Three- and two-dimensional topological insulators in Pb2Sb2Te5, Pb2Bi2Te5, and Pb2Bi2Se5 layered compounds

The electronic structure of ternary compounds Pb₂Sb₂Te₅, Pb₂Bi₂Te₅, and Pb₂Bi₂Se₅, which have a layered structure that consists of nine-layer atomic blocks separated by van der Waals gaps, has been theoretically studied. It has been shown that all studied compounds are three-dimensional topological insulators. The possibility of the existence of a two-dimensional topological insulator has been found in ultrathin (0001) Pb₂Sb₂Te₅ and Pb₂Bi₂Te₅ films. Oscillations of the ℤ₂ topological invariant with an increase in the film thickness have been observed in the latter compound.     

Multicomponent GeSe2-CdI2-TeO2 (Bi2O3) systems: glass formation and properties

The glass-forming regions of the GeSe₂-CdI₂-TeO₂ (I), GeSe₂-CdI₂-Bi₂O₃ (II) and GeSe₂-TeO₂-Bi₂O₃ (III) systems have been determined. The obtained glassy phases have been characterized by their basic physicochemical parameters such as temperatures of glass transition, crystallization, and melting, density and microhardness. The phase T-X diagram of the GeSe₂-CdI₂ system, which is the basic joint line for systems I and II, has been specified. Three non-variant equilibria (two eutectic and one syntectic) have been observed at temperatures 350, 280 and 375 °C for compositions containing 15, 95 and 33.3 mol% GeSe₂, respectively. A new intermediate phase with probable composition of 2CdI₂·GeSe₂ has been formed.     

Opto-galvanic spectroscopy of some molecules in discharges: NH2, NO2, H2 and N2

The change of the discharge voltage when laser light crossing the discharge is tuned to a molecular transition has been measured. Experiments have been performed in the wavelength region between 570 nm and 620 nm with discharges in NH₃, NO₂, H₂, N₂, O₂ and argon. Transitions from the ground states of NH₂ and NO₂ and transitions from metastable states of N₂ and H₂ have been detected. The spacial dependence of the opto galvanic in a low pressure dc-discharge of H₂ and N₂ has been studied.     

Photo-synthesis of As3S2 crystal from AsAs2S2 mixture

Existence of As₃S₂ compound which is stable in a certain temperature range higher than room temperature has been confirmed in AsAs₂S₂ system. Photo-synthesis effect of As₃S₂ crystal from As and As₂S₂ mixture, namely light induced conversion from a crystalline state to another crystalline state, has also been investigated.     

非晶态锂离子导体B2O3-0.7Li2O-0.7LiCl-xAl2O3

木文研究了Al₂O₃对B₂O₃-0.7Li₂O-0.7LiCl非晶态的形成和电学性能的影响,我们发现:加入适量的Al₂O₃后,无需借助液氮骤冷技术,直接将熔体倾倒在室温下的紫铜板上就很容易形成大块非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃。Al₂O₃的加入使B₂O₃-0.7Li₂O-0.7LiCl的电导率有所降低,但在高温下不太明显,电导激活能略微升高,实验发现:Al₂O₃含量x=0.03是较合适的剂量,较容易形成大块非晶态,对电导率的影响也不大。 关键词：     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Emission analysis of Dy and Pr:Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses

In this paper, we present the spectral results of Dy³⁺ and Pr³⁺ (1.0 mol%) ions doped Bi₂O₃-ZnF₂-B₂O₃-Li₂O-Na₂O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy³⁺:glass has shown two emission transitions ⁴F_7/2→⁶H_15/2 (482 nm) and ⁴F_7/2→⁶H_13/2 (576 nm) with an excitation at 390 nm wavelength and Pr³⁺:glass has shown a strong emission transition ¹D₂→³H₄ (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy³⁺ and Pr³⁺ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.     

