Thermal expansion in single crystals of κ-(BEDT-TTF)2Cu(NCS)2 in magnetic fields up to 6 Tesla

The linear thermal expansion coefficients _i (i = a^*,b,c) of single crystals of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ (T_c = 9.15K) have been measured in the temperature range from 2 to 300K, by means of capacitive dilatometry. At T_c, we resolved second order jumps Δ_i, which disappear in magnetic fields above H_c2 From the initial slopes dH_c2/dT parallel and perpendicular to the superconducting layers the Ginzburg-Landau coherence lengths ξ(O) = 67 å and ξ(O) = 7.3 å were determined. Via Ehrenfest's relation it is possible to deduce the uniaxial stress dependence of T_c which was found to be negative in the a^*-direction ( ∂ T_c/ ∂ σ_a* = −3.20 K/kbar), positive in the c-direction ( ∂ T_c/ ∂ σ_c = 1.46 K/kbar) and zero in the b-direction.     

Thermal expansion studies on Th(MoO4)2, Na2Th(MoO4)3 and Na4Th(MoO4)4

Thermal expansion behavior of Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ was studied under vacuum in the temperature range of 298–1123 K by high temperature X-ray diffractometer. Th(MoO₄)₂ was synthesized by reacting ThO₂ with 2 mol of MoO₃, at 1073 K in air and Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were prepared by reacting Th(MoO₄)₂ with 1 and 2 mol of Na₂MoO₄, respectively at 873 K in air. The XRD data of Th(MoO₄)₂ was indexed on orthorhombic system where as XRD data of Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were indexed on tetragonal system. The lattice parameters and cell volume of all the three compounds, fit into polynomial expression with respect to temperature, showed positive thermal expansion (PTE) up to 1123 K. The average value of thermal expansion coefficients for Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were determined from the high temperature data.     

-NMRC-NMR study on κ-(BEDT-TTF)2X

The electronic state of the κ phase family of BEDT-TTF compounds, κ-(BEDT-TTF)₂X [X = Cu(NCS)₂, Cu[N(CN)₂]Br and Cu[N(CN)₂]Cl], has been investigated by -NMR¹³C-NMR as well as H-NMR.¹-NMR. The ¹³C isotope was substituted selectively into the central double bonded carbon sites of BEDT-TTF. The ¹³C nuclear spin-lattice relaxation rate and line spectra have been measured for the Cu(NCS)₂ and Cu[N(CN)₂]Br salts. The Knight shift evaluated from the the width of the spectra of the powdered samples is temperature-insensitive and scaled to the uniform susceptibility. However, the relaxation rate, T₁⁻¹, does not obey the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T₁⁻¹ is in excess of the Korringa relation. These behaviors are considered to come from strong aniferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the H¹ NMR and magnetization measurements gave an unambiguous evidence for antiferromagnetic transition with spin canting at 25 K in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The κ phase family, situated in the vicinity of metal-insulator and magnetic transitions, is characterized by strong electron correlations.     

Magnetic properties of novel compounds R3(Fe,Cr)29Xy (R=Nd, Sm and X=N, C)

The novel ternary rare-earth iron-rich interstitial compounds R₃(Fe,Cr)₂₉X_y (R=Nd, Sm and X=N, C) with the monoclinic Nd₃(Fe,Ti)₂₉ structure have been successfully synthesized. Introduction of the interstitial nitrogen and carbon atoms led to a relative volume expansion ΔV/V of about 6% and an enhancement of Curie temperatures T_c about 268 K for the nitride and about 139 K for the carbide, respectively. The Nd₃Fe_24.5Cr_4.5X_y compounds have a planar anisotropy at room temperature. A first-order magnetization process (FOMP) with critical field B_cr=4.4 T and 3.1 T at room temperature were observed for the Nd-nitride and carbide compounds, respectively. The Sm₃Fe₂₄Cr₅X_y compounds were found to have a large uniaxial anisotropy of about 18 T at 4.2 K and about 11 T at 293 K. A FOMP with B_cr=2.3 T was also observed in the Sm-nitride compounds at 4.2 K. Magnets with coercivity of μ_OjH_c0.8 T at 293 K has been successfully developed from the Sm₃Fe₂₄Cr₅X_y (X---N and C) phases.     

