Structural,elastic, electronic,optical and thermal properties of c-SiGe<Subscript>2</Subscript>N<Subscript>4</Subscript>

We have investigated the structural, elastic, electronic, optical and thermal properties of c-SiGe₂N₄ by using the ultrasoft pseudopotential density functional method within the generalized gradient approximation. The calculated structural parameters, including the lattice constant, the internal free parameter, the bulk modulus and its pressure derivative are in agreement with the available data. The independent elastic constants and their pressure dependence, calculated using the static finite strain technique, satisfy the requirement of mechanical stability, indicating that c-SiGe₂N₄ compound could be stable. We derive the shear modulus, Young’s modulus, Poisson’s ratio and Lamé’s coefficients for ideal polycrystalline c-SiGe₂N₄ aggregate in the framework of the Voigt-Reuss-Hill approximation. We estimate the Debye temperature of this compound from the average sound velocity. Band structure, density of states, Mulliken charge populations and pressure coefficients of energy band gaps are investigated. Furthermore, in order to understand the optical properties of c-SiGe₂N₄, the dielectric function, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 40 eV. Thermal effects on some macroscopic properties of c-SiGe₂N₄ are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account. We have obtained successfully the variations of the primitive cell volume, volume expansion coefficient, heat capacities and Debye temperature with pressure and temperature in the ranges of 0–40 GPa and 0–2000 K. For the first time, the numerical estimates of the elastic constants and related parameters, and the thermal properties are performed for c-SiGe₂N₄.     

Optical properties of thin Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript>, films produced by RF magnetron sputtering

Optical properties of thin Cu₂ZnSnS₄ films produced by RF magnetron sputtering of preliminarily synthesized material are studied. Transmission and reflection coefficients are studied in a range from 0.4 to 26 μm. The optical band-gap width depending on substrate temperature is estimated; in optimal modes, it is equal to 1.47 eV. The study of electrical properties shows that Cu₂ZnSnS₄ possesses low charge-carrier mobility, μ = 1.9 cm²/(V s), at room temperature and hole concentration р = 5 × 10¹⁸ cm^–3. Electron microscopy shows that the film possesses a polycrystalline structure with a crystallite size on the order of 100 nm.     

Growth and optical properties of paraelectric K<Subscript>1-y</Subscript>Na<Subscript>y</Subscript>Ta<Subscript>1-x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>3</Subscript> single crystals

We report the successful growth of an electroholographic crystal, potassium sodium tantalate niobate (KNTN), by a top-seeded solution growth method. Both blue and colorless crystals were obtained. The structure, optical absorption, and refractive dispersion properties of the as-grown crystals have been investigated. Furthermore, the Kerr coefficients R₁₁ and R₁₂ of paraelectric K_0.95Na_0.05Ta_0.61Nb_0.39O₃ single crystal were determined by using an automated scanning Mach–Zehnder interferometer. The crystal has large Kerr coefficients with R₁₁= 2.8×10^-16 m²/V² and R₁₂= -0.3×10^-16 m²/V² at the wavelength of 632.8 nm near its cubic–tetragonal phase boundary. PACS 81.10.Dn; 42.70.Nq     

Thermal expansion and thermal conductivity of single-crystal Cu<Subscript>x</Subscript>Ag<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>InS<Subscript>2</Subscript> solid solutions grown by the moving solvent method

Single crystals of Cu_xAg_1−x InS₂ solid solutions are grown by the moving solvent method. The compositions and structures of the single crystals are determined. The thermal expansion coefficients of these crystals are determined with a dilatometer. The thermal expansion coefficients are found to vary linearly with concentration x. The thermal conductivity of the crystals is measured by the absolute method, and the concentration dependence of the thermal conductivity is constructed. This dependence is shown to have a minimum near the equimolar composition.     

Acoustic attenuation in ferroelectric Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals

The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn₂P₂S₆, which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of Sn₂P₂S₆ crystals.     

Thermoelectric properties of La<Subscript>0.75</Subscript>Ca<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> manganite at ultrahigh pressures up to 20 GPa

Pressure dependences of the thermopower and electrical resistivity of the La_0.75Ca_0.25MnO₃ manganite are measured in the pressure range 0–20 GPa at room temperature. The absolute value of the thermopower increases in the pressure range 0–3 GPa and decreases at higher pressures. At the same time, the electrical resistivity decreases over the entire pressure range. It is found that the competing effect of the closing of the bandgap, which is determined by the activation energy for the thermopower, and the pressure broadening of the d bands is the cause of the observed behavior of the thermoelectric properties of La_0.75Ca_0.25MnO₃, which is untypical for the majority of dielectrics and semiconductors with single-band unipolar conductivity in the absence of phase transitions and is accompanied by a change in the sign of the pressure coefficient of the thermopower. The interrelation between the magnetic and thermoelectric properties of manganites under pressure is analyzed in the framework of the double exchange model. The causes of the considerable decrease in the pressure coefficients of the insulator-metal transition and Curie temperatures under pressure experimentally observed in manganites are discussed.     

