Surface crystallization and second-order optical non-linearity in Gd2O3–Bi2O3–B2O3 glasses

Some ternary Gd₂O₃–Bi₂O₃–B₂O₃ glasses are prepared, and crystallization behavior and second harmonic intensity are examined to develop new non-linear optical crystallized glasses. The glasses with Gd₂O₃ contents of 8–14 mol% have large densities of over 6 g/cm³ and large refractive indices of ∼1.9. Transparent surface crystallized glasses consisting of two kinds of crystalline phases with different morphologies, i.e. plate shape and needle shape crystals, are fabricated by heat-treatment at temperatures between glass transition and crystallization temperatures. From second harmonic generation microscope observations, micro-Raman scattering spectra and XRD analyses, plate shape crystals are determined to be non-linear optical Gd_xBi_1−xBO₃ and needle shape crystals are Bi₃B₅O₁₂ having no second-order optical non-linearity. Since crystallized glasses consisting of Gd_xBi_1−xBO₃ crystals exhibit relatively strong SHGs, they have a high potential for application to light control devices.     

Derivatives of α-zirconium phosphate with two different functional groups

The syntheses of derivatives of α-zirconium phosphate having the general formula Zr(RPO₃)_x(R′PO₃)_2−x (where R and R′ are generally organic radicals, but may also indicate −H or −OH groups) have been achieved by precipitating a mixture of two phosphonic acids with a zirconium salt in the presence of hydrofluoric acid. Different mixed compounds, classified in the following three classes

• 1.(a) Zr (HPO₄)_x (R′PO₃)_2−x (R′ = −H; −C₆H₅; −CH₂CH ₂COOH)
• 2.(b) Zr (RPO₃)_x (R′PO₃)_2−x (R′ = −CH₂COOH;R′ = −CH₂OH)
• 3.(c) Zr (HPO₃)_x (R′PO₃)_2−x (R′ = −CH₆H₅; −CH₂OH; −CH₂COOH; −CH₂CH₂COOH)

were prepared and characterized with regard to their compositions, X-ray powder patterns and densities. It was found that the system is discontinuous, not all x values from 0 to 2 being possible. No single crystals for X-ray structure determination were obtained; however, some chemical evidence shows that the mixed compounds possess a layered structure similar to that of α-zirconium phosphate. Some considerations on the reciprocal disposition of the R and R′ pendent groups in the layered structure are reported. Many other derivatives are expected to be obtained easily by using other mixtures of phosphonic acids than those employed in this work. Owing to the large variety of possible combinations of R and R′ groups, the mixed compounds should exhibit a large spectrum of properties and show promise for application in the field of ion exchange, adsorption, intercalation and heterogeneous catalysis.     

Monophase domain, fibers single crystals grown by the micro-pulling down technique and optical characterisation of LiGd1−xYbx(WO4)2

We have determined the single phase domain of LiGd_1−xYb_x(WO₄)₂. The lattices parameters decrease as a function of Yb³⁺ substitution in Gd³⁺ sites. Transparent LiGd_1−xYb_x(WO₄)₂ fibers single crystals were successfully grown by the micro-pulling down technique (μ-PD). The Yb³⁺-doped LiGd(WO₄)₂ fibers single crystals have been pulled under stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to 120 μm. The fibers diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Fibers single crystals free of defects are observed for Ytterbium concentration in the melt up to 5 at%. Above this limit, inclusions and cracks appear and the optical quality of the fibers were deteriorated. The emission spectra of Yb³⁺-doped LiGd(WO₄)₂ were investigated.     

Unusual structural properties of (Na0.5La0.5)MoO4:Er,Ce crystals

The structure of single crystals grown by the Czochralski technique from (Na_0.500La_{0.495 ? x} Ce_xEr_0.005)MoO₄ (x = 0.10, 0.125, 0.15, 0.175, 0.20) melts in a weakly oxidizing atmosphere (99–99.5% N₂ + 0.5–1% O₂) (NLM-0) was studied by x-ray diffraction. Some of the crystals received short-term annealing at 1000°C (NLM-2) and/or long-term annealing at 700°C (NLM-1). The oxygen in the structure of NLM:Er,Ce-0 and NLM:Er,Ce-1 with x = 0.20 was found to occupy two crystallographic sites at random. In the structure of the NLM:Er,Ce-2 crystal with x = 0.20, the oxygen atoms are fully ordered into one site, as in the scheelite structure. It cannot be ruled out that these crystals contain both Ce³⁺ and Ce⁴⁺ ions.     

