Electrical anisotropy of α-(BEDT-TTF)2KHg(SCN)4

The transverse magnetoresistance of α-(BEDT-TTF)₂KHg(SCN)₄ was investigated with the magnetic field rotated within a conducting ac-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance in the weak-field limit, ΔR(B,θ), has the form ΔR(B,θ) = B²(psin²(θ − θ_min) + qcos²(θ − θ_min), where θ is the angle betweeen a-axis and magnetic field direction and (p, q, θ_min) are temperature-dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical anisotropy within be-plane is 3.5 4.5 and 2.5 3.0 above and below the phase transition at 10 K, respectively.     

-NMRC-NMR study on α-(BEDT-TTF)2MHg(SCN)4

The low temperature electronic states of the organic conductors, α-(BEDT-TTF)₂MHg(SCN)₄ [M = K, Rb], have been studied by -NMR¹³C-NMR with the use of ¹³C labeled samples. We observed anomalies in the nuclear spin-lattice relaxatin rate, T₁, and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some phase transition. However, absence of critical fluctuation near the transition temperature and absense of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW systems such as the TMTSF compounds. The present results imply that the electronic density of states decreases to a half below the transition.     

The spectroelectrochemical, magnetic, and structural characterization of reduced hexaazatriphenylenehexacarbonitrile, HAT(CN)6

The cyanoazacarbon, hexaazatriphenylenehexacarbonitrile, or HAT(CN)₆, is readily reduced and the spectroelectrochemical properties associated with the multiple reductions are described. The singly reduced radical monoanion forms stable salts and we report the crystal structure and magnetic properties for the tetrabutylammonium salt. [Tetra-n-butylammonium] [HAT(CN)₆] behaves ferromagnetically below 3 K and follows Curie Law behavior at higher temperatures after correction for Pauli-type susceptibility. The room temperature conductivity of the powdered salt is 3.8 × 10⁻⁸ S/cm. The crystal structure shows closely bound pairs of radical anions with slip-stacking of these pairs to form a staircase, features that elucidate the observed properties. Thin films of HAT(CN)₆ were found to support negative charge transport by electron time-of-flight measurements, yielding electron mobilities of 10⁻⁴ cm²/Vs at room temperature.     

Angle-dependent magnetoresistance oscillations and Fermi surface reordering at high magnetic fields is α-(ET)2KHg(SCN)4

Angle dependent magnetoresistance oscillations (AMRO) have been studied in the charge transfer salt α-(ET)₂KHg(SCN)₄ for magnetic fields in the range 0 - 30T. This salt exhibits the onset of antiferromagnetic order at temperatures T_N 8-10 K and the presence below this temperature of a region of sharp negative magnetoresistance at a field around 22 T known as the 'kink'. AMRO have been measured in this salt for a wide range of applied fields since the period, amplitude, and nature of the oscillations can be used to directly infer the character of the Fermi surface (FS) as a function of field. The data indicate that a profound change in the band structure occurs at this kink transition; the high field phase is characterised by quasi-2D oscillations from a closed cylindrical FS which is elongated in the c direction; the low field phase appears to be a spin density wave groundstate, with a FS consisting of a sheet (which is quasi-1D in character and tilted at an angle of 21° to the b^*c plane) and small closed 2D pockets. It is suggested that the breakdown orbits between the pockets and the 1D sheets are able to account for the various Shubnikov-de Haas frequencies observed below the kink.     

High field magnetization measurements on α-(BEDT-TTF)2KHg(SCN)4

We report measurements of the steady and oscillatory part of the magnetization by means of a torquemeter at temperatures down to 0.4K and magnetic fields up to 28T. Below 24T we find the well known splitting in the de Haas van Alphen- (dHvA-) oscillations for T ≤ 1K and Θ ≤ 25°, where Θ is the angle between the magnetic field and the normal to the layers. The phase transition at 24T is indicated by a strong increase of the dHvA amplitude. At low fields the steady torque shows a quadratic dependence on B, whereas in the high field regime a complex dependence on the temperature and the angle θ is observed. For Θ ≥ 60° several field induced anomalies occur up to the maximal field of 28T for temperatures well below 4K. But also for small values of θ our experiments show evidence for a complex magnetic groundstate at 28T for temperatures below 1K.     

