Scintillation properties of the Ce-doped multicomponent garnet epitaxial films

(Lu,Y,Gd)₃(Al,Ga)₅O₁₂:Ce garnet scintillator single crystalline films were grown onto LuAG, YAG and GGG substrates by liquid phase epitaxy method. Absorption, radioluminescence spectra and photoluminescence excitation, emission spectra, and decay kinetics were measured. Photoelectron yield, its dependence on amplifier shaping time and energy resolution were determined to evaluate scintillation performance. Most of the samples exhibited strong UV emission caused by trapped excitons and/or Gd³⁺ 4f–4f transition. However, emission spectrum of the best performing Gd₂YAl₅O₁₂:Ce is dominated by the Ce³⁺ fast 5d–4f luminescence. This sample has outperformed photoelectron yield of all the garnet films studied so far.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃(Ga,Al)₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect––host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

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.     

Terbium-doped garnet single crystals as X-ray-sensitive phosphors

The spectra of luminescence induced by synchrotron radiation in the fundamental absorption range were measured at 10 and 300 K for Czochralski-grown bulk Y₃Al₅O₁₂ single crystals doped with Tb³⁺ ions and for (Tb,La,Gd)₃Ga₅O₁₂ single crystal films grown by liquid phase epitaxy from a PbO-B₂O₃ based supercooled solution melt on Gd₃Ga₅O₁₂ substrates.     

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.     

Luminescence of bulk and thin-film single crystals of gadolinium gallium garnet excited by UV radiation

A difference between the luminescence spectra of bulk and thin-film single crystals of Gd₃Ga₅O₁₂ gadolinium gallium garnet excited by UV radiation from a deuterium lamp has been studied. The films were grown by liquid phase epitaxy from supercooled melts based on the PbO-B₂O₃ and Bi₂O₃-B₂O₃ solid solution systems.     

Synthesis and luminescence of ultrafine Er3+- and Yb3+-doped Gd11SiP3O26 and Gd14B6Ge2O34 particles for cancer diagnostics

In search of new contrast materials for NMR and fluorescence diagnostics and neutron capture therapy of cancer, we have synthesized ultrafine Er³⁺- and Yb³⁺-doped Gd₁₁SiP₃O₂₆ and Gd₁₄B₆Ge₂O₃₄ particles and studied their luminescence properties. We measured the Er³⁺ upconversion luminescence spectra of the gadolinium erbium ytterbium phosphosilicates and borate germanates in the visible range and evaluated the absolute quantum yield of their luminescence. The quantum yield of luminescence in the gadolinium phosphosilicate Gd₁₁SiP₃O₂₆ doped with 5.0 at % Yb and 2.5 at % Er is comparable to that in known Yb³⁺/Er³⁺ codoped fluorides. The nonradiative Yb³⁺ ?? Er³⁺ energy transfer efficiency is evaluated.     

Hydrothermal synthesis and photoluminescence properties of Gd2Zr2O7:Tb phosphors

This paper presents hydrothermal synthesis, characterization, and photoluminescence (PL) properties of novel green-emitting phosphors, Gd₂Zr₂O₇:Tb³⁺. Their crystal structure, morphology and photoluminescence properties were investigated by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and fluorescence spectrophotometer. The results revealed that one-dimensional Gd₂Zr₂O₇:Tb³⁺ nanorods with diameter of about 30 nm and length of 150-300 nm were formed, and the products exhibited a fluorite-type structure. PL study revealed that Gd₂Zr₂O₇:Tb³⁺ phosphors presented dominant green emission luminescence, which was attributed to the transitions from ⁵D₄ excited states to ⁷F_J (J = 3-6) ground states of Tb³⁺. The luminescence intensity of Gd₂Zr₂O₇:Tb³⁺ with different Tb³⁺ concentration was also investigated and reported, and an obvious concentration quenching was observed when Tb³⁺ ion concentration was 5 at.%.     

Effective lattice stabilization of gadolinium aluminate garnet (GdAG) via Lu3+ doping and development of highly efficient (Gd,Lu)AG:Eu3+ red phosphors

Abstract

The metastable garnet lattice of Gd₃Al₅O₁₂ is stabilized by doping with smaller Lu³⁺, which then allows an effective incorporation of larger Eu³⁺ activators. The [(Gd