Thermal and magnetothermal properties of La0.5Pr0.5Mn2Si2

The thermal and magnetothermal properties of La_0.5Pr_0.5Mn₂Si₂ and isostructural LaFe₂Si₂ intermetallic compounds in the temperature range 4.5-303 K are reported with and without applied magnetic field. The electronic, lattice, and magnetic contributions to the heat capacity of La_0.5Pr_0.5Mn₂Si₂ are determined and analyzed. We have determined and from heat capacity experiments; the values are in line with those from the magnetization measurements. We conclude that in order to observe the anomaly in the heat capacity data around in the system, the transition around should occur in a narrow temperature interval.     

Ac magnetotransport in La0.7Sr0.3Mn0.95Fe0.05O3 at low dc magnetic fields

We report the ac electrical response of La_0.7Sr_0.3Mn_1−xFe_xO₃(x=0.05) as a function of temperature, magnetic field (H) and frequency of radio frequency (rf) current (). The ac impedance (Z) was measured while rf current directly passes through the sample as well as in a coil surrounding the sample. It is found that with increasing frequency of the rf current, Z(T) shows an abrupt increase accompanied by a peak at the ferromagnetic Curie temperature. The peak decreases in magnitude and shifts down with increasing value of H. We find a magnetoimpedance of for at around room temperature when the rf current flows directly through the sample and when the rf current flows through a coil surrounding the sample. It is suggested that the magnetoimpedance observed is a consequence of suppression of transverse permeability which enhances skin depth for current flow. Our results indicate that the magnetic field control of high frequency impedance of manganites is more useful than direct current magnetoresistance for low-field applications.     

Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na₂Al₂B₂O₇ samples aged for over 3 months, we found that Na₂Al₂B₂O₇ at room temperature is a mixture of two phases with space group and P6₃/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al₂B₂O₇]²⁻_∝ double layers stacking alternatively with Na⁺ ions along the c-direction, but differ at in-plane bond orientations of the BO₃/AlO₄ groups within the double layers: in P6₃/m phase B-O₁/Al-O₁ bonds of the two layers are perfectly aligned, whereas in phase they are twisted by 46.4/41.6° around c-axis against each other. It is also found that a freshly prepared sample contains only the phase, but part of the phase will transfer to P6₃/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.     

Relaxor behavior of K0.5La0.5Bi2Ta2O9 ceramics

Potassium lanthanum bismuth tantalate (K_0.5La_0.5Bi₂Ta₂O₉), a new relaxor ferroelectric was synthesized via the solid-state reaction route. X-ray structural studies along with Rietveld refinement confirmed it to be an n=2 member of the Aurivillius family of oxides and the refined lattice parameters are , and . The appearance of 1/2{h00} and 1/2{hk0} type superlattice reflections in the electron diffraction patterns reflected the presence of ordered polar regions. A broad dielectric peak associated with frequency dependent dielectric maximum temperature was observed. The value of the diffuseness parameter γ=1.93, obtained from the fit of a modified Curie-Weiss law established the relaxor nature of the title compound. The dielectric relaxation obeyed the Vogel-Fulcher relation wherein and . The relaxor behavior was attributed to the local polar ordering on A-sites.     

Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric applications.     

Low field magnetocaloric effect and heat capacity of A-site ordered NdBaMn2O6 manganite

The magnetocaloric effect (MCE) in the A-site ordered manganite NdBaMn₂O₆ is studied. The MCE in this compound has an anomalous behavior. In low magnetic fields, the abrupt transitions between the direct and inverse magnetocaloric effects are observed. In a relatively strong magnetic field , the direct and inverse effects are observed only at cooling, while the heating mode reveals only an inverse MCE. The value of the MCE ( and for  kOe) does not reach high values, but the proximity of the effects occurring at room temperatures expects the use of both effects in the magnetic cooling technology.     

Behaviour of discontinuous gold films on SrTiO3 substrates under annealing

Morphological changes of thin, discontinuous gold films on SrTiO₃ substrates, resulting from evaporation in the temperature range of 1143-1278 K, have been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). If the gold covered fraction of the surface is small, the evaporation kinetics can be related to the desorption of adatoms. Measuring the density of the gold beads and the time dependence of the effective thickness of the film as calculated from the diameter of the beads, the following parameters have been determined: the surface diffusion length of the gold adatoms, , the mass transfer surface diffusion coefficient, and the evaporation flux, .     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

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.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.     

Possible coexistence of superconductivity and magnetism in intermetallic NiBi3

NiBi₃ polycrystals were synthesized via a solid state method. X-ray diffraction analysis shows that the main phase present in the sample corresponds to NiBi₃ in a weight fraction of 96.82 % according to the refinement of the crystalline structure. SEM - EDS and XPS analysis reveal a homogeneous composition of NiBi₃, without Ni traces. The powder superconducting samples were studied by performing magnetic measurements. The superconducting transition temperature and critical magnetic fields were determined as , Oe and Oe. The superconducting parameters were , , and κ=5.136. Isothermal measurements below the transition temperature show an anomalous behavior. Above the superconducting transition the compound presents ferromagnetic characteristics up to 750 K, well above the Ni Curie temperature.     

The kinetic energy dependent effective Debye temperature for CoS2 (100)

The effective Debye temperatures of the highly spin-polarized material CoS₂ were measured using temperature dependent low energy electron diffraction and shown to be dependent upon electron kinetic energy. The normal dynamic motion of the (100) surface results in the effective surface Debye temperature of compared to a bulk Debye temperature of . Similar values for the bulk Debye temperature have been obtained through LEED I(V) analysis and core level photoemission with a lower value for the bulk Debye temperature found from heat capacity measurements.     

On photoluminescence in HgGa2S4

In this paper, we investigate the kinetics of photoluminescence in excited crystals of HgGa₂S₄ which have recently been proposed for implementing tunable luminescent devices. From photoluminescence experiments, performed at various temperatures and excitation powers, it appears that two kinds of radiative recombination processes take place during crystal excitation. These originate two bands in emission spectra which were resolved by means of a fitting procedure. The dependencies of these bands on temperature and excitation power density are explained by means of a specific kinetic model. A broad band, peaking at about 1.8 eV, is ascribed to electron-hole tunnel recombinations occurring in associated donor-acceptor pairs, according to a Prener-Williams scheme. The second narrow band, peaking at about 2.3 eV, is ascribed to electron-hole recombinations occurring in centres presenting short () and long-life () excited states. At room temperature, owing to thermally activated relaxation from short- to long-life states, these centres saturate under relatively low excitation powers. The tunability of photoluminescence is a consequence of competition between monomolecular and bimolecular recombination processes.     

Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa₂Se₄ and ZnGa₂S₄. Most of the compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry (defect stannite), has been reported. For ZnGa₂Se₄ we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group for the investigated crystals of ZnGa₂Se₄.Regarding ZnGa₂S₄ we have found three modes exclusively showing Raman activity (2A⊕1B₁), and only eight modes showing infrared as well as Raman activity (3B₂⊕5E). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa₂S₄ most likely crystallizes in space group .