EPR and optical investigation of ultraviolet-emitting Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet

Gd₃Ga₅O₁₂ garnet was prepared by a solution combustion method and characterized using powder X-ray diffraction, electron paramagnetic resonance (EPR) and optical spectroscopic techniques. EPR spectrum of the samples at ambient and low temperatures exhibited resonance signal at g?≈?2 attributed to Gd³⁺ ions disposed in an octahedral symmetry. The optical absorption spectrum showed a band centered at 274 nm attributed to ⁸S_7/2→⁶I_J transition of the Gd³⁺ ions. The excitation spectrum showed a maximum at 273 nm along with two relatively weaker peaks at 276 and 279 nm. These three excitation bands were assigned to the transitions ⁸S_7/2→⁶I_J. The emission spectrum showed two peaks centered at 311 and 306 nm that were assigned to the transitions ⁶P_7/2→⁸S_7/2 and ⁶P_5/2→⁸S_7/2 of Gd³⁺, respectively. To evaluate the phosphor performance, several magnetic and thermodynamic parameters for the system were calculated adopting standard procedure.     

Effect of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> concentration in Bi-containing high-temperature solutions on the luminescence of epitaxial Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> films

Single-crystal gadolinium gallium garnet films have been grown by liquid-phase epitaxy on (111) Gd₃Ga₅O₁₂ substrates from supercooled Bi₂O₃-B₂O₃ fluxed melts at different Gd₂O₃ concentrations. The luminescence spectra of the films have been measured at 10 and 300 K under unmonochromatized synchrotron X-ray excitation and selective UV synchrotron excitation. The Bi³⁺ luminescence is discussed.     

Cathodo- and photoluminescence of Cu<Subscript>3</Subscript>Ga<Subscript>5</Subscript>Se<Subscript>9</Subscript> single crystals

The cathodo- and photoluminescence spectra of Cu₃Ga₅Se₉ single crystals have been measured at different excitation intensities and temperatures. The results indicate that the radiative recombination of nonequilibrium charge carriers occurs primarily through impurity levels due to anion and cation vacancies. The anion and cation vacancy concentrations can be controlled by thermal and laser anneals.     

Synthesis,structural, photoluminescence and mechanoluminescence properties of Tb<Superscript>3+</Superscript>: Ca<Subscript>2</Subscript>Gd<Subscript>2</Subscript>W<Subscript>3</Subscript>O<Subscript>14</Subscript> novel green nanophosphors

Novel green nanophosphors Ca₂Gd₂W₃O₁₄: Tb³⁺ were synthesized by solid state reaction method. From the X-ray diffraction profiles it is observed that Tb³⁺: Ca₂Gd₂W₃O₁₄ phosphors were crystallized in the form of tetragonal structure. The scanning electron microscopy (SEM) image shows that the particle size is at around 300 nm. In addition to these the prepared powder phosphors were also examined by the energy dispersive X-ray analysis (EDAX), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and mechanoluminescence (ML) spectra. Emission spectra of Tb³⁺: Ca₂Gd₂W₃O₁₄ nanophosphors have shown bright green emission at 545 nm (⁵D₄ → ⁷F₅) with an excitation wavelength λ_exci = 374 nm (⁷F₆ → ⁵G₆). ML spectra shows the radiation effect on the Ca₂Gd₂W₃O₁₄: Tb³⁺ nanophosphors and from that it was observed that these phosphors are very less sensitive for lower exposure.     

Electron traps in Gd<Subscript>3</Subscript>Ga<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>12</Subscript>:Ce garnets doped with rare-earth ions

The curves of thermally stimulated luminescence of Gd₃Ga₃Al₂O₁₂:Ce³⁺ ceramics (a nominally pure sample and samples doped with rare-earth ions) are measured in the temperature range of 80–550 K. The depth and the frequency factor of electron traps established by Eu and Yb impurities are determined. An energy-level diagram of rare-earth ions in the bandgap of Gd₃Ga₃Al₂O₁₂ is presented.     

Effect of Pb ions on the optical absorption in Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> epitaxial films

We have studied the effect of Pb²⁺ impurities and Pb²⁺-Pb⁴⁺ pairs on the optical absorption between 200 and 860 nm in single-crystal gadolinium gallium garnet films grown by liquid-phase epitaxy from supercooled PbO-B₂O₃ fluxed melts containing 0.1–0.5 mol% gadolinia.     

Physicochemical properties of La<Subscript>3</Subscript>Ga<Subscript>5.5</Subscript>Ta<Subscript>0.5</Subscript>O<Subscript>14</Subscript>

We report the optical and dielectric properties and microhardness of La₃Ga_5.5Ta_0.5O₁₄ lanthanum gallium tantalate (langatate) crystals. Analysis of the optical transmission spectra of the crystals in relation to their refined compositions indicates that the bands at 34000–35000 and 27000–28000 cm⁻¹ are due to lanthanum and oxygen vacancies, respectively, and that the band at 20000–21000 cm⁻¹ is responsible for the yellow (orange) coloration of the crystals. Their resistivity and microhardness decrease with increasing oxygen vacancy concentration.     