Magnetic study of Ti-substituted NiFe2O4 ferrite

The Ni_1+xTi_xFe_2−2xO₄ (0 ≤ x ≤ 0.1) ferrite systems prepared by a semi-chemical route, have been studied by electron paramagnetic resonance (EPR) at X-band, Mössbauer spectroscopy and magnetization measurements at various temperatures. EPR spectra of these samples comprise generally a broad and asymmetric EPR signal. The variation of g_eff and peak-to-peak line width ΔH_pp, with Ti concentration and temperature are attributed to the variation of dipole–dipole interaction and the superexchange interaction. Mössbauer spectra comprise two sets of sextet attributed to Fe³⁺ at two distinct sites-A and -B. Ti⁴⁺ ions are concluded to occupy the octahedral B-sites. Magnetic moment is found to decrease with the increase of Ti⁴⁺ concentration. The effective magnetic field H_eff at the A-sites also follows a similar trend. The reason is attributed to the canted structure of spins in the Ti-doped samples. An anomalous behavior at x = 0.015 is observed in the properties studied here and some sort of phase change is believed to occur at 473 K in these ferrites.     

Magnetic properties of a new iron lead vanadate Pb2FeV3O11

Temperature dependence of dc/ac magnetization and electron paramagnetic resonance (EPR) spectra of Pb₂FeV₃O₁₁ iron lead vanadate has been investigated. The dc magnetic measurements have shown the presence of antiferromagnetic interactions with Curie-Weiss temperature, T_CW = −15.2 K, in the high temperatures range while the field cooled (FC) magnetization revealed a maximum at T_N = 2.5 K which coincides with a long range magnetic ordering. Temperature dependence of χ′ has shown a maximum at the same temperature. EPR spectrum of Pb₂FeV₃O₁₁ at room temperature is dominated by nearly symmetrical, very intense and broad resonance line centered at g_eff ∼ 2.0 that could be attributed to the correlated system of iron ions. The temperature dependence of magnetic resonance parameters (amplitude, g-factor, linewidth, integrated intensity) has been determined in the 4-300 K range and it suggests the existence of short range correlated spin system up to high temperatures. The temperature dependence of the amplitude of the resonance line has shown a pronounced maximum at 12.5 K that indicates on the existence of two subsystems of weakly and strongly coupled iron pairs. Comparison of dc magnetic susceptibility and EPR integrated intensity points to the presence of correlated spin agglomerates that play an important role in determination of the magnetic response of Pb₂FeV₃O₁₁.     

Electrochemical performances of Ag–(Bi2O3)0.75(Y2O3)0.25 composite cathodes

The electrochemical performances of Ag–Y_0.25Bi_0.75O_1.5 (YSB) composite cathodes on Ce_0.8Sm_0.2O_1.9 (SDC) electrolytes have been investigated at intermediate temperature using AC impedance spectroscopy. The results indicated that the electrochemical performances of these composites are quite sensitive to the compositions and the microstructures of the cathode. The optimum YSB addition to Ag resulted in 10 times lower area specific resistance. The ASR of Ag-50 vol.% YSB was about 0.12 Ωcm² at 700 °C as compared to 3.9 Ωcm² for Ag cathodes. The observed high performance of Ag–YSB composite cathodes might be due to the high oxygen-ion conductivity of YSB and its high catalytic activity for oxygen reduction.     

Specific heat study of α-(BEDT-TTF)2MHg(SCN)4

The low-temperature specific heat of α-phase of (BEDT-TTF)₂MHg(SCN)₄ (M = K, Rb, NH₄), where BEDT-TTF is bis-(etylenedithio) tetrathiafulvalene, is studied. The electronic specific heat coefficient (γ) estimated from the data between 1K and 2.3K amounts to 5.3, 10.3, 28.8mJ/mol K² for M = K, Rb and NH₄, respectively. The K-salt does not have distinct anomally around 8K within the precision of our experiments, implying that the transition occurs with a gradual change of electronic structure.     

Crystal structures and magnetic properties of iron (III)-based phosphates: Na4NiFe(PO4)3 and Na2Ni2Fe(PO4)3

Crystal structures from two new phosphates Na₄NiFe(PO₄)₃ (I) and Na₂Ni₂Fe(PO₄)₃ (II) have been determined by single crystal X-ray diffraction analysis. Compound (I) crystallizes in a rhombohedral system (S. G: R-3c, Z = 6, a = 8.7350(9) Å, c = 21.643(4) Å, R₁ = 0.041, wR₂=0.120). Compound (II) crystallizes in a monoclinic system (S. G: C2/c, Z = 4, a = 11.729(7) Å, b = 12.433(5) Å, c = 6.431(2) Å, β = 113.66(4)°, R₁ = 0.043, wR₂=0.111). The three-dimensional structure of (I) is closely related to the Nasicon structural type, consisting of corner sharing [(Ni/Fe)O₆] octahedra and [PO₄] tetrahedra forming [NiFe(PO₄)₃]⁴⁺ units which align in chains along the c-axis. The Na⁺ cations fill up trigonal antiprismatic sites within these chains. The crystal structure of (II) belongs to the alluaudite type. Its open framework results from [Ni₂O₁₀] units of edge-sharing [NiO₆] octahedra, which alternate with [FeO₆] octahedra that form infinite chains. Coordination of these chains yields two distinct tunnels in which site Na⁺.The magnetization data of compound (I) reveal antiferromagnetic (AFM) interactions by the onset of deviations from a Curie-Weiss behaviour at low temperature as confirmed by Mössbauer measurements performed at 4.2 K. The corresponding temperature dependence of the reciprocal susceptibility χ⁻¹ follows a typical Curie-Weiss behaviour for T > 105 K. A canted AFM state is proposed for compound (II) below 46 K with a field-induced magnetic transition at H ≈ 19 kOe, revealed in the hysteresis loop measured at 5 K. This transition is most probably associated with a spin-flop transition.     

