Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

Effect of Mn-doping on the electrical properties of Cu2GeSe3

In this work we report the temperature dependence of the resistivity ρ of p-Cu₂GeSe₃ and manganese-doped p-Cu₂GeSe₃ at low temperature. It was found that for a intrinsic sample ρ obeys the Shklovskii-Efros-type variable-range hopping resistivity law in the temperature range from 4 to 63 K. This behaviour is governed by generation of a Coulomb gap Δ=78 meV in the density of localized states. We find a low activation term T₀=0.24 K, which is an indication of a large localization length ξ. For Mn-doped sample a metal-insulator transition (MIT) is observed at T=65 K. On the basis of the Mott criterion for metal-insulator transition, the critical carrier density n_c is determined. From the analysis of resistivity data it is concluded that Mn acts as acceptor impurity.     

Spin-Peierls and incommensurate states in layered S=1/2 system TiOCl

We demonstrate that TiOCl is a good model inorganic system to investigate spin-Peierls state. Our ³⁵Cl and ^47,49Ti NMR data show that a pseudo spin-gap behavior below T*=135 K precedes successive phase transitions at T_c=94 K and into a singlet spin-Peierls ground state with a large energy gap E_g/k_B=430 K.     

Thermal expansion and kinetic coefficients of crystals

Electrical (ρ) and thermal (W) resistivities and thermal expansion coefficient (β) of Cu, Zn, Al, Pb, Ni, β-brass, Al₂O₃, NaCl, Si, SiO₂(∥), and SiO₂(⊥) were simultaneously measured with standard four-probe, absolute steady-state, and quartz dilatometer techniques. Measurements of Ni and β-brass were performed at temperatures from 300 to 1100 K and measurements of all other samples were made between 90 and 500 K. This temperature range includes the range below and above the Debye temperature (T_D). The total uncertainties of the specific electrical and thermal resistivities and thermal expansion coefficient (TEC) measurements are 0.5%, 3.0%, and (1.5-4.0%), respectively. The universal linear relationship between the electrical and thermal resistivities and βΤ over the wide temperature range was found experimentally. Using the Landau criterion for convection development for ideal phonon and electron gases in the solids, the universal relations, ρ^ph/ρ^*≡βT and W^ph/W^*≡βT (where ρ^ph is the phonon electrical resistivity, is the characteristic electrical resistivity, W^ph is the phonon thermal resistivity, and W^*=k_BG/qc_p is the characteristic thermal resistivity) between relative phonon electrical and phonon thermal resistivities and βΤ were derived. The derived universal relations provide a new method for estimating the kinetic coefficients (electrical and thermal resistivities) from TEC measurements.     

Magnetic properties of PrRh2Si2: A neutron diffraction study

The magnetic properties of polycrystalline PrRh₂Si₂ sample have been investigated by neutron diffraction measurements. Antiferromagnetic transition with an anomalously high ordering temperature (T_N∼68 K) is clearly observed in magnetic susceptibility, specific heat, electrical resistivity and neutron diffraction measurements. Neutron diffraction study shows that Pr³⁺ ions carry an ordered moment of 2.99(7)μ_B/Pr³⁺ and align along the crystallographic±c-directions for the ions located at the (0,0,0) and positions. The magnetoresistance at 2 K and 10 T is rather large (∼35%).     

In-situ control of different oxygen fugacity experimental study on the electrical conductivity of lherzolite at high temperature and high pressure

At pressure 1.0-4.0 GPa and temperature 1073-1423 K and under the control of oxygen fugacity (Mo+MoO₂, Fe+FeO and Ni+NiO), a YJ-3000t multi-anvil solid high-temperature and high-pressure apparatus and Solartron-1260 impedance/Gain-Phase analyzer were employed to analyze the electrical conductivity of lherzolite. The experimental results showed that: (1) within the range of the selected frequencies (10³-10⁶ Hz), either as viewed from the relationship between the real or imaginary part of complex impedance and the frequency, or from the relationship between modulus, phase angle and frequency, it can be seen clearly that the complex impedance has a strong dependence on frequency; (2) with the rise of temperature (T), the electrical conductivity (σ) increased, and Lg σ and 1/T follows the Arrhenius relationship; (3) with the rise of pressure, the electrical conductivity decreased, and activation enthalpy and temperature-independent pre-exponential factor decreased as well. And the activation energy and activation bulk volume of the main charge carrier in the lherzolite have been obtained for the first time, which are 1.68±0.02 eV and 0.04±0.01 cm³/mol, respectively; (4) under the given pressure and temperature, the electrical conductivity tends to increase with increasing oxygen fugacity, and under the given pressure, the activation enthalpy and pre-exponential factor tend to decrease with the rise of oxygen fugacity; (5) at 2.0 GPa and the control of the three solid buffers, Mo+MoO₂, Fe+FeO and Ni+NiO, the exponential factors of electrical conductivity variation range with oxygen fugacity are , and the theoretical model for the relationship between the electrical conductivity of lherzolite and the oxygen fugacity under high pressure has been established for the first time; (6) the electrical conduction mechanism of small polarons provides a reasonable explanation to the variation of conductivity of lherzolite with oxygen fugacity.     