Optical properties,defects, and composition of La<Subscript>3</Subscript>Ga<Subscript>5.5</Subscript>Ta<Subscript>0.5</Subscript>O<Subscript>14</Subscript> crystals

La₃Ga_5.5Ta_0.5O₁₄ crystals were grown in pure argon and in an argon + 2% oxygen atmosphere. The growth atmosphere significantly effects the elemental composition of the crystals, their color, and the genesis and intensity of absorption bands in the ultraviolet, visible, and infrared spectral regions. Two types of defects were identified in the crystals: planar interstitial defects and nearly spherical vacancy-type defects.     

Heat Capacity and Thermodynamic Functions of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> from 14 to 320 K

The heat capacity of Ga₂Se₃ is measured from 14 to 320 K by adiabatic calorimetry. The smoothed heat capacity data are used to evaluate temperature-dependent thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) of gallium selenide. Under standard conditions, the thermodynamic properties of Ga₂Se₃ are C _p ⁰ (298.15 K) = 120.8 ± 0.2 J/(K mol), S⁰(298.15 K) = 180.4 ± 0.4 J/(K mol), H⁰(298.15 K) - H⁰(0) = 25.32 ± 0.05 kJ/mol, and Φ⁰(298.15 K) = 95.52 ± 0.19 J/(K mol). The Debye characteristic temperature of Ga₂Se₃ evaluated from heat capacity data is 340 ± 10 K.     

Combustion synthesis of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

Nanophase of Ga₂O₃ has potentially important applications in photocatalysis. We report the synthesis of nanophase of the metastable γ- and stable β-Ga₂O₃ and demonstrate that it is possible to prepare a continuously varying mixture starting from the pure metastable γ- to the pure β-phase. This is achieved by employing a facile and reliable combustion route, using urea as a fuel. Typical grain sizes, as estimated from XRD studies, are about 3 nm. Given the importance of surface chemistry for potential applications, thermogravimetric coupled with mass spectrometry is used in conjunction with FTIR to elucidate the chemistry of the adsorbed surface layer. Studies on the γ-Ga₂O₃ phase indicate the occurrence of weight loss of 8.1% in multiple steps. Evolved gas analysis and FTIR studies show presence of physisorbed H₂O molecules and chemisorbed –(OH) ions bonded to active surface states and accounts predominantly for the observed weight loss.     

Tailoring magnetic and dielectric properties of Co<Subscript>0.9</Subscript>Cu<Subscript>0.1</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> with substitution of small fractions of Gd<Superscript>3+</Superscript> ions

This paper describes the structural, magnetic, and dielectric properties of Gd³⁺ substituted cobalt–copper ferrite. The influence of Gd³⁺ substitution on the structural, magnetic and electrical properties of cobalt–copper ferrite was investigated through various characterization techniques. Thermal analysis was carried out on the prepared gel to know the combustion and calcination temperature. The detailed structural analysis suggests that the substitution of a Fe³⁺ ion with a Gd³⁺ ion at B site results in lattice distortion, modification in crystallite size and grain size of the material. X-ray photoelectron spectroscopy confirmed the oxidation states of the elements present. Magnetic measurement performed at 300 and 50 K depicts the decrease in saturation magnetization (Ms) and increase in coercivity (Hc) with Gd³⁺ substitution in the cobalt–copper spinel ferrite. The dielectric measurements acquired over a wide range of frequencies and temperature showed an increase in dielectric constant with increasing Gd³⁺ concentration.     

Bridgman growth and defects of Nd<Superscript>3+</Superscript> : Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> laser crystals

Nd³⁺ : Sr₃Ga₂Ge₄O₁₄ crystals have been grown by the modified Bridgman method. The growth defects, such as striations, scattering particles and dislocations were investigated. Some featherlike striations were observed in as-grown crystals. EPMA analysis suggested that these inclusions were caused by the segregation of Nd₂O₃ from the melt. Chemical etching results showed that the dislocation density was in the range of 10³ ∼ 10⁵/cm².     

Microstructure and phase transformations in a Ni<Subscript>50</Subscript>Mn<Subscript>29</Subscript>Ga<Subscript>16</Subscript>Gd<Subscript>5</Subscript> alloy with a high transformation temperature

A Heusler Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy with a high transformation temperature has been obtained by substituting 5 at% Gd for Ga in a ternary Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloy. The microstructure and phase transformations in the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy have been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. It is shown that the microstructure of the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy consists of matrix and hexagonal Gd (Ni,Mn)₄Ga phase, which indicates a eutectic structure composed of these two phases. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy exhibits a martensite transformation start temperature up to 524 K, approximately 200 K higher than that of Ni₅₀Mn₂₉Ga₂₁ alloy. At room temperature, non-modulated martensite with twin substructure is observed in Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy.     