Thin films of the organic superconductor α-(BEDT-TTF)2I3

α-(BEDT-TTF)₂I₃ is a quasi-two-dimensiona1 organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80°C leads to the metallic system α₁-(BEDT-TTF)₂I₃ which becomes superconducting below 8 K. Thin films of the α-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α₁- phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α₁-films become superconducting with an onset at 9 K.     

Resistive transition in an extremely Two-Dimensional superconductor, α-(BEDT-TTF)2NH4Hg(SCN)4

In order to investigate the superconductivity in α-(BEDT-TTF)₂NH₄Hg(SCN)₄, resistivity was measured as a function of temperature, magnetic field and exciting current. In addition, simultaneous measurements of I-V characteristics and ac susceptibility were carried out. The results indicate highly two-dimensional character of the superconductivity.     

Superconductivity ia α(BEDT-TTF)2MHg(SCN)4 (M = K, Rb, Tl, NH4)

The layered organic metals (α-(BEDT-TTF)₂MHg(SCN)₄ (M = K, Rb, Tl) show resistance decrease with the onset at 0.3 K, 0.5 K and 0.1 K, respectively. These decreases are suppressed by a magnetic field less than 0.2 T and considered as an occurence of superconductivity, although zero resistance is not achieved. We discuss this incomplete superconducting transition in terms of the vicinity to a superconductor-insulator transition caused by the elevated sheet resistance due to the antiferromagnetic ordering at 8 10 K.     

Anisotropic Hc2, Shubnikov de Haas, and angular dependent magnetoresistance in the organic superconductor α-(BEDT-TTF)2MHg(SCN)4

We report measurements on the anisotropic critical field, the angular dependent magnetoresistance, and Shubnikov de Haas oscillations in α-(BEDT-TTF)₂NH₄Hg(SCN)₄, the only ambient pressure superconductor (T_c = 1.2K) in the α-(BEDT-TTF)₂MHg(SCN)₄ organic conductor family. The anisotropic critical field is well described by a 2-D model for layered supercondcutors where the anisotropy ε = Hc\\ / H_c is 25 at low temperatures, and rises to 100 near T_c. Beating behavior is observed in the SdH effect in tilted magnetic fields.     

Low-temperature thermoelectric properties of α- and β-Zn4Sb3 bulk crystals prepared by a gradient freeze method and a spark plasma sintering method

The low temperature thermoelectric properties of Zn₄Sb₃ samples prepared by the gradient freeze (GF) method and sintering have been characterized. With decreasing temperature a dramatic rise in the thermal expansion is observed associated with the structural transition from β- to α-phase; Δl/l=2.8×10⁻⁴ at T_s^GF=257.4 K for GF and Δl/l=1.6×10⁻⁴ at T_s^S=236.5 K for sintered samples. Enhancement is observed in electrical conductivity and p-type thermopower at T_s^GF and T_s^S, while a reduction is observed in the magnetic susceptibility. The GF sample exhibits higher thermoelectric performance than the sintered sample. The power factor of the α-phase in the GF sample is twice as large as that of the β-phase; it exceeds 20 μW/cm·K² between 120 and 240 K, indicating that the α-phase Zn₄Sb₃ is one of the prime candidates for thermoelectric materials for cryogenic use.     

Electrical and electrochemical properties of γ-Li2CuZrO4

The γ-Li₂CuZrO₄ with a double rock-salt structure, as a lithium-ion compound, has never been reported on their physical and physicochemical properties. The γ-Li₂CuZrO₄ was prepared by a solid-state reaction process. X-ray diffraction was used to analyze the structure of the products. Its electrical properties were characterized by both DC and AC measurements in the temperature range from 133 to 1273 K. Cyclic voltammetry and galvanostatic cell cycling were also employed to evaluate its electrochemical performance. It is found to be a pure electronic semiconductor with a fairly high conductivity (10⁻⁵ S/cm at 133 K, 10⁻² S/cm at 300 K and 10⁻¹ S/cm at 1173 K). The average activation temperature is 14.4 kJ/mol. When increasing the temperature above 1073 K, a phase transition from γ to β takes place. When reacting with lithium in an electrochemical cell, γ-Li₂CuZrO₄ decomposes into three phases during the initial discharge process, and possesses a reversible capacity of about 70 mAh/g.     