CMR and related properties of single crystals of cation-deficient LaMnO3

Single crystals of LaMnO₃, La_1−xMnO₃ (x=0, 0.04, 0.1) and LaMn_1−xO₃ (x=0.1) have been grown by the floating-zone melting technique. Electrical and magnetic properties of these crystals have been examined. Only La_0.9MnO₃ crystals exhibit ferromagnetism, (T_c∼250 K), an insulator–metal transition around 250 K and CMR (58% at T_c).     

Magnetic properties and NMR studies of the SrAlM hexagonal ferrite system

The magnetic parameters for single crystals of SrAl_xFe_12−xO₁₉ (x=0, 0.8) were measured at 77 and 295 K. The nuclear magnetic resonance (NMR) experimental data of ⁵⁷Fe nuclei in domains of these samples were determined in a wide range of temperatures using the spin-echo technique. Influence of substitution of Fe³⁺ ions by Al³⁺ ions on magnetic properties have been studied with the aid of NMR spectra of ⁵⁷Fe nuclei. It is shown that only 82% of Al³⁺ exist in b-sublattice (site 2a) and 18% of Al³⁺ ions are statistically distributed in the other sublattices for SrAl_xFe_12−xO₁₉. The percentage 82% Al³⁺ in sites 2a experimentally increases the value of anisotropy constant K₁ by 0.60×10⁶ erg cm⁻³. Thus, the contribution of Fe³⁺ ion in the position 2a to the magnetic anisotropy constant of SrFe₁₂O₁₉ in low temperatures is estimated. It is shown that the value K₁(2a)=−0.90×10⁶ erg cm⁻³.     

Stability and transport properties of microcrystalline Si1−xGex films

The crystallization evolution of boron and phosphorus doped amorphous Si_1−xGe_x films (5×10¹⁷–5×10²⁰ cm⁻³), deposited on SiO₂/Si(001) substrates by molecular beam in high vacuum at room temperature, were studied by XRD, TEM and SEM. The amorphous Si_1−xGe_x films were fully crystallized at ∼600°C. Up to 800°C no morphology changes were observed. Between 800 and 950°C, voids and hillocks were gradually developed in the films, which consequently collapsed. The Hall concentration and mobility were characterized in the Si_1−xGe_x films, annealed between 600 and 800°C. The mobility and conductivity of p-Si_0.5Ge_0.5 films at room temperature were found to be relative high: 60 cm²/V s and 2000 (Ω cm)⁻¹, respectively.     

Frequency up-conversion luminescence in Yb3+–Ho3+ co-doped PbxCd1−xF2 nano-crystals precipitated transparent oxyfluoride glass-ceramics

Oxyfluoride glasses were developed with composition 30SiO₂·15AlO_1.5·28PbF₂·22CdF₂·(4.9−x)GdF₃·0.1HoF₃·xYbF₃ (x=0, 0.1, 0.2, 0.5, 1, 2, 3, and 4) in mol%. Powder X-ray diffraction analysis revealed that the heat-treatments of the oxyfluoride glasses at the first crystallization temperature cause the precipitation of Yb³⁺–Ho³⁺ co-doped fluorite-type nano-crystals of about 17.8 nm in diameter in the glass matrix. These transparent glass-ceramics exhibited very strong green up-conversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂)→⁵I₈ transition under 980 nm excitation. The intensity of the green up-conversion luminescence in the glass-ceramics was much stronger than that in the precursor oxyfluoride glass. The reasons for the highly efficient Ho³⁺ up-conversion luminescence in the oxyfluoride glass-ceramics are discussed.     