Antiferromagnetic transition and electrical conductivity in α-Gd2S3

Magnetic susceptibility and electrical resistivity of α-Gd₂S₃ with an orthorhombic structure (space group: Pnma) have been measured for powder and single-crystal samples. While the magnetic susceptibility of powder sample exhibits a broad peak having a maximum at 4.2 K, the susceptibility for a single crystal with an applied magnetic field along the b-axis demonstrates a sharp drop below 10 K. Nevertheless, the susceptibility with the field perpendicular to the b-axis keeps increasing with decreasing temperature even below 10 K. The electrical resistivity ρ for the powder sample of 4.2×10³ Ω cm around room temperature increases with decreasing temperature and shows a slight discontinuity at about 65 K. In both regions above and below 65 K, is proportional to T^−1/4 with respective coefficients, which is associated with Mott variable-range hopping conductivity. The resistivity of a single crystal along the b-axis is considerably smaller than the value for the powder sample as 0.35 Ω cm at room temperature, and its temperature dependence is fairly weak. While cooling, the resistivity first decreases down to 240 K and then keeps the value independent of the temperature down to 140 K, and subsequently rises gently below 140 K.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

The second hyperpolarizability of CdTe nanocrystals using polarization-resolved degenerate four-wave mixing

Polarization-resolved forward degenerate four-wave mixing (DFWM) in a nonresonant region revealed the effective third-order nonlinear susceptibility of colloidal CdTe nanocrystals (NCs) with the size near the Bohr radius and various concentrations. The second hyperpolarizabilities, and , of the CdTe NCs were ∼1.15 × 10⁻⁴¹ m⁵/V² and ∼3.01 × 10⁻⁴² m⁵/V² from the measurement of the concentration-dependent third-order nonlinear susceptibility of CdTe NCs, respectively. The ratio (/) of the hyperpolarizabilities was ∼0.26, which indicated a large contribution of an electronic polarization process to the third-order nonlinearity of CdTe NCs.     

Nanophases in mechanochemically synthesized AgI-CuI system: structure, phase stability and phase transitions

Nanoscale crystallites of Ag-rich (Ag_1−xCu_xI, x=0.05, 0.10, 0.15 and 0.25), Cu-rich (Cu_1-yAg_yI, y=0.05, 0.10, 0.15 and 0.25) and intermediate Ag_1-xCu_xI (x=0.50) solid solutions and end members AgI, CuI with sizes in the range of 46-13 nm were synthesized by attrition at ambient temperature in a soft mechanochemical reaction (MCR) of Ag, Cu and I. Monophasic γ-AgI (zincblende, ) with disordered Ag⁺ sublattice and the crystallite size of about ∼31 nm was realized in the case of Ag_0.75Cu_0.25I (x=0.25) composition. Lattice parameter decreases linearly from 649 to 604 pm with increasing Cu concentration in the AgI-CuI system validating Vegard's law. Smallest size (∼13 nm) agglomerated nanocrystals were realized in the Cu-rich composition Cu_0.75Ag_0.25I (), while unagglomerated uniform-sized (∼17 nm) and spherical shape nanocrystallites of Ag_0.50Cu_0.50I () with maximum strain were synthesized for sensor applications using MCR. Differential scanning calorimetry study shows the systematic changes in the phase transition temperature with Cu substitution. Ag-rich composition posses less enthalpy (ΔH (x or Cu=0.05, 0.10, 0.15, 0.25)=6.0, 6.11, 6.6, 6.3 in kJ/mol) and entropy (ΔS (y or Ag=0.05, 0.10, 0.15, 0.25)=14.15, 14.1, 15.03, 13.6 in J/mol K) when compared to undoped AgI () implying greater thermal stability of γ-phase due to Cu-strengthened Ag-I bond. Enhanced entropy () in Cu_0.75Ag_0.25I (Cu-rich) solid solutions relative to CuI () indicates Ag-induced cation disorder. Fifteen percent Ag-doped CuI (Cu_0.85Ag_0.15I) nanocrystals apparently behave like microscopic p-n junctions with currents in the range of 10⁻⁶-10⁻⁸ A characterized by a non-linear I-V curve.     

Pressure cycle of superconducting Cs0.8Fe2Se2 : A transport study

We report measurements of the temperature and pressure dependence of the electrical resistivity (ρ) of single-crystalline iron-based chalcogenide Cs_0.8Fe₂Se₂. In this material, superconductivity with a transition temperature develops from a normal state with extremely large resistivity. At ambient pressure, a large “hump” in the resistivity is observed around 200 K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden T_c suppression around . Even at 9 GPa a metallic resistivity state is not recovered, and the ρ(T) “hump” is still detected. A comparison of the data measured upon increasing and decreasing the external pressure leads us to suggest that the superconductivity is not related to this hump.     