Structural and magnetic properties of a novel compound with Y3(Fe, V)29 stoichiometry and disordered CaCu5-type structure

A novel phase with nominal composition Y₃Fe_27.5V_1.5 has been synthesized by arc melting elemental constituents, followed by annealing at 1123 K. The samples were analyzed by electron microprobe analysis and the stoichiometric composition was Y_3.04Fe_27.56V_1.44. X-ray diffraction from powder samples shows that the structure is of a disordered CaCu₅-type (Space Group: P6/mmm) with lattice parameters a=4.8769(1) Å, c=4.1729(3) Å. This disordered structure, revealed using standard difference Fourier methods, can be considered as a TbCu₇-type, containing iron dumbbell sites. The disorder is not restricted to the rare earth sites and the dumbbell atoms, but is observed at other sites as well. The stoichiometric formula derived by Rietveld analysis is Y_3.00(3)Fe_28.5(3). Magnetic measurements give a Curie temperature of 439(5) K, and a saturation magnetization of 156.4 and 125.5 A m² kg⁻¹ at 5 and 300 K, respectively. The anisotropy field is 4.76 T at 5 K and 2.43 T at 300 K. X-ray powder diffraction patterns of magnetically aligned powder samples indicate that the magnetocrystalline anisotropy is of easy plane type. This result is supported from the analysis of the magnetization measurements on aligned powders which give K₁<0, K₂>0 and K₁+2K₂<0, a result that confirms the easy plane anisotropy. Mössbauer spectra were also obtained and fitted with a model consistent with the disorder. The average hyperfine fields are 28.4 and 21.5 T at 4.2 and 300 K, respectively.     

Mechanical and thermal properties of Tb2(MoO4)3 crystals

The mechanical and thermal properties of single crystal Tb₂(MoO₄)₃ have been systematically studied. The result of microhardness measurement indicates that the crystal belongs to the soft materials category. The thermodynamic parameters obtained from DTA analysis were used for determining the type of liquid-solid interface during crystal growth. Negative thermal expansion along the c-axis was observed, and this behavior was attributed to the bent Tb-O-Mo bonds. The specific heat of the crystal was measured to be 0.122 cal g⁻¹ K⁻¹ at 293.15 K. The thermal conductivity of Tb₂(MoO₄)₃ at room temperature was found to be smaller than that of representative ferroelectric LiNbO₃.     

Synthesis, photoluminescence, thermoluminescence and dosimetry properties of novel phosphor KSr4(BO3)3:Ce

Polycrystalline powder sample of KSr₄(BO₃)₃ was synthesized by high-temperature solid-state reaction. The influence of different rare earth dopants, i.e. Tb³⁺, Tm³⁺ and Ce³⁺, on thermoluminescence (TL) of KSr₄(BO₃)₃ phosphor was discussed. The TL, photoluminescence (PL) and some dosimetric properties of Ce³⁺-activated KSr₄(BO₃)₃ phosphor were studied. The effect of the concentration of Ce³⁺ on TL intensity was investigated and the result showed that the optimum Ce³⁺ concentration was 0.2 mol%. The TL kinetic parameters of KSr₄(BO₃)₃:0.002 Ce³⁺ phosphor were calculated by computer glow curve deconvolution (CGCD) method. Characteristic emission peaking at about 407 and 383 nm due to the 4f⁰5d¹ → ²F_{(5/2, 7/2)} transitions of Ce³⁺ ion were observed both in PL and three-dimensional (3D) TL spectra. The dose–response of KSr₄(BO₃)₃:0.002 Ce³⁺ to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of KSr₄(BO₃)₃:0.002 Ce³⁺ was also investigated.     

Synthesis, structure determination and magnetic susceptibilities of the oxides Ln3Li5Sb2O12 (Ln ≠ Pr, Nd, Sm)

The oxides Ln₃Li₆Sb₂O₁₂ (Ln≠Pr, Nd, Sm) were prepared in air by heating a mixture of Ln₂O₃, LiNO₃and Sb₂O₃ in the temperature range 1023–1243 K. Lattice parameters as well as atomic coordinates in the space group 12,3 (Z≠8) are established from X-ray powder diffraction data by the Rietveld method. Magnetic susceptibilities from 4.2 to 300 K follow a Curie-Weiss law above 50 K (Ln≠Pr) or 70 K (Ln≠Nd). This is attributed to the splitting of the ground state associated with the Ln³⁺ ions by the influence of the crystal field. The magnetic moments, 3.54 and 3.61 μ_D for praseodymium and neodymium respectively, agree with those calculated by Hund's formula. The Sm³⁺ behaviour can be explained by taking into account that the splitting of the multiplets is not too large compared to kT. The magnetic moment observed at room temperature for this cation is 1.6 μ_B.