Structure study of MoO3-ZnO-B2O3 glasses by Raman spectroscopy and formation of α-ZnMoO4 nanocrystals

Molybdenum oxide (MoO₃)-containing glasses of xMoO₃-50ZnO-(50−x)B₂O₃ (x = 10, 20, and 30) are prepared using a conventional melt quenching method, and the glass structure and crystallization behaviour are clarified. It is found that the thermal stability against crystallization of the glasses decreases drastically with increasing MoO₃ content. The main valence of Mo ions in the glasses is found to be Mo⁶⁺ from X-ray photoelectron spectroscopy measurements. The Raman bands observed at ∼860 cm⁻¹ and 950 cm⁻¹ suggest that the coordination state of Mo⁶⁺ ions in the glasses is mainly (MoO₄)²⁻ tetrahedral units. All glasses examined in this study give the formation of α-ZnMoO₄ as the initial crystalline phase. In particular, 30MoO₃-50ZnO-20B₂O₃ glass shows the bulk crystallization of α-ZnMoO₄ nanocrystals with a diameter of ∼5 nm. The crystallized glasses consisting of Eu³⁺-doped ZnMoO₄ crystals are synthesized, and enhanced photoluminescence emissions (i.e., the quantum yield is 9%) due to the 4f transitions ⁵D₀ → ⁷F_J (J = 0-4) of Eu³⁺ ions is observed.     

Yb-doped NaBi(WO4)2 fibre single crystals grown by the micro-pulling down technique and emission spectroscopic characterization

NaBi_1−xYb_x(WO₄)₂ fibres single crystals were successfully grown by micro-pulling down technology (MPD). The Yb³⁺-doped NaBi(WO₄)₂ fibres single crystals have been pulled using MPD technique with controlled diameter and stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to the fibre radii and pulling rate range [6-48 mm h⁻¹]. We have determined the monophased field of NaBi_1−xYb_x(WO₄)₂ for x ≤ 0.3. The lattices parameters decrease as a function of Yb³⁺ substitution in Bi³⁺ sites. The melt behaviour has been study by DTA/TG analysis. We have found that the stoichiometric compounds NaBi(WO₄)₂ melt congruently at 935 °C. The fibre diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Complementary Yb³⁺ spectroscopic characterization in the NaBi(WO₄)₂ lattice has been done by IR emission measurements under laser pumping at room temperature.     

Platelet-shaped nanoparticles of PbI2 and PbMnI2 embedded in polymer matrix

Nanocrystals of PbI₂ and Pb_1−xMn_xI₂ (0<x<0.15) embedded in transparent polymer matrix have been studied by optical absorption and Faraday rotation spectroscopy. Average size of semiconductor nanoparticles was controlled by choosing polymer material and growth conditions. The obtained nanoparticles are platelet-like, with thickness up to 0.7 nm. The observed exciton peaks in the absorption spectra for nanocrystals into gelatin matrix correspond to the discrete thickness of the nanoparticles, which include finite number of atomic layers. For the Faraday rotation of Pb_1−xMn_xI₂ nanocrystals, reversal negative sign and additional enhancement has been revealed as compared with bulk crystals.     

Second-order optical nonlinearities of metastable BiBO3 phases in crystallized glasses

Metastable BiBO₃ and rare-earth (RE)-doped RE_xBi_1−xBO₃ crystals were prepared through the crystallization of 50Bi₂O₃ · 50B₂O₃ and xRE₂O₃ · (50−x)Bi₂O₃ · 50B₂O₃ glasses (RE = Y, La, Gd, Yb), and their second harmonic (SH) intensities were examined using the Kurtz powder method. It was confirmed that BiBO₃ has two different crystalline forms, i.e., BiBO₃(I) and BiBO₃(II), and that BiBO₃(I) transforms into BiBO₃(II) at temperatures around 500 °C. Both phases were found to be nonlinear optical crystals with a phase matchable character, and the SH intensity of BiBO₃(II) powders was approximately 110 times as large as α-quartz powders. The incorporation of RE³⁺ into BiBO₃(II) was confirmed from X-ray diffraction analysis, and in particular, Gd_xBi_1−xBO₃(II) showed an enhanced SH intensity of approximately 145 times as large as α-quartz.     