Theoretical investigation of optical spectra and covalent effect of Cr in Y2Ti2O7 and Y2Sn2O7

In this work, the complete matrix of optical spectral levels in trigonal symmetry of 3d² (3d⁸) ions are established on basis of strong field coupling mechanism by using two spin–orbit coupling parameters model. The contribution of the spin–orbit coupling of ligand to the optical spectra has been included in these formulas. As an application, the optical spectra of Cr⁴⁺ in Y₂Ti₂O₇ and Y₂Sn₂O₇ have been studied by the complete diagonalization (energy matrix) method. The covalent effect has been studied and the difficulty about Dq parameter in explanation of optical spectra of Cr-doped Y₂Ti₂O₇ and Y₂Sn₂O₇ is removed. The theoretical results are in good agreement with observed data.     

Crystal field interactions in DyFe2Si2 and DyFe2Ge2

Two sets of crystal field (CF) parameters have been proposed for DyFe₂Si₂, none of which could provide a simultaneous explanation of the available experimental data, particularly at low temperatures (below 100 K). The set derived from magnetic studies could not even explain the thermal variation of the magnetic specific heat reported in the same work. Although the set of CF parameters, obtained from a fit to the Mossbauer spectra, could provide a fairly good explanation of the thermal variation of the magnetic susceptibilities along the c-axis, it could not explain the observed thermal variation of other reported experimental findings. In the present work, an appraisal of the CF parameters proposed earlier has been done and a set of CF parameters has been derived, which provide a simultaneous explanation of all the available experimental data. The effect of substitution of Ge for Si on the magnetic properties and the magnetic specific heat of DyFe₂Si₂ has been studied in the framework of one electron crystal field model. The inelastic neutron scattering studies and EPR measurements are required to check the predicted Stark energies and the paramagnetic resonance g-values.     

Ionic conductivity of sodium zinc silicates in the compositional region Na2ZnSiO4-Na2ZnSi2O6

The compositional region Na₂ZnSiO₄-Na₂ZnSi₂O₆ has been investigated by means of X-ray powder diffraction measurements on samples quenched from elevated temperatures. Four different phases have been observed. The powder pattern of the high and low temperature modification of Na₂ZnSiO₄ could thus be indexed on the basis of an orthogonal cell, while for Na_1.6Zn_0.8Si_1.2O₄ a cubic unit cell could be used. These phases are all structurally related to high-cristobalite. The powder pattern of the compound Na₂ZnSi₂O₆ has been indexed on the basis of a monoclinic unit cell. The variation of the ionic conductivity x in Na_xZn_0.5xSi_2-0.5xO₄ (1.25 ? x ? 2) has been determined. The best conductivity, 1 × 10^-2 (Ω cm)^-1 at 600 K is found for the composition Na_1.85Zn_0.925Si_1.075O₄.     

Third-order nonlinear optical properties of GeSe2-Ga2Se3-PbI2 glasses

The third-order nonlinear optical (NLO) properties of new selenium-based GeSe₂-Ga₂Se₃-PbI₂ glasses have been measured using the optical Kerr effect (OKE) technique, with picosecond and femtosecond laser pulses. The 0.70GeSe₂-0.15Ga₂Se₃-0.15PbI₂ glass has the largest third-order optical nonlinear susceptibility in GeSe₂-Ga₂Se₃-PbI₂ glass system with χ⁽³⁾ of 5.28×10¹² esu. In addition, the response time of glasses is sub-picosecond, which is predominantly associated with electron cloud. Local structure of the glasses has been identified by using Raman studies, while the origins of the observed nonlinear optical response are discussed. The [Ge(Ga)Se₄] tetrahedral and lone-pair electrons from highly polarizable Pb atom in glasses play an important role in enhanced NLO response. These results as well as their good chemical stability indicate that GeSe₂-Ga₂Se₃-PbI₂ glasses are promising materials for photonic applications of third-order nonlinear optical signal processing.     

Effect of Bi2O3 content on physical and optical properties of 15Li2O-15K2O-xBi2O3-(65−x) B2O3: 5V2O5 glass system

Multi-component bismuth borate glasses doped with vanadium ions 15Li₂O-15K₂O-xBi₂O₃-(65−x) B₂O₃: 5V₂O_5, (x=3, 5, 7, 10, 12 and 15) have been prepared using conventional melt quench technique. Characterization of the prepared glasses has been done using X-ray diffraction, differential scanning calorimetry and density measurements. The effect of Bi₂O₃ content on the optical properties of the present glass system is studied from the optical absorption spectra recorded in the wavelength range 200-800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The origin of the Urbach energy is associated with the phonon-assisted indirect transitions. The density and molar volume studies indicate that Bi₂O₃ in these glasses is acting partly as network modifier and partly as network former. The variations in the optical band gap energies, density and molar volume with Bi₂O₃ content have been discussed in terms of changes in the glass structure. Values of the theoretical optical basicity, average crosslink density and the average electronic polarizability are also reported.     