Current-voltage characteristics of polycrystalline (La<Subscript>0.5</Subscript>Eu<Subscript>0.5</Subscript>)<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The current-voltage characteristics of the polycrystalline substituted lanthanum manganite (La_0.5Eu_0.5)_0.7Pb_0.3MnO₃ have been measured at temperatures close to the metal-insulator transition temperature and at low temperatures. In both cases, the current-voltage characteristics exhibit nonlinear properties that are strongly dependent on the strength of an applied magnetic field. The mechanisms responsible for the nonlinear properties at these temperatures are found to be different: near the metal-insulator transition, the current-voltage characteristics are determined by the phase layering inside granules, while at low temperatures, they are determined by tunneling of carriers through insulating interlayers of the granules.     

EPR Studies of Copper-Doped Potassium Dihydrogen Citrate (C<Subscript>6</Subscript>H<Subscript>7</Subscript>KO<Subscript>7</Subscript>) Single Crystal

The magnetic environments of Cu²⁺-doped potassium hydrogen citrate (C₆H₇KO₇) complex have been identified by electron paramagnetic resonance (EPR) technique. The angular variation of the EPR spectra has shown that three different Cu²⁺ complexes are located in different chemical environments, and each environment contains one magnetic Cu²⁺ site occupying substantial positions in the lattice and showing a very high angular dependence. The principal g and the hyperfine structure parameter (A) values of three sets of Cu²⁺ complex groups are determined. The covalency parameter, mixing coefficients and Fermi-contact term of the complex are obtained, and the ground-state wave function of the Cu²⁺ ion in the lattice has been constructed.     

Vibrational spectra of disordered oxyfluoride (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>WO<Subscript>3</Subscript>F<Subscript>3</Subscript>

Comparative analysis of the IR absorption and Raman scattering spectra of a polycrystalline sample of perovskite-type oxyfluoride (NH₄)₃WO₃F₃ has been performed in the frequency range 370–4000 cm^?1 at temperatures from 92 to 303 K, including the transition between the orientationally disordered cubic and low-symmetry phases. The conformation of WO₃F₃ octahedral groups is established and transitional anomalies of the internal modes of these groups and ammonium ions are revealed. Comparative analysis of the IR and Raman spectra suggests that the phase transition under study is mainly related to the ordering of octahedral groups and formation of W-O…H-N hydrogen bonds.     

Acoustic and magnetic properties of La<Subscript>0.825</Subscript>Sr<Subscript>0.175</Subscript>MnO<Subscript>3</Subscript> lanthanum manganites

This paper reports on measurements of the acoustic, magnetic, and electrical properties and on an x-ray microprobe analysis of a La_0.825Sr_0.175MnO₃ single-crystal sample. The acoustic studies were made with a pulsed acoustic spectrometer operating on a 770-MHz carrier. The studies revealed anomalies in the damping coefficients and sound velocity near 300, 200 K, and the Curie temperature T_C (283 K) where the colossal magnetoresistance occurs. The effect of a magnetic field on the magnetic texture of lanthanum manganites cooled below T_C, observed earlier in samples of other composition, is confirmed. In addition, a region was found wherein the magnetic susceptibility of an unclamped sample behaves anomalously. The electrical resistivity was observed to decrease substantially below T_C; this effect exhibits a hysteretic pattern in the interval 200–180 K.     

Anomalous absorption in H<Subscript>2</Subscript>CN and CH<Subscript>2</Subscript>CN molecules

Structures of H₂CN and CH₂CN molecules are similar to that of H₂CO molecule. The H₂CO has shown anomalous absorption for its transition 1₁₁–1₁₀ at 4.8 GHz in a number of cool molecular clouds. Though the molecules H₂CN and CH₂CN have been identified in TMC-1 and Sgr B2 through some transitions in ortho as well as in para species, here we have investigated the condition under which transitions 1₁₁–1₁₀ and 2₁₂–2₁₁ of these molecules may show anomalous absorption. For the present investigation, we have calculated energy levels and radiative transition probabilities. However, we have used scaled values for collisional rate coefficients. We found that relative values of collisional rate coefficients can produce the required anom-alous absorption in 1₁₁–1₁₀ and 2₁₂–2₁₁ transitions in the molecules.      