Production of Sr-deficient bismuth tantalates from microwave-hydrothermal derived precursors: Structural and dielectric properties

Strontium bismuth tantalates were produced for the first time from microwave-hydrothermal precursors at , for 2 h. Structural and dielectric properties were investigated by X-ray diffraction and complex impedance spectroscopy. A high ferroelectric-paraelectric transition temperature of was observed, together with two different dielectric regimes for the ac electrical conductivity below T_c. The activation energies were calculated as 0.155 and 0.531 eV, and are related to conduction by oxygen vacancies. It was concluded that the low activation energies showed by these materials could contribute to their fatigue-free nature.     

Young's modulus surface and Poisson's ratio curve for cubic metals

The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l, θ) along arbitrary loading direction [h k l] are given for cubic crystals. The representation surface for which the length of the radius vector in the [h k l] direction equals to E(h k l) and representation curve for which the length of the radius vector with angle θ deviated from the reference directions , and equals to υ(100, θ), υ(110, θ) and υ(111, θ) for example, are constructed for six FCC metals Ag, Al, Au, Cu, Ni, Pb and seven BCC metals Cr, Fe, Mo, Nb, Ta, V, W.     

Crystal structure and compressibility of a high-pressure Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81, isomorphous with cubic zirconia

A Ti-rich oxide, (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)_∑=1.0O_1.81, was synthesized at 8.8 GPa and 1600 °C using a multi-anvil apparatus. Its crystal structure at ambient conditions and compressibility up to 10.58 GPa were determined with single-crystal X-ray diffraction. This high-pressure phase is isomorphous with cubic zirconia (fluorite-type) with space group Fm3¯m and unit-cell parameters a=4.8830(5) Å and V=116.43(4) Å³. Like stabilized cubic zirconia, the structure of (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 is also relaxed, with all O atoms displaced from the (, , ) position along 〈1 0 0〉 by 0.319 Å and all cations from the (0, 0, 0) position along 〈1 1 1〉 by 0.203 Å. No phase transformation was detected within the experimental pressure range. Fitting the high-pressure data (V vs. P) to a third-order Birch-Murnaghan EOS yields K₀=164(4) GPa, K′=4.3(7), and V₀=116.38(3) Å³. The bulk modulus of (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 is significantly lower than that (202 GPa) determined experimentally for cubic TiO₂ or that (~210 GPa) estimated for cubic ZrO₂. This study demonstrates that cubic TiO₂ may also be obtained by introducing various dopants, similar to the way cubic zirconia is stabilized below 2370 °C. Furthermore, (Ti_0.50Zr_0.26Mg_0.14Cr_0.10)O_1.81 has the greatest ratio of Ti⁴⁺ content vs. vacant O²⁻ sites of all doped cubic zirconia samples reported thus far, making it a more promising candidate for the development of electrolytes in solid oxide fuel cells.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

Theoretical investigation of zero-field splitting and local structure distortion for a substitution of Fe ion in CdCl2 crystal

A theoretical method for studying the inter-relation between electron and molecule structure is proposed on the basis of the complete energy matrices of the electron-electron repulsion, the ligand-field and the spin-orbit coupling for d⁵ configuration ion in a trigonal ligand-field. As an application, the local distortion structure of (FeCl₆)^3- coordination complex for Fe³⁺ ions doped into CdCl₂ is investigated. Both the second-order zero-field splitting parameter and fourth-order zero-field splitting parameter are considered simultaneously in the structural investigation. By diagonalizing the complete energy matrices, the local structure distortion parameters ΔR=−0.24 Å, Δθ=2.137° at 26 K and ΔR=−0.203 Å, Δθ=2.515° at 225 K for Fe³⁺ ions in CdCl₂ are determined. These results elucidate a microscopic origin of various ligand-field parameters which are usually used empirically for the interpretation of electron paramagnetic resonance results. It is found that the theoretical results are in good agreement with the experimental values.     

Temperature and pressure effects on the spin state of ferric ions in the [Fe(sal2-trien)][Ni(dmit)2] spin crossover complex

Thermal and pressure effects have been investigated on the [Fe(sal₂-trien)][Ni(dmit)₂] spin crossover complex by means of Mössbauer spectroscopic, calorimetric, X-ray diffraction and magnetic susceptibility measurements. The complex displays a complete thermal spin transition between the and spin states of Fe^III near 245 K with a hysteresis loop of ca. 30 K. This transition is characterised by a change of the enthalpy, ΔH_HL=7 kJ/mol, entropy, ΔS_HL=29 J/Kmol, and the unit cell volume, ΔV_HL=15.4 Å³. Under hydrostatic pressures up to 5.7 kbar the thermal transition shifts to higher temperatures by ca. 16 K/kbar. Interestingly, at a low applied pressure of 500 bar the hysteresis loop becomes wider (ca. 61 K) and the transition is blocked at ∼50% upon cooling, indicating a possible (irreversible) structural phase transition under pressure.