New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS = bis(ethlenedithio)tetraselenafulvalene)

Crystal structutres and electronic properties of BETS compounds with tetrahedral anions, MX₄ (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Ga, Fe, In; X = Cl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)₂GaCl₄ demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)₂FeCl₄ with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl₄ and λ-FeCl₄ salts is very small. The structure analysis of κ-(BETS)₂FeCl₄ at 18 K indicated that the FeCl₄ anion has approximately C_3v symmetry. The ESR measurements revealed the high-spin state of Fe³⁺ in λ-FeCl₄ salt and and low-spin state in κ-FeCl₄ salt. ESR intensities of κ- and λ-FeCl₄ salts suggested antiferromagnetic interaction between Fe³⁺ ions.     

Structural and physical properties of θ -(BEDT-TTF)2Cu2(CN)[N(CN)2]2

The title compound was obtained during the crystal growth of two κ-type superconductors, κ-(BED-TTF)₂Cu(CN)[N(CN)₂] and κ′ -(BEDT-TTF)₂ Cu₂(CN)₃. This compound is a magnetic semiconductor, whose spin susceptibility is expressed by quadratic-layer Heisenberg antiferromagnet model (QLAF) between 32.6K and 220K. At 220K, a semiconductor-semiconductor transition into a high conductivity phase was observed by transport measurements.     

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Low-frequency Raman spectra in κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br

The ordering of the terminal ethylene groups of the BEDT-TTF molecule, i.e., the staggered or eclipsed conformation, in κ-(BEDT-TTF)₂Cu(NCS)₂ and κ-(BEDT-TTF)₂Cu[N(CN)₂]Br was studied between 18 and 295 K by Raman scattering. The low-frequency spectra of these compounds are similar to each other. Broad peaks at about 55 cm⁻¹ extremely broadened and their intensities became weak with decreasing temperature. The anomalous behaviour was interpreted in terms of critical dynamics of the pseudospin-phonon coupled system, where the spin states represent the conformations of the terminal ethylene groups. It was found that the ordered state is formed around the superconducting critical temperature.     

Studies of the microwave resistivity of κ-(BEDT-TTF)2CU[N(CN)2]Cl

We have performed temperature dependent measurements of the surface impedance of κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl in both directions, parallel and perpendicular to the highly conducting planes. In both orientations the microwave resistivity basically follows the dc behavior for T > 15 K: the crystal undergoes a broad metal-insulator transition at around 50 K. While there are no indications of superconductivity at ambient pressure found in the dc resistiviy and by ac susceptibility, the microwave resistivity clearly drops at 13 K and again at 5 K, indicating two supercondcuting phases. When pressure is applied by embedding it in grease, only the well known superconducting phase transition at T_c= 13 K remains.     

Structural and magnetic properties of Nd3(Fe,Ti)29Cx carbide

Nd₃(Fe,Ti)₂₉C_x, carbide has been synthesized by gas-solid reaction. An enhancement of the Curie temperature T_c from 437 K to 575 K is observed, reflecting a lattice expansion of 4.1% upon carbonation. The room temperature saturation magnetization M_s of the carbide is 145.5 A m² kg ¹ and the average hyperfine field, H_eff, 24.8 T. The magnetic structure of Nd₃(Fe,Ti)₂₉C_x carbide changes from easy-cone-like in the case of the parent compound to axial-like after carbonation with a room temperature anisotropy field H_A of 8 T.     

New low thermal expansion ceramics: Sintering and thermal behavior of Ln1/3Zr2(PO4)3-based composites

Materials with the general formula M_xZr₂(PO₄)₃ are known to possess low coefficients of thermal expansion (CTE). The present work investigates the thermal properties of new composite materials issued from the decomposition at high temperature of Ln_1/3Zr₂(PO₄)₃ (Ln=La, Gd). The decomposition process was studied and showed that the resulting powder was a LnPO₄, Zr₂P₂O₉ and ZrO₂ mixture. Composite materials made of that mixture were sintered and characterized. The effect of sintering aids such as ZnO was considered. Final densities of the composites were about 90% of theoretical density and these materials presented low CTE in the 10⁻⁶ °C⁻¹ range.     