Visible-light activities of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>/BiVO<Subscript>4</Subscript> composite photocatalysts

Gd₂O₃/BiVO₄ composite photocatalysts were hydrothermal synthesized and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and UV–vis diffusion reflectance spectra; all the composite photocatalysts exhibited enhanced photocatalytic activities than the pure BiVO₄ for degradation of methyl orange under visible-light irradiation. The improved activity of composites was discussed and ascribed to the electron-scavenging effect of dopants.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Photoluminescence of Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>:Eu nanocrystals

The photoluminescence (PL) of Ga₂S₃-5 mol % Eu₂O₃ nanocrystals prepared by mechanical comminution of the initial compound has been studied in the temperature range from 77 to 300 K. It is established that the PL spectrum of nanocrystals, in comparison to that of a massive sample, extends over a broader wavelength interval (430–620 nm) and has two maxima (at 507 and 556 nm) instead of one. The intensity of emission from nanocrystals is significantly higher than that from the massive crystal. The halfwidth in both cases varies with the temperature in proportion to T ^1/2. The intensity of emission at 556 nm for nanocrystals depends on the temperature as lgI ～ 1/T, this dependence having three linear portions corresponding to an activation energy of 0.04, 0.16, and 0.43 eV. The PL bands with maxima at 507 and 556 nm are assigned to the intracenter 4f ⁶5d4f ⁷ transitions in Eu²⁺ ions.     

Synthesis and luminescent characterization of YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>:Tb<Superscript>3+</Superscript> phosphors

YAl₃(BO₃)₄:Tb³⁺ phosphors were fabricated by the sol–gel method. The phosphor showed prominent luminescence in green due to the magnetic dipole transition of ⁵D₄–⁷F₅. Structural characterization of the luminescent material was carried out with X-ray powder diffraction (XRD) analysis. Luminescence properties were analyzed by measuring the excitation and photoluminescence spectra. Photoluminescence measurements indicated that the phosphor exhibited bright green emission at about 541 nm under UV excitation. It is shown that the 11% of doping concentration of Tb³⁺ ions in YAl₃(BO₃)₄:Tb³⁺ phosphors is optimum.     

Synthesis,photoluminescence and mechanoluminescence properties of Eu<Superscript>3+</Superscript> ions activated Ca<Subscript>2</Subscript>Gd<Subscript>2</Subscript>W<Subscript>3</Subscript>O<Subscript>14</Subscript> phosphors

A new series of Eu³⁺ ions-activated calcium gadolinium tungstate [Ca₂Gd₂W₃O₁₄] phosphors were synthesized by conventional solid-state reaction method. The X-ray diffraction patterns of the powder samples indicate that the Eu³⁺: Ca₂Gd₂W₃O₁₄ phosphors are of tetragonal structure. The prepared phosphors were well characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL), and mechanoluminescence (ML) spectra. PL spectra of Eu³⁺: Ca₂Gd₂W₃O₁₄ powder phosphors have shown strong red emission at 615 nm (⁵D₀ → ⁷F₂) with an excitation wavelength λ _exci = 392 nm (⁷F₀ → ⁵L₆). The energy transfer from tungstate groups to europium ions has also reported. Mechanoluminescence studies of Eu³⁺: Ca₂Gd₂W₃O₁₄ phosphors have also been explained systematically.     

Organic additive assisted hydrothermal synthesis and photoluminescence properties of CeF<Subscript>3</Subscript>:Tb<Superscript>3+</Superscript> and NaCeF<Subscript>4</Subscript>:Tb<Superscript>3+</Superscript> nanoparticles

A series of Tb³⁺ doped CeF₃ and NaCeF₄ nanoparticles with different morphology and dimension were synthesized via hydrothermal method. Different organic additives, including sodium dodecyl sulfonate (SDS), polyvinylpyrrolidone (PVP), cetyltrimethyl ammonium bromide (CTAB), oleic acid (OA), polyethylene glycol (PEG), trisodium citrate (Cit) were introduced to control the crystallite size and morphology. Powder X-ray diffraction (PXRD), fourier transform infra-red spectra (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and down-conversion (DC) photoluminescence spectra were used to characterize the samples. The emission peaks of all the prepared samples centered at 490, 545, 585 and 621 nm which can be ascribed to the ⁵D₄→⁷F_J (J?=?6, 5, 4, 3) transitions respectively of Tb³⁺ ion. However, emission intensities are strongly controlled by morphology and particle sizes which are influenced by different organic additives used in synthesis. Moreover, the crystal growth process was monitored through a series of time-dependent experiments and a possible formation mechanism has been proposed.     

Photoelectric properties of ordered-vacancy Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals

We have studied the photoconductivity spectrum, thermally stimulated current, current-light characteristics, and temperature-dependent photocurrent in Bridgman-grown ordered-vacancy Ga₂Se₃ crystals. The observed temperature quenching of photoconductivity and two regions of its thermal activation in Ga₂Se₃ crystals are interpreted in terms of a multicenter recombination model which incorporates an s-channel of active recombination, r-centers of photosensitivity, and traps for nonequilibrium majority carriers.