NdxLa2−xCaB10O19: Synthesis and characterization

Solid solutions of Nd_xLa_2−xcaB₁₀O₁₉ with different Na³⁺ concentration have been synthesized by substituting Nd ³⁺ for La³⁺ in La₂CaB₁₀O₁₉ Powder X-ray diffraction analysis shows that Nd³⁺ is easy to incorporate into the crystal. Single crystal nd_xLa_2−xCaB₁₀O₁₉ (NLCB) in centimeter size has been grown by Kyropoulos method. The crystal has strong absorption around 580nm and 805nm. The fluorescence spectra indicate that there is an energy transition at 1.06μm. And the SHG of NLCB is about the twice as that of KDP. These favorable features make NLCB a candidate for laser NLO multifunctional materials.     

Electrical and structural properties of y(MnxFe3−xO4)−(1 − y)B2O3 glasses between 400 and 1000 K

The structure of silica glass was investigated by high-resolution electron microscopy. Observed dark field images were compared with those calculated by a kinematical electron diffraction theory on the basis of random and crystallite models. Bright small spots with sharp contrast were observed in the dark field image, which were found to originate from crystallites. It was indicated that small crystallites of about 17 Å in size were present in silica glass.     

Hg-rich liquid-phase epitaxy of Hg1−xCdxTe

Over the past decade, liquid-phase epitaxy (LPE) has become an established technique for the growth of HgCdTe. This article reviews one of the successful LPE technologies developed for HgCdTe, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions.

In spite of the relatively low solubility of Cd in Hg-rich solutions and the relatively high Hg pressure at the usual growth temperatures, this approach has been found to offer superior results for growth of HgCdTe suitable for various compositions and layer structures.

An historical perspective and the current status of VLPE technology are presented. Particular emphasis is placed on the important role of the thermodynamic parameters (phase diagram), on control of stoichiometry (defect chemistry) and on impurity doping (distribution coefficient) for growth of HgCdTe layers from Hg solutions. Critical material characteristics, such as transport properties, minority-carrier lifetime, morphology and crystal structure, are also discussed.     

EPR study of RLi2O.V2O5, RNa2O.V2O5, RCaO.V2O5, and RBaO.V2O5 modified vanadate glass systems

Experimental EPR spectra in several modified vanadate glass systems reveal hyperfine structure (hfs) lines whose widths vary with the molar ratio of modifier to vanadium pentoxide, R. In the RNa₂O.V₂O₅ system, for example, hfs lines show no resolution at low R values (near 0.1); by contrast, these lines exhibit dramatic narrowing as R approaches 0.5. In the model proposed here, this narrowing is due to an increase in hopping time for small polarons associated with V⁴⁺ ions in these systems. Increases in polaron hopping times are accompanied by increases in electron spin-spin relaxation times T₂'s, and, an associated narrowing of EPR linewidths. Experiments confirm that spectral widths are limited by electron T₂'s due to the fact that EPR linewidths do not vary with temperature down to 4.2 K. Resolved spectra in RNa₂O.V₂O₅ at R = 0.5 reveal a hyperfine coupling parameter of 0.0177 ± 0.0008 T, corresponding to an upper-limit polaron hopping frequency of 487 ± 20 MHz. By similar analyses, the systems of RCaO.V₂O₅, RBaO.V₂O₅, and RLi₂O.V₂O₅ exhibit comparable polaron hopping frequencies limits of 480 ± 20 MHz, 469 ± 20 MHz, and 468 ± 20 MHz, respectively, when R is near 1.0. In addition to the relaxation effects discussed here, results of modeling of resolved spectra to obtain hyperfine coupling constants A_|| and A_┴, and g_┴ values g_|| and g_┴ are presented and discussed.     

Contact-limited conduction in V2O5P2O5 glasses

The electrical properties of V₂O₅P₂O₅ glass-electrode interfaces have been investigated. Gold, graphite and aluminium, in that order, form contacts of increasing resistance. The dependence of barrier height on the work function of the electrode material is not consistent with the concept of Schottky barriers at the interfaces. The phenomenon may be explained by the downward bending and filling of a narrow band normally situated in the vicinity of the Fermi level, caused by injection of electrons from the less noble metals. This gives rise to a surface layer, which can, in principle, be insulating. However, alternative mechanisms, in the case of the investigated glasses most probably a Poole-Frenkel type field enhanced conduction via separated traps, lead to finite interface resistance.     

Preparation of silica microspheres coated with V2O5 xerogel and V2O5/WO3 composite xerogel via adapted Stöber process

In the present study, we have synthesized spherical silica particles covered with either layered V₂O₅ xerogel or V₂O₅/WO₃ mixed xerogel via an adapted Stöber method using an ammonia solution of methyltriethoxysilane (MTES). The SEM images revealed the presence of a lamellar V₂O₅ xerogel layer coating the silica particles, as evidenced by the (0 0 1) diffraction lines. Both materials have voltammetric behavior that is quite similar to that of the V₂O₅ xerogel. The catalytic activity of the silica particles coated with the V₂O₅/WO₃ mixed xerogel was evaluated in the epoxidation of styrene using iodosylbenzene as an oxidant.     

