Persistent luminescence of Zn<Subscript>2</Subscript>GeO<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript>/Pr<Superscript>3+</Superscript> phosphors

Zn₂GeO₄, Zn₂GeO₄:Mn²⁺, Zn₂GeO₄:Pr³⁺ and Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphors were fabricated by a solid state reaction. The phase and luminescent properties of the fabricated phosphors were investigated. The XRD patterns show that all of the fabricated phosphors have an orthorhombic structure. The fabricated Zn₂GeO₄ shows an emission band in the range of 350–550 nm. The fabricated Zn₂GeO₄:Mn²⁺ and Zn₂GeO₄:Pr³⁺ phosphors show emission bands corresponding to Mn²⁺ and Pr³⁺ ions, respectively. The fabricated Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphor shows the emission band results from Mn²⁺ and the codoped Pr³⁺ enhances the emission intensity of Mn²⁺. Moreover, Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphor exhibits longer decay time than that of Zn₂GeO₄:Mn²⁺. The higher intensity and longer lifetime of Mn²⁺ emission are induced by the energy transfer from Pr³⁺ of various vacancies to Mn²⁺ in Zn₂GeO₄:Mn²⁺/Pr³⁺ phosphors.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

Electric-field effect on crystal growth in the Li<Subscript>3</Subscript>PO<Subscript>4</Subscript>-Li<Subscript>4</Subscript>GeO<Subscript>4</Subscript>-Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-LiF system

We have studied the electric-field effect on crystallization processes in the Li₃PO₄-Li₄GeO₄-Li₂MoO₄-LiF system. In zero field, Li_3+x P_1?x Ge _x O₄ (x = 0.31) crystals were grown on the cathode under the conditions of this study. At low applied voltages (≤ 0.5 V), we obtained Li₂MoO₄, Li₂GeO₃, and Li_1.3Mo₃O₈. In the range V = 0.5–1 V, crystals of Li_3+x P_1?x Ge _x O₄ solid solutions with x = 0.17, 0.25, 0.28, 0.29, and 0.36 were obtained. An applied electric field was shown to reduce the melting temperature of the starting mixtures and the crystallization onset temperature.     

Synthesis of pure Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript>:Mn green phosphors by simple PVA–metal complex route

Green light emitting Zn_2−x Mn _x SiO₄ (willemite) particles were synthesized by a simple and cost-effective poly(vinyl alcohol) (PVA)-complex route. Microstructural studies on the calcined products show that particles are pure, single phase, nano-crystalline, and agglomerated morphology. A pure-phase willemite structure was obtained with calcination between 850 and 1,175 °C for 2 h in air. Particle size analysis indicates that the average particle size is ∼1 μm. The photoluminescence properties of these 4 and 12 mol% Mn-doped Zn₂SiO₄ powders were measured by fluorescence spectroscopy. Particles with 4 mol% Mn doping prepared at 1,150 °C, with an emission decay time, I ₀/e, of 13.4 ms showed the highest relative peak emission intensities. The emission intensity at this doping level was measured to be ∼110% of a representative commercial product’s, doped with 11.2 mol% Mn, and exhibiting a decay time of 7.1 ms. The effect of calcination temperature on the photoluminescence and crystallinity properties of synthesized green powders was also investigated.     

Far infrared reflectance of sintered Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript>

Zinc-titanate ceramics were obtained by initial mechanical activation in a high-energy planetary mill for 15 min followed by sintering at temperatures 900–1100 °C for 2 h. Room temperature far infrared reflectivity spectra for all samples were measured in the range 100–1200 cm⁻¹. The same ionic oscillators were present in the measured spectra, but their intensities increased with the sintering temperature in correlation with the increase in sample density and microstructure changes. Optical parameters were determined for seven oscillators belonging to the spinel structure using the four-parameter model of coupled oscillators. Born effective charges were calculated from the transversal/longitudinal splitting.     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Chlorine impurity content of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> and GeO<Subscript>2</Subscript>

Bismuth(III) and germanium(IV) oxides are used as precursors for the crystal growth of bismuth orthogermanate. The chlorine content of the starting oxides is critical to the engineering performance of this material. We propose a simple, high-speed technique for determining the chlorine content of bismuth(III) and germanium(IV) oxides, down to 5 × 10^?4 wt %, using capillary electrophoresis.     

Synthesis and conductivity of Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-based cermet material

SHS synthesis of Zn₂SnO₄-based cermet material from Zn + NiO + SnO₂ compacted powder mixtures has been studied. The final product is obtained as a monolithic cylindrical block composed of a ZnO-based outer layer and Zn₂SnO₄-based central part, in which the metal phase is distributed. The phase composition and microstructure of combustion products have been studied by x-ray phase analysis (XPA), electron microscopy, and microprobe techniques. It is established that the structure of the obtained cermet material has a dramatic effect on their conductivity.     