A study of vacancy-type defects in pure and boron-doped Ni3Al alloys

Positron lifetimes have been measured for two sets of pure and boron-doped Ni₃Al alloys. The alloys were large-grain polycrystals and had compositions of Ni_75+xAl_25−x (x = −1, 0, + 1) with 0, 100 and 500 wt ppm boron added. Lifetime parameters for samples of composition Ni_75+xAl_25−x (x= ± 1) with 0 and 500 wt ppm boron added were measured after initial thermal conditioning and after a subsequent cold-work anneal treatment. Positron trapping (≈20%) was observed in all unprocessed alloys. The vacancy concentration was calculated to be ≈ 5 × 10⁻⁶ and showed little, if any, systematic dependence on either alloy composition or boron concentration. Cold-worked fully annealed samples contained no detectable vacancies, i.e. the trapped state intensity was observed to be zero. The results are at variance with previously published data. During the annealing procedure (> 350°C) carbon was observed to diffuse out of the cold-worked samples. It is therefore possible that carbon stabilizes vacancies in Ni₃Al alloys. There is, however, no evidence to suggest that boron interacts with constitutional vacancies in Ni₃Al.     

Materials with layered structures XIII: electrical and optical properties of layered chalcogenides in the system CoIn2S4-CoIn2Se4

The system CoIn₂S_4xSe_4(1−x) has been investigated by X-ray powder methods on samples quenched at 700 °C. The spinel type phase has a phase width of 1≥x>0.9. A new layered compound is formed for 0.9>x>0.45 which crystallizes with the α-FeGa₂S₄-type with a=392.6 pm and c=1270.3 pm (x=0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750→700 °C. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10⁻⁵ Ω⁻¹ cm⁻¹.     

Dielectric properties of TlGa1 − x MnxS2 (0 ≤ x ≤ 0.03) single crystals

The electrical properties (loss tangent, real (ε) and imaginary (ε″) parts of complex dielectric permittivity, and ac conductivity across the layers) of TlGa_{1 ? x} Mn_xS₂ (0 ≤ x ≤ 0.03) layered single crystals have been studied in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the TlGa_{1 ? x} Mn_xS₂ crystals has a relaxation nature. Partial substitution of manganese for gallium reduces the dielectric permittivity of TlGaS₂ and changes the shape of the ε″(f) curve. Over the entire frequency range studied, the ac conductivity of the TlGa_{1 ? x} Mn_xS₂ crystals varies as f ^0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states, the spread of their energies, and the mean hop distance and time have been estimated.     

Preparation and properties of Ba2−xSrxSmTaO6 (x=0–2): a group of new perovskite materials

A new group of complex oxide materials were synthesized by the conventional solid state method. These compounds have the general formula Ba_2−xSr_xSmTaO₆ (x=0, 0.5, 1.0, 1.5 or 2) and complex perovskite structures. The compounds belonging to this Ba_2−xSr_xSmTaO₆ group exhibited a regular transition from the ideal cubic structure to a pseudocubic structure as the strontium content is increased. Pure Ba₂SmTaO₆ gave an ideal cubic perovskite crystal structure of A₂(BB′)O₆ type, while pure Sr₂SmTaO₆ was found to have a pseudocubic structure. The unit cell dimensions of these compounds varied from 8.4687 to 8.3232 Å as the amount of Sr (x) in Ba_2−xSr_xSmTaO₆ increased from 0 to 2. The theoretical density of these samples also decreased from 7.6756 to 6.9401 gm/cm³ as the value of x is changed from 0 to 2. All these powders could be sintered to more than 90% theoretical density at 1650 °C in 4 h. These compounds were found to be thermally stable and did not show any phase change. They had dielectric constants in the range 14–18 and the loss factor values were about 2×10⁻³ at a frequency of 10⁷ Hz. These Ba_2−xSr_xSmTaO₆ ceramics gave moderate values of dielectric constant and loss factor, in the range suitable for use as substrates for superconductors.     

Effect of Ta incorporation on the microstructure,electrical and optical properties of Hf1−xTaxO high-k film prepared by dual ion beam sputtering deposition

The microstructure, electrical and optical properties of Hf_1−xTa_xO (x = 0, 0.18, 0.28, 0.36 and 0.43) high-k thin films deposited by a novel deposition technique—dual ion beam sputtering deposition (DIBSD) have been investigated. From the O1s and Si 2p spectra of X-ray photoelectron spectroscopy (XPS), it is worth noting that the thickness of the interfacial layer significantly decreases after doping appropriate content Ta, and the formation of metal silicate components (M–O–Si) can be effectively suppressed by doping 43% Ta concentration into Hf_1−xTa_xO system. Compared to the pure HfO₂ sample, Hf_1−xTa_xO with 43% Ta after post-deposition annealing (PDA) exhibits the highest k-value (∼21.0 ± 0.2) and crystallization temperature (950 °C), the smallest root mean square (RMS) surface roughness of R_a ∼ 0.12 nm, Max height–depth R_p−v ∼ 1.5 nm and C–V hysteresis of 50 mV, the lowest leakage current density of 1.13 × 10⁻⁸ A/cm² at V_g=(V_fb−1) and an acceptable value of E_g ∼ 4.68 ± 0.1 eV.     

