Luminescence of Cu ions in phosphate glass

Time-resolved fluorescence and lifetime measurements of Cu⁺ ions in phosphate glass with different copper concentrations have been performed at room temperature and 77 K. The experimental results are explained by energy transfer.     

Estimation of inter-atomic force constants and phonon dispersion using correlation effects among thermal displacement of atoms in Ge

Neutron diffraction measurements have been performed on powder Ge at 6, 150 and 300 K. Oscillatory diffuse scattering intensity is clearly observed at 150 and 300 K. The diffuse scattering theory including correlation effects among thermal displacements of atoms is applied to background function in the Rietveld analysis. The inter-atomic distance and temperature dependence of the values of correlation effects are discussed. The force constants among the first, second and third nearest neighboring atoms in Ge are obtained from the values of correlation effects and Debye-Waller temperature parameters. A phonon dispersion spectrum in Λ [111] direction for Ge crystal has also been calculated at 80 K using inter-atomic force constants and the crystal structure.     

Calorimetric investigation of the structural relaxation and crystallization of amorphous Ge16Te84 alloys

Differential scanning calorimetric measurements have been performed on amorphous Ge₁₆Te₈₄ alloys; the study of the dependence of the cooling rate on the reciprocal fictive temperature yields the activation energy of the relaxation process; this value fits well that deduced from shear viscosity measurements. The enthalpy difference between the supercooled liquid and the crystal has been investigated by drop calorimetry on samples annealed at different temperatures below T_g, during different times. These results, combined with the former obtained by DSC lead to the enthalpy values of the metastable liquid system from 390 to 510 K.     

Photoemission (UPS and XPS) study of local structures of amorphous GeTe and GeSe films

In situ photoemission (UPS and XPS) measurements have been performed for amorphous GeTe and GeSe films deposited onto a cooled substrate during thermal annealing and crystallization of the films. It has been found that an amorphous film prepared at room temperature has a 4-2 coordinated local structure while a highly disordered film deposited onto a 77 K substrate is largely 3-3 coordinated and relaxes into the 4-2 coordinated structure upon thermal annealing.     

Paramagnons in the PrNi system with an induced magnetic moment

The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (T _C ～ 21 K), softening of some part of magnetic excitations occurs near the Γ point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchang interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition.     

Electrical Resistivity and Thermally Stimulated Current in CuIn5Se8

From electrical resistivity measurements in the range from 240 to 450 K and from optical absorption measurements the energy gap value of CuIn₅Se₈ has been found to be 1.13 eV. Thermally stimulated current and electrical measurements at high temperature performed in indium annealed samples show deep levels at 0.55 and 0.79 eV, respectively. These defects are expected to be associated with interstitial indium or indium in copper site because of the In‐rich condition.     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

The solidification behavior and phase equilibrium of o-Nitroaniline-β-Naphthol binary eutectic system

Phase diagram studies of o-nitroaniline-β-naphthol system have been made by the thaw melt method. Simple eutectic type phase diagram was formed. Formation of eutectic mixture occurred at 0.7222 mole fractions of o-nitroaniline and melted at 326.85 K. The heat of fusion of components and eutectic mixtures were determined with the help of DSC technique and excess thermodynamic functions were calculated. Experiments were performed to study the undercooling, microstructure and linear velocities of crystallization. Flexural strength measurements were also made. Eutectic mixture showed higher flexural strength as compared to the components.     

Electrical conductivity of sulfamic acid single crystals

Single crystals of sulfamic acid have been grown by the method of slow evaporation at constant temperature. DC electrical conductivity was measured in the temperature range 300 ‐ 440 K along a, b and c‐axes. Conductivity measurements show slope change near 330 K and 410 K. The slope change observed around 330 K may be attributed as due to a phase transition which has been well supported by the DSC and DTA measurements. Slope change observed around 410 K is attributed as the onset of the thermal decomcoposition as evidenced by TGA curve. TGA studies show the crystal is very stable up to 440 K. Activation energies for the conduction process are calculated for all measured crystallographic directions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on a new Rb3H9(PO4)4 crystal

A new type of trirubidium nonahydrogen tetraphosphate, Rb₃H₉(PO₄)₄, crystal has been crystallized. Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on the title compound. The crystal does not undergo any structural phase transition in the temperature range from 120 to 440 K except a melting transition at 399 K. The compound belongs to a monoclinic system with space group C2/c at room temperature. The structure is built on a framework of PO₄ tetrahedra linked by five types of OH‐O hydrogen bonds in the length range from 2.472(4) to 2.679(4) Å. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Manifestation of microcrystalline structure in transport properties of p‐quinquophenyl films

The electrical conductivity measurements of the p‐quinquephenyl films possessing microcrystallite structure are done at temperature between 10 K–300 K. The investigated film thickness varies within 0.2–2.5 µm. The measurements were performed with Al(Au) electrodes for the films deposited by spin coating in argon atmosphere on glass substrates. The molecular dynamics simulations as well as quantum chemical calculations have shown that three different mechanisms contribute to the observed electrical feature, which are: the conductivity originated from proper crystalline states; this of the amorphous‐like inter‐grain region and the one created in the grain boarder interfaces. Comparing the experimental data and theoretically calculated dependencies, we have shown that the main contribution to the observed DC conductivity of this material results from the interfaces located at the borders of polycrystalline grains, although they possess a relatively low part of the total film volume. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural study on a new Cs2S2(SO4)3 crystal