Crystal structures and structural relationships of KFe2(SeO2OH)(SeO3)3 and SrCo2(SeO2OH)2(SeO3)2

The new phases KFe₂(SeO₂OH)(SeO₃)₃ and SrCo₂(SeO₂OH)₂(SeO₃)₂ have been synthesized under low-hydrothermal conditions and their structures were determined by single-crystal X-ray methods. Both compounds are monoclinic; KFe₂(SeO₂OH)(SeO₃)₃: space group P2, A = 9.983(4), B = 5.270(1), C = 10.614(4) Å, β = 97.42(2)°, V = 553.7 ų, Z = 2; SrCo₂(SeO₂OH)₂(SeO₃)₂: space group P2ln, A = 14.984(2), B = 5.286(1), C = 13.790(2) Å, β = 94.72(1)°, V = 1088.5 ų , Z = 4. The refinements converged to R-values of 2.9 and 3.6% respectively.

The atomic arrangement in KFe₂(SeO₂OH)(SeO₃)₃ and SrCo₂(SeO₂OH)₂(SeO₃)₂ is based on isolated MO₆ octahedra (M = Fe³⁺, Co²⁺), which are corner-linked via trigonal pyramidal selenite groups to a framework structure. Interstitials are occupied by potassium or strontium atoms in ten- or eight-coordination respectively, and by the lone-pair electrons of the Se⁴⁺ atoms. Both compounds are not isotypic but are closely related and may be interpreted as different distortions of an idealized structure type in space group P2/m, which was modelled for a theoretical compound SrFe₂(SeO₃)₄ by distance least squares refinement (program ).     

Study of the antiferromagnetic phase in Sm(Mn1−xCrx)2Ge2

Magnetic and thermal expansion measurements have been carried out on the polycrystalline Sm(Mn_1−xCr_x)₂Ge₂ samples to see how the antiferromagnetie (AFMII) region in SmMn₂Ge₂ is affected by Cr substitution. It is found that the antiferromagnetic region disappears for samples with less than 2 at.% of Cr. Sharp changes in the thermal expansivity (Δl/l) at FMI–AFMII and AFMII–FMII transitions are observed, indicating first order transitions. The decrease in relative thermal expansivity at the two transitions with the increase of Cr concentration is related to the decrease in the stability and the temperature-range of the AFMII phase observed in magnetization measurements. A spin reorientation transition (T_SR) has been observed for x=0, at 148 K. It is found that the T_SR increases with the increase of Cr concentration. A magnetic phase diagram as a function of Cr concentration in Sm(Mn_1−xCr_x)₂Ge₂ has been constructed.     

Revisiting the superconducting phase diagram of(TMTSF)2ClO4

We present results of a search for evidence of reentrance at high magnetic field of the superconducting state in the quasi-one dimensional organic superconductor (TMTSF)₂ClO₄, for a specific orientation of the field at low temperature. This search was motivated by the theoretical work of Lebed' and of Dupuis and Montambaux, and in part by that of Te anovic and coworkers, who predict novel magnetic field-temperature phase diagrams in quasi-1-D and quasi-2-D superconductors in a strong magnetic field. In the quasi-1-D case, a field directed both normal to the chains and along the layers is proposed to induce exotic oscillations of the critical temperature at high field, while for the 2-D case, quantum effects associated with Landau quantization lead to similar predictions. We have investigated the superconducting transition in a quasi-1-D candidate material, (TMTSF)₂ClO₄, to 0.06 K with field oriented along the b′-axis (± 0.02°). From mostly constant field temperature sweeps, we find positive curvature in H_c2(T) below about T_c/2. In addition, a resistivity decrease is observed to persist to 7 T, the highest field employed, with a slightly positive slope ∂H/∂T above 5T. The possibility of these features being representative of high field superconductivity is discussed.     

Structural chemistry and magnetism of dicyclopentadienidehalides of lanthanides Part 8: terbiumdicyclopentadienidebromide [Tb(C5H5)2Br]2