DC conduvtivity of V2O5TeO2 glasses

The dc conductivity of semiconducting vanadium tellurite glasses of compositions in the range 50 to 80 mol% V₂O₅ has been measured in the temperature region 77 to 400 K. Measurements have been made on annealed samples at different annealing temperatures. Annealing the samples at temperature of about 250°C causes the appearance of a complex crystalline phase resulting in an increase of conductivity. Results are reported for amorphous samples of different compositions. The conductivity of tellurite glasses is slightly higher than the corresponding composition of phosphate glasses, but the general trend of the increase of conductivity and decrease of high temperature activation energy with increasing V₂O₅ content is similar in the two systems. The data have been analysed in the light of existing models of polaronic hopping conduction. A definite conclusion about the mechanics of conduction (adiabatic or nonadiabatic) is difficult in the absence of a precise knowledge of the characteristic phonon frequency v₀. Adiabatic hopping is indicated for v₀～10¹¹ Hz, however this value leads to unreasonably low value for the Debye temperature θ_D, and higher values for v₀～10¹³ hz satifiies the conditions for nonadiabatic hopping which appears to be the likely mechanism of conduction in V₂O₅TeO₂ glasses. The low temperature data (< 100 K) can be fitted to Mott's variable range hopping, which when combined with ac conductivity data gives reasonable values of α, but a high value for the disorder energy.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

Solubility of YBa2Cu3O7−δ and Nd1+xBa2−xCu3O7±δ in the BaO/CuO flux

The knowledge of the phase relations and solubilities in the Y–Ba–Cu–O and Nd–Ba–Cu–O systems are of fundamental importance for crystal growth and liquid-phase epitaxy of YBa₂Cu₃O_7−δ (YBCO) and Nd_1+xBa_2−xCu₃O_7±δ (NdBCO). The determination of the solubility curve of YBCO and NdBCO in a BaO/CuO flux containing 31 mol% BaO was done by observation of the formation and dissolution of crystals on the surface of the high-temperature solution. The heat of the solution of YBCO at 1000°C was found to be 34.7 kcal/mol, and for NdBCO at 1060°C, it was found to be 28.1 kcal/mol. The determination of the solubility curves requires special care, and the problems of the time-dependent shift of the solution composition due to the corrosion of the crucible is discussed. The scatter of the solubility data published by different authors could be due to the use of solutions with different Ba : Cu ratios, different determination methods, i.e. different crystallization mechanisms, different crucibles and starting chemicals.     

The morphology of α-Al2O3 and α-Fe2O3: The importance of surface relaxation

Current methods for predicting crystal morphology from structure ignore the fact that real crystal surfaces are not necessarily exact terminations of the bulk structure. Instead, surfaces relax and in so doing minimise the total surface energy. This lowering of the surface energy and its influence on crystal morphology are here calculated and illustrated for α-Al₂O₃ (corundum) and α-Fe₂O₃ (haematite). The results are compared with previous studies based on Hartman-Perdok theory.     

The specific heat capacities of V2O5/P2O5-based glasses

The specific heat capacities of a series of V₂O₅/P₂O₅-based glasses have been measured in the approximate temperature range 3–300 K. The proportions of the host constituents were varied and the effects produced by adding oxides of copper, titanium and sodium were examined. The measurements were extended to investigate the influence of fast neutron irradiation upon the specific heat capacity of pure, initially crystalline V₂O₅.The observations are consistent with a spectrum of background vibrations, the form of which is temperature dependent. At intermediate temperatures the vibrations were consistent with the glasses displaying predominantly chain-like characteristics. With reduction to lower temperatures the vibrational behaviour increasingly resembled that of solids displaying three-dimensional characteristics. The temperature dependence of the specific heat capacity at the lowest temperatures was consistent with the presence of a narrow band of low-frequency modes centred upon 1.67 × 10¹¹ s^?1, the density of which increased with phosphorus content. From observations of the density of these modes in the doped specimens, it was concluded that the role of copper wass largely that of a network former, whereas titanium and sodium were essentially network modifiers.     

Growth twins in self-frequency doubling laser crystal YbxY1−xAl3(BO3)4

Using chemical etching.method, the growth twins in self-frequency doubling laser crystal Yb_xY_1−xAl₃(BO₃₄ have been observed. The etching pits on both sides of growth twin boundaries in the (10 1) slice are of the triangles with different orientations. The structure of growth twins is investigated by transmission synchrotron topography. In the transmission synchrotron topograph, the growth twins are visible not by ‘domain contrast’ but by ‘boundary contrast’, i.e. the twins appear in the topograph in form of X-ray kinematical diffraction contrast due to the lattice strain stemming from the impurity incorporation in the boundaries. The growth twins in Yb_xY_1−xAl₃(BO₃)₄ crystal are of inversion types, since no domain contrast was observed.     

Characterisation of semiconducting V2O5–Bi2O3–TeO2 glasses through ultrasonic measurements

Tellurite containing vanadate (50−x)V₂O₅–xBi₂O₃–50TeO₂ glasses with different bismuth (x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi₂O₃ content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi₂O₃ content. The observed results confirm that the addition of Bi₂O₃ modifier changes the rigid formula character of TeO₂ to a matrix of regular TeO₃ and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO₄ trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi₂O₃ content are more stable than with high Bi₂O₃ content.