GeO<Subscript>2</Subscript>-rich low-loss single-mode optical fibers

We have optimized the MCVD process for the fabrication of single-mode optical fibers with a heavily doped (up to 30 mol % GeO₂) core for Raman lasers and amplifiers. Removing the central index dip in the core, we were able to reduce the optical losses in such fibers to the lowest level for the MCVD technology.     

Synthesis and x-ray diffraction study of solid solutions in the Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript>-Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript>-Zn<Subscript>2</Subscript>ZrO<Subscript>4</Subscript> system

The multicomponent refractory oxide system Zn₂(Ti_aSn_b)_{1 ? x} Zr_xO₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 1: 0, 2: 1, 3: 1, 4: 1; x = 0?1.0; Δx = 0.05) has been studied by x-ray diffraction, using samples prepared by melting appropriate oxide mixtures in a low-temperature hydrogen-oxygen plasma. Two phases, both with wide homogeneity ranges, have been identified: α-phase, with a cubic inverse spinel structure, and β-phase, with a tetragonal spinel structure. The phase boundaries in the system have been determined. Structural data are presented for about 100 solid solutions of different compositions.     

X-Ray luminescence of polycrystalline TbO<Subscript>2</Subscript>-doped Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

This paper examines the effect of doping level on the X-ray luminescence of TbO₂-doped polycrystalline lithium tetraborate. It is shown that, when interpreting such spectra, it is convenient to proceed from the terms of free activator and constituent ions. We demonstrate that the emission lines of Tb³⁺ in doped polycrystalline lithium tetraborate are effectively excited in the band between 350 and 650 nm, which is predominantly due to electron transitions from the ⁵ D ₃ and ⁵ D ₄ excited states to spin-orbital levels of the ⁷ F _J ground multiplet. The emission lines of lithium and boron in single-crystal and polycrystalline undoped lithium tetraborate are effectively excited in the band between 274 and 550 nm.     

Synthesis and characterization of magnetic and microwave absorbing properties in polycrystalline cobalt zinc ferrite (Co<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>) composite

In this research work, magnetic and microwave absorption loss and other response characteristics in cobalt zinc ferrite composite has been studied. Cobalt zinc ferrite with the composition of Co_0.5Zn_0.5Fe₂O₄ was prepared via high energy ball milling followed by sintering. Phase characteristics of the as-prepared sample by using XRD analysis shows evidently that a high crystalline ferrite has been formed with the assists of thermal energy by sintering at 1250 °C which subsequently changes the magnetic properties of the ferrite. A high magnetic permeability and losses was obtained from ferrite with zinc content. Zn substitution into cobalt ferrite has altered the cation distribution between A and B sites in spinel ferrite which contributed to higher magnetic properties. Specifically, Co_0.5Zn_0.5Fe₂O₄ provides electromagnetic wave absorption characteristics. It was found that cobalt zinc ferrite sample is highly potential for microwave absorber which showed the highest reflection loss (RL) value of ??24.5 dB at 8.6 GHz. This material can potentially minimize EMI interferences in the measured frequency range, and was therefore used as fillers in the prepared composite that is applied for microwave absorbing material.     

Optical and structural properties of ZnO + Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript> thin films prepared by the sol–gel method

ZnO + Zn₂TiO₄ thin films were obtained by the sol–gel method using precursor solutions with different Ti/Zn ratios in the 0.18–2.13 range. The films were deposited on glass substrates and annealed in an open atmosphere at 550 °C. The oxide was characterized by X-ray diffraction and photoacoustic (PA) spectroscopy. The films were constituted of polycrystalline ZnO for the lowest Ti/Zn ratio (0.18), polycrystalline Zn₂TiO₄ for the 0.70 and 1.0 ratios, and mixes of both oxides for the intermediate ratios (0.32 and 0.50). For the highest ratios studied (1.44 and 2.13), the films were amorphous. The energy band gap (E_g) values were determined from optical absorption spectra, measured by means of the PA technique spectra. E_g varied in the 3.15 eV (ZnO) to 3.70 eV (Zn₂TiO₄) range.     

Local amorphization of atomic structure in Mg<Subscript>0.54</Subscript>Zn<Subscript>0.46</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites

The structure of Mg_0.54Zn_0.46Fe₂O₄ ferrites has been studied using x-ray diffraction techniques. It is established that high-temperature annealing (T = 1280°C, τ = 0.5–8.0 h) leads to a change in the crystal structure of samples, which is accompanied by their local amorphization.     