A single-crystal X-ray structural investigation of [Tb(C₅H₅)₂Br]₂ revealed the [Sc(C₅H₅)₂Cl]₂-type structure, space group P2₁/c, with a = 1407.6(2) pm, b = 1644.7(2) pm, c = 1370.6(9) pm, β = 93.46(3)°, V = 3167(2) × 10⁶ pm³, D_c = 2.322 g cm⁻³ and Z = 6 dimers (R = 0.036 for 4627 reflections with I > 3σ(I)). The metal centres have the pseudosymmetry C_2v. Magnetic susceptibility data show Curie-Weiss behaviour between 213 and 6 K with θ_p = −4.5(3) K and a magnetic moment μ = 9.8(1) μ_B close to the Tb³⁺ free-ion value (9.72 μ_B). Below 6 K, deviations from Curie-Weiss behaviour are observed, and at 5.3 K a maximum in the susceptibility is detected which may be caused by intradimer antiferromagnetic spin coupling. The magnetic properties are compared with the prediction of various models, starting from cubic crystal fields and isotropic intramolecular exchange interactions, followed by extension to lower crystal field symmetry (orthorhombic) and anisotropic contributions to the spin coupling. However, a reasonable agreement between the measured and calculated data was not obtained. As in [Gd(C₅H₅)₂Br]₂, the low-temperature behaviour is governed by effects which cannot be described by spin coupling models in the generally accepted form.     

Magnetocrystalline anisotropy and exchange interactions in the novel R3(Fe, V)29 compounds (R = Y, Nd, Sm)

A type of magnetocrystalline anisotropy and exchange interactions of the novel ternary R₃(Fe, V)₂₉ compounds (R = Y, Nd, Sm) have been investigated. The compounds are uniaxial ferromagnets with easy magnetization direction along the [ 0 1] axis of the monoclinic lattice at room temperature. The temperature variations of the magnetic moment and the first anisotropy constant for Y₃(Fe, V)₂₉ are presented. The first order magnetization process along the hard magnetization direction takes place for Sm₃(Fe, V)₂₉ at T < 120 K. A magnetic anomaly is detected in the temperature dependence of the a.c. susceptibility for Nd₃(Fe, V)₂₉ which can be related to a spin reorientation.     

Temperature dependence (300 to 10K) of the structural and electronic properties of the molecular charge-transfer salt (BEDT-TTF)3 Cl2,2H2O

The crystal and electronic structure of the molecular charge-transfer salt (BEDT-TTF)₃ Cl₂ 2H₂O was determined at 10, 130, 190 and 295K. The thennal expansion has been evaluated from measurements of cell parameters between 10 and 295K.     

AC ionic conductivity investigations on the CsK(SO4)·Te(OH)6 material

Single crystals of CsK(SO₄)·Te(OH)₆ are obtained by slow evaporation at room temperature. Differential scanning calorimetry (DSC) measurements have been carried out in the temperature range 350–650 K. It reveals three anomalies in the studied temperature range. Complex impedance measurements are performed on this material as a function of both temperature and frequency. The electric conduction and dielectric relaxation have been studied. The temperature dependence of DC conductivity indicates that the sample became an ionic conductor at high temperature. The frequency dependence of conductivity follows the Jonscher’s dynamic law. The high level of conductivity at high temperature is explained by the dynamical disordering of protons between the neighboring SO₄ groups. This behavior indicates the presence of superprotonic phase transition in CsK(SO₄)·Te(OH)₆.     

Magnetic fluctuation effects on Raman scattering in the organic superconductor κ-ET2Cu[N(CN)2]Br

We present a study of the 503, 880, 890, 901, 920, and 1475 cm⁻¹ Raman active κ-ET₂Cu[N(CN)₂]Br single crystal phonons, around T=40 K, where an anomaly in the longitudinal sound velocity and antiferromagnetic spin fluctuations have been detected. Doublets, narrowing of phonon lines and non-monotonic intensity variations, as a function of temperature, are observed indicating a possible interaction between phonon and correlated electrons.     

Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

The phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ alloys were studied. The pseudobinary C36-type Laves phase Sr(Al,Mg)₂ was found as a structural intermediate between the Zintl phase and the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase, were determined to be x=0–0.10, 0.45–0.68 and 0.80–1, respectively. The Mg-substituted Zintl phase Sr(Al_0.95Mg_0.05)₂ can be hydrogenated to Sr(Al,Mg)₂H₂ at about 473 K. However, the Sr(Al,Mg)₂H₂ directly decomposes into SrH₂ and Sr(Al,Mg)₄ starting at 513 K. When the temperature is 573 K, the C36 Laves phase Sr(Al_0.5Mg_0.5)₂ can be hydrogenated into SrMgH₄ and Al, while the C14 Laves phase Sr(Al_0.1Mg_0.9)₂ is hydrogenated into SrMgH₄, Mg₁₇Al₁₂ and Mg.