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.     

Long-lasting afterglow in Tb-doped SiO2-Ga2O3-CaO-Na2O glasses and its sensitization by Yb

Long-lasting afterglow due to Tb³⁺ ions has been observed in a Tb³⁺-doped SiO₂-Ga₂O₃-CaO-Na₂O glass, where a 4s empty orbital of Ga³⁺ probably works as an electron-trapping center. The sensitization effect of Yb³⁺ on the afterglow has been noticed.     

The ν2 fundamental vibration-rotation band of T2O

The ν₂ fundamental vibration-rotation band of T₂O vapor has been measured at grating resolution, and the rotational structure has been analyzed. The band center and the values of the rotational constants A, B, and C for the ground state and excited state have been determined. These values are consistent with the data for J through 6, and with extrapolation from H₂O and D₂O.     

The magnetic structure of U2N2Te

The magnetic structure of tetragonal U₂N₂Te has been studied by means of neutron diffraction on polycrystalline sample. A ferromagnetic alignment of the magnetic moments below 68 K has been confirmed. The best agreement between the calculated and observed intensities of the magnetic reflections has been obtained for the moment direction forming an angle 70 ± 5° to the tetragonal axis. The magnitude of the uranium ordered moment was found to be 2.50 ± 0.05 μ_B.     

Millimeter- and Submillimeter-Wave Spectra of CD2F2

The millimeter- and submillimeter-wave spectrum of ¹³CD₂F₂ present in natural abundance in methylene fluoride-d₂ (CD₂F₂) has been measured in the region 230-380 GHz. The spectrum was recorded using a frequency-modulated millimeter- and submillimeter-wave spectrometer. More than 200 rotational transitions in the ground state of ¹³CD₂F₂ with J≤45 and K_a≤8 have been assigned. A combined weighted least squares fit of the newly assigned transitions with previously reported microwave data has been carried out in the Watson's A- and S-reduced Hamiltonian. The data have been fitted with a standard deviation approaching the experimental accuracy, to provide improved values for the rotational and quartic centrifugal distortion constants, including sextic distortion constants for the ground state of ¹³CD₂F₂.     

Magnetic ordering in the garner NaCa2Mn2V3O12

The magnetic structure of the garnet NaCa₂Mn₂V₃O₁₂ with Mn²⁺ ions at 16a positions has been determined by neutron diffraction. Also measurements of Young's modulus and the inverse susceptibility have been made. The magnetic ordering is the same as in the corresponding Co, Ni, Cr-garnets.     

Optical absorption and thermoluminescence properties of ZnF2-MO-TeO2 (MO=As2O3, Bi2O3 and P2O5) glasses doped with chromium ions

Optical absorption, thermoluminescence, infrared spectra and differential thermal analysis of three different tellurite glass systems viz., ZnF₂-As₂O₃-TeO₂, ZnF₂-Bi₂O₃-TeO₂ and ZnF₂-P₂O₅-TeO₂ containing 0.4% of Cr₂O₃, have been investigated. Results have been analysed in the light of different oxidation states of chromium ion and the most suitable host for lasing Cr³⁺ ions has been identified and reported.     

Structural information from EPR powder spectra of Mn(II)-doped Rb2MgCl4, Rb2MgCl3Br and Rb2MgCl2Br2

The structure of the compounds Rb₂MgCl₄, Rb₂MgCl₃Br and Rb₂MgCl₂Br₂ has been determined to be the tetragonal K₂NiF₄ structure. EPR spectra at X and Q-band frequencies of the polycrystalline samples of the Mn(II)-doped compounds, indicate that the Br^? ions prefer to order at special sites of the structure, as concluded from the fact that only octahedral groups [MgCl₆] and tetragonal groups [MgCl₄Br₂] are found.