₄Br at 100°C. The structure of 1, as determined by x-ray single crystal analysis, consists of cations and anions of hexa-aqua manganese [Mn(H₂O)₆]²⁺, tetramethyl ammonium [N(CH₃)₄]⁺ and decavanadate [V₁₀O₂₈]^6–. The extended H-bonding between the [Mn(H₂O)₆]²⁺ and [V₁₀O₂₈]^6– ions gives rise to a pseudo- two-dimensional network in the crystal lattice x-ray crystallographic data for 1: monoclinic P2₁/n, a = 9.1499(5), b = 12.8725(7), c = 18.625(1) Å, = 92.252(1)°, V = 2192.0(2) ų, MZ = 2, and D _calcd = 2.22 g cm^–3.     

Triplet Excitions in Crystalline Trans-Stilbene I. Spectroscopy of the So → T1 Transition

Abstract

The first singlet → triplet absorption of trans-stilbene has been studied between 10 and 300 K. The triplet exciton energy (energy of the lowest 0–0 line) is 17380 cm^?1 at 10 K, and does not change significantly with temperature. Vibrational A_g modes of 206. 1250 and 1570 cm^?1 are active. The Franck-Condon factor of the origin region is small. The low temperature spectra of the 0–0 and 206 cm^?1 regions show a doublet structure with a splitting of 82 ± 1 cm^?1 which is attributed to the site splitting. Further structure shown by the lines is discussed. The lines are approximately lorentzian. From curve fitting, line-widths and exciton-phonon coupling constants, increasing linearly with T, are deduced. Exciton-phonon coupling appears to be different on the two sites. The product of absorption coefficient at 4880 Å, α, by the total triplettriplet interaction rate constant has been measured at 295 K: αγ=2.3 ± 0.3. 10^?15 cm² sec^?1, corresponding for γ_total to a value of a few 10^?12 cm³ sec^?1.     

Preparation and crystal structures of the Ce(IV) β-diketonates, [Ce(tmhd)4] and [Ce(pmhd)4] (tmhd =2,2,6,6-tetramethylheptane-3,5-dionate and pmhd = 1-phenyl-5-methylhexane-1,3-dionate)

The cerium(IV) -diketonate compounds [Ce(-diket)₄] [where -diket = tmhd (2,2,6,6-tetramethylheptane-3,5-dionate) 1, pmhd (1-phenyl-5-methylhexane-1,3-dionate) 2] were prepared by reacting cerium(IV) ammonium nitrate [CAN; Ce(NH₄)₂(NO₃)₆] with the respective Na(-diket) compound in ethanol, and structurally characterized by single crystal X-ray diffraction. Compound 1 crystallizes in the triclinic space group with a = 12.472(4), b = 19.972(5), c = 21.436(3) Å, = 97.05(7), = 90.16(2), = 106.55(3)°, V = 5076(2) ų, Z = 4, T = 150(2) K. Compound 2 crystallizes in the monoclinic space group P2₁/n. with a = 14.817(6), b = 17.123(6), c = 19.146(3) Å, = 105.46(4)°, V = 4682(3) ų, Z = 4, T = 150(2) K. Crystals of 1 contain two independent [Ce(tmhd)₄] molecules, with four chelating tmhd ligands bonded to each metal in a distorted dodecahedral arrangement; the cerium atom in 2 is also bonded to four chelating pmhd ligands but in this case the coordination geometry is closer to square antiprism. Both complexes are air and moisture stable. Sublimation studies reveal that 1 sublimes almost quantitatively, while 2 is comparatively involatile.