Ce and Eu activated Na<Subscript>2</Subscript>Zn<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>4</Subscript>, a new promising novel phosphor

A new efficient phosphor, Eu²⁺/Eu³⁺ and Ce³⁺ activated Na₂Zn₅(PO₄)₄ has been synthesized by solid-state reaction technique at high temperature. X-ray powder diffraction analysis confirmed the formation of Na₂Zn₅(PO₄)₄ host lattice. Scanning electron microscopy indicated that the microstructure of the phosphor consisted of irregular fine grains with a size of about 0·5–2 μm. Photoluminescence excitation spectrum measurements of Ce³⁺ activated Na₂Zn₅(PO₄)₄ show that the phosphor can be efficiently excited by UV-Vis light from 280 to 310 nm to realize emission in the visible (blue) range due to the 5d-4f transition of Ce³⁺ ions which is applicable for scintillation purpose, whereas Eu²⁺/Eu³⁺ activated Na₂Zn₅(PO₄)₄ phosphor emits blue, green and red emission spectrum shows at 487 nm, 546 nm with a dominant peak at 611 nm respectively, due to Eu²⁺/Eu³⁺ ions which is promising candidate for solid state lighting. Therefore, newly synthesised, by low cost and easy technique prepared, novel phosphors may be useful as RGB phosphor for solid state lighting application.     

New,thermally stable Gd<Subscript>11</Subscript>(GeO<Subscript>4</Subscript>)(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>O<Subscript>10</Subscript>-based upconversion phosphors

A series of Gd_11–x–y Yb _x Er _y GeP₃O₂₆ germanate phosphates differing in the ratio of the Yb³⁺ and Er³⁺ active ions have been synthesized, and their luminescence spectra have been measured. According to X-ray diffraction characterization results, all of the synthesized germanate phosphates are single-phase and have a triclinic structure (sp. gr. P1). We have measured upconversion luminescence spectra due to the Er^{3+ 2}H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 radiative transitions in the synthesized gadolinium ytterbium erbium germanate phosphates and determined the luminescence upconversion energy yield (B _en) in Gd_11–x–y Yb _x Er _y GeP₃O₂₆. The effects of the concentrations and ratio of the dopants in the Gd₁₁(GeO₄)(PO₄)₃O₁₀ germanate phosphate host on B _en and the ratio of the luminescence intensities in the red and green spectral regions (R/G) have been assessed.     

Mechanochemical Synthesis of Crystalline Compounds in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–GeO<Subscript>2</Subscript> System

Crystalline mechanochemical synthesis products in the Bi₂O₃–GeO₂ system are studied by x-ray diffraction. The results indicate the formation of sillenite (Bi₁₂GeO₂₀), eulytite (Bi₄Ge₃O₁₂), and Aurivillius (Bi₂GeO₅) phases. The Aurivillius phase is shown to be in mechanochemical equilibrium with the sillenite phase in the 2Bi₂O₃ + GeO₂ system and with the eulytite phase in the Bi₂O 3 + GeO 2 system. The structural parameters of the synthesized metastable solid solutions are determined. The three phases contain high concen-trations of vacancies. In addition, the sillenite and Aurivillius phases are characterized by compositional disordering. Structural and ESR data point to partial reduction of the oxides, which accounts for the formation of the Aurivillius phase. According to x-ray photoelectron spectroscopy results, mechanical activation of bismuth oxide produces reduced binding energy states of Bi and O, which is tentatively attributed to clustering and the formation of complex radicals.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 711–719.Original Russian Text Copyright © 2005 by Zyryanov, Smirnov, Ivanovskaya.     

A novel approach for preparation of Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript>:Tb nanoparticles by sol-gel-microwave heating

Zn₂SiO₄:Tb nanoparticles were prepared by sol-gel-microwave heating for the first time. X-ray powder diffraction (XRD) analysis confirmed the formation of Zn₂SiO₄ in willemite structure. Field-emission scanning electron microscopy showed a narrow size distribution, small size (40–50 nm) and spherical shape of the particles. Energy dispersive spectroscopy result indicated that the ratio of Tb³⁺/Zn²⁺ was in agreement with that of the feed. Photoluminescence measurement indicated that the phosphor emitted strong green light centered at 545 nm under UV light excitation. The excitation spectra confirmed the energy transfer from the host material to the Tb³⁺ ions. This is in favor of the effective green emission of Zn₂SiO₄:Tb nanoparticles.     

Photoluminescence of Si<Subscript>0.9</Subscript>Ge<Subscript>0.1</Subscript>O<Subscript>2</Subscript> and GeO<Subscript>2</Subscript> films irradiated with silicon ions

We have studied the photoluminescence (PL) of GeO₂ and 90 mol % SiO₂-10 mol % GeO₂ films synthesized by method of RF magnetron sputtering and then irradiated with silicon ions and annealed. The PL of silicon-implanted GeO₂ films, related to the presence of Si nanocrystals (nc-Si), was observed for the first time. It is established that the transformation of the defect centers responsible for the PL in the spectral range 350–600 nm, as well as the formation of nc-Si emitting in the region of 700–800 nm, significantly depend on the matrix type. In particular, the PL intensity at 700–800 nm in 90 mol % SiO₂-10 mol % GeO₂ films is weak. The role of the isovalent substitution of Si and Ge atoms in the transformation of defect centers and the formation of nc-Si is discussed.