Nonlinear magnetic resonance in the (CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>)<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> crystal

The nonlinear microwave absorption in the (CH₃NH₃)₂CuBr₄ antiferromagnetic crystal is investigated experimentally. The temperature and angular dependences of the parameters of nonlinear resonance and the dependences of these parameters on the microwave pump power are analyzed. It is found that the nonlinear properties deteriorate with decreasing temperature and the linear and nonlinear contributions are competitive in character.     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.     

Ion transfer in solid solutions of Cu<Subscript>2</Subscript>Se and Ag<Subscript>2</Subscript>Se superionic conductors

The cationic conductivities of Cu₂Se and Ag₂Se superionic conductor solid solutions in the composition region from Cu₂Se to Cu_0.7Ag_1.3Se are measured. It is demonstrated that the activation energy of ionic conduction depends only slightly on the chemical composition, varies from 0.14 to 0.17 eV, and exhibits a weakly pronounced maximum for the Ag_0.44Cu_1.56Se solid solution. The ionic Seebeck coefficients are measured for the Ag_0.23Cu_1.757Se solid solution. The heat of cation transfer in this solution is found to be equal to 0.144±0.014 eV from the Seebeck coefficients.     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO<Subscript>2</Subscript> varistors

Ca,Ta-doped TiO₂ varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350 °C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 10⁵. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO₂ varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO₂ system are explained by analogy to a grain-boundary atomic defect model. Received: 24 October 2001 / Accepted: 8 January 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/826-49531, E-mail: wangwanyan@yahoo.com.cn     

Multiple magnetic transitions in single crystals of Ce<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript> and La<Subscript>12</Subscript>Fe<Subscript>57.5</Subscript>As<Subscript>41</Subscript>

Measurements of magnetic and transport properties were performed on needle-shaped single crystals of Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁. The availability of a complete set of data enabled a side-by-side comparison between these two rare earth compounds. Both compounds exhibited multiple magnetic orders within 2–300 K and metamagnetic transitions at various fields. Ferromagnetic transitions with Curie temperatures of 100 and 125 K were found for Ce₁₂Fe_57.5As₄₁ and La₁₂Fe_57.5As₄₁, respectively, followed by antiferromagnetic type spin reorientations near Curie temperatures. The magnetic properties underwent complex evolution in the magnetic field for both compounds. An antiferromagnetic phase transition at about 60 K and 0.2 T was observed merely for Ce₁₂Fe_57.5As₄₁. The field-induced magnetic phase transition occurred from antiferromagnetic to ferromagnetic structure. A strong magnetocrystalline anisotropy was evident from magnetization measurements of Ce₁₂Fe_57.5As₄₁. A temperature-field phase diagram was present for these two rare earth systems. In addition, a logarithmic temperature dependence of electrical resistivity was observed in the two compounds within a large temperature range of 150–300 K, which is rarely found in 3D-based compounds. It may be related to Kondo scattering described by independent localized Fe 3d moments interacting with conduction electrons.     

An Ellipsometric Investigation of the Optical Properties of Ru<Subscript>2</Subscript>Ge<Subscript>3</Subscript> and Ru<Subscript>2</Subscript>Sn<Subscript>3</Subscript> Compounds

Ellipsometric investigations of the optical properties of Ru₂Ge₃ and Ru₂Sn₃ intermetallic compounds are carried out in the wavelength range from 0.22 to 15 μm. The nature of interband light absorption is analyzed based on a comparative analysis of the experimental and theoretical frequency dependences of an optical conductivity. The obtained results confirm the existence of energy gaps at the Fermi level in the electronic spectra of these materials predicted earlier by the band-structure calculations.     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.     

Ab initio study of thermoelectric properties of Cu<Subscript>3</Subscript>PSe<Subscript>4</Subscript> and Cu<Subscript>3</Subscript>PS<Subscript>4</Subscript>: alternative materials for thermoelectric applications

This letter discusses the thermoelectric properties of Cu₃PSe₄ and Cu₃PS₄ compounds, using the Ab initio calculations. These compounds are predicted to be good thermoelectric materials thanks to the nature of their band edge states. Seebeck coefficient of Cu₃PSe₄ exhibits a maximum value of 1256 µV/K at roopm temperature, whereas it is 2389 µV/K for Cu₃PS₄. Furthermore, the electrical conductivity is significantly enhanced with doping level while the electronic thermal conductivity is weakly increased. Besides, the factor of merit of these compounds shows a value around the unity only at low doping levels. Hence, this predicts that these compounds may present excellent thermoelectric properties, therefore they could be considered as alternatives for thermoelectric applications.