Synthesis and properties of Ce1−xGdxO2−x/2 solid solution prepared by flame spray pyrolysis

Flame spray pyrolysis, which produces ultrafine particles, was applied to the synthesis of Ce_1−xGd_xO_2−x/2 solid solutions by substituting Gd from a mole fraction of 0–0.40. The solubility limit of Gd in the Ce_1−xGd_xO_2−x/2 solid solution produced by flame spray pyrolysis was between 0.25 and 0.30, which is consistent with the reported value. The as-prepared Ce_1−xGd_xO_2−x/2 particles had a square morphology and a nanometer range in the equivalent diameter. The small particle size made it possible to reduce the sintering temperature of the Ce_1−xGd_xO_2−x/2 solid solution from 1650 °C to 1400 °C for the ceria-based solid electrolytes produced by the solid state preparation. The maximum ionic conductivity was achieved when the mole fraction of Gd was 0.25. The mole fraction for the highest ionic conductivity was the same as the particles produced by hydrothermal synthesis. However, the ionic conductivity of the Ce_1−xGd_xO_2−x/2 prepared by the flame spray pyrolysis (1.01 × 10⁻² S/cm at 600 °C) was higher than that prepared by the hydrothermal synthesis (7.53 × 10⁻³ S/cm at 600 °C).     

Structural aspects of adamantine like multinary chalcogenides

The present state of knowledge of structure, phase relations and metal ordering in 2(ZnX)_x(CuBX₂)_1 − x (B = Ga, In and X = S, Se, Te) and Cu₂Zn_xFe_1 − xSnS₄ multinary compounds is discussed. The chemical disorder process in 2(ZnX)_x(CuBX₂)_1 − x alloys leads to a phase separation, i.e. in a certain composition range (2-phase field) two phases, tetragonal domains and a cubic matrix, coexist. Its width depends on the three-valent cation only and is independent from the size of anion. In the subsolidus region of the 2(ZnX)_x(CuBX₂)_1 − x system the stability range of tetragonal mixed crystals as well as the miscibility gap is decreasing, the stability range of cubic mixed crystals is increasing. The process of structural disorder in 2(ZnX)_x(CuBX₂)_1 − x as well as Cu₂Fe_1 − xZn_xSnS₄ alloys is connected to the cation substructure. In tetragonal 2(ZnX)_x(CuInX₂)_1 − x alloys a non-random Zn distribution on the both cation positions of the chalcopyrite-type structure was revealed, whereas a random distribution of Zn and Cu on two different sites of the kesterite type structure was obtained in Cu₂ZnSnS₄ in contradiction to literature. The crossover from stannite (x = 0) to kesterite (x = 1) in Cu₂Fe_1 − xZn_xSnS₄ is considered as a three-stage process of cation restructure involving Cu⁺, Zn²⁺ and Fe²⁺, whereas Sn⁴⁺ does not take part in this process. In tetragonal 2(ZnX)_x(CuInX₂)_1 − x alloys the anion displacement is decreasing with increasing ZnX content in CuInX₂ indicating a decreasing tetragonal distortion. Here the disorder process in the cation substructure and the displacement process in the anion substructure are coupled.     

Mechanism of AC charge transport in TlSb<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0 and 0.03)

The dielectric properties and ac electrical conductivity of TlSb_1–xGa _x S₂ (x = 0, 0.03) single-crystals have been measured in the frequency range 5 × 10⁴ to 3.5 × 10⁷ Hz. Experimental data on the frequency dispersion of the dielectric coefficients and electrical conductivity of the TlSb_1–xGa _x S₂ (x = 0, 0.03) single crystals have allowed us to identify the nature of the dielectric loss and the mechanism of charge transport and evaluate parameters of localized states in the band gap. The incorporation of gallium atoms into the crystal lattice of TlSbS₂ crystals has been shown to lead to an increase in the Fermi-level density of states and mean hop time and distance.