A new type of dicesium disulfur trisulfate, Cs₂S₂(SO₄)₃, crystal has been crystallized. Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on this compound. The crystal does not reveal any structural phase transition in the temperature range from 120 to 920 K. The compound belongs to a cubic system with space group P2₁3 at room temperature. It is found that both Cs and S atoms located on special positions are 93% occupied and 7% in exchange with each other. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vibrational entropy near glass transition in a chalcogenide glass and supercooled liquid

Raman spectroscopic measurements have been performed on Ge₂₀Se₈₀ glass and supercooled liquid at temperatures ranging between 298 and 500 K. Temperature dependent softening of vibrational mode frequencies has been used in conjunction with the available vibrational density of states data at ambient temperature to estimate the relative contributions of vibrational and configurational entropies across glass transition. Nearly 20% of the additional entropy above glass transition is estimated to be vibrational. Thermal expansion effect on vibrational mode softening is found to be insufficient to account for the anharmonic component of vibrational entropy implying possible coupling between the vibrational and configurational entropies at temperatures above T_g. These results may have important consequences in shaping our understanding of various aspects of glass transition.     

New Structural Phases of Polymer Battery Anode Materials: Alkali-Metal-Doped Polyacetylene and Polyphenylene

New ordered phases of alkali-metal complexes of polyacetylene and poly(p-phenylene) have been identified by x-ray diffraction studies and by quasi-equilibrium measurements of potential vs. composition during electrochemical reduction with Li+, Na+, and K+ counterions.     

GaN films studied by near-field scanning optical microscopy, atomic force microscopy and high resolution X-ray diffraction

Spatially resolved photoluminescence measurements from GaN films were achieved using a near-field scanning optical microscope (NSOM). We have studied GaN films grown by metalorganic vapor phase epitaxy on sapphire substrates. Spatial scans of topography, band-edge and yellow luminescence have been performed with submicron spatial resolution. Spatial variations in the photoluminescence characteristics are clearly observed at the submicron scale. Measurements by atomic force microscopy and high resolution X-ray diffraction were also performed and compared with the NSOM measurements.     

Crystal structure and phase transition of single crystalline CsNH2SO3

Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on cesium sulfamate CsNH₂SO₃ single crystal. Two distinct endothermic peaks in the DSC curves are observed at 330 and 436 K. It is pointed out that the peak at 330 K is attributed to the structural phase transition, and the other peak at 436 K is associated with the thermal decomposition of the crystal. The structures in room‐ and high‐temperasture phases are determined, and the space group of the sample crystal is found to change from monoclinic P 2₁/c to orthorhombic Pnma. The structure of the room‐temperature phase consists of two different types of N‐H···O hydrogen bond, but in the high‐temperature phase there is no specific hydrogen bond between the NSO₃ pseudo‐tetrahera. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of optical parameters of Ga0.75In0.25Se layered crystals

The optical properties of the Ga_0.75In_0.25Se crystals have been investigated by means of transmission and reflection measurements in the wavelength range of 380–1100 nm. The analysis of the results performed at room temperature revealed the presence of optical indirect transtions with band gap energy of 1.89 eV. The variation of the band gap energy as a function of temperature was also studied in the temperature range of 10–300 K. The rate of change of band gap energy (γ = –6.2 × 10^–4 eV/K) and absolute zero value of the band gap (E_gi(0) = 2.01 eV) were reported. The wavelength dependence of the refractive index was analyzed using Wemple and DiDomenico, Sellmeier and Cauchy models to find the oscillator energy, dispersion energy, oscillator strength and zero‐frequency refractive index values. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Metamagnetic phase transition in Nd1−x TbxCo2 compounds

The electric resistivity and neutron diffraction measurements of the intermetallic compounds Nd_1?x Tb_xCo₂ (x = 0, 0.2, 0.3, 0.5, 0.7, or 1) have been performed. All compounds have a cubic structure at room temperature and a rhombohedral structure at 4.2 K. The magnetic structure of these compounds is described by the wave vector k = 0. The magnetizations of the rare earth and cobalt sublattices increase with increasing x. The concentration dependence of the cobalt sublattice magnetization is explained on the basis of the model of metamagnetic band subsystem in compounds of the RCo₂ type.     

Time-resolved fluorescence spectra of titanium-containing fluorophosphate glass

The visible luminescence due to Ti³⁺ ions in titanium-containing fluorophosphate glass has been observed with UV excitation. Time-resolved fluorescence and lifetime measurements have also been performed at room and low temperature, respectively.     

Electrical and optical properties of natural and synthetic melanin biopolymer

Natural and synthetic melanin have been investigated by means of optical, electrical and photoelectronic measurements. Optical measurements evidence absorption curves which allow to estimate optical gap for synthetic melanin, by using Tauc’s method. Dark conductivity and photoconductivity measurements were performed as a function of temperature and for different duration of thermal treatments. It has been evidenced that both quantities are thermally activated and thermal treatments play a very important role as far as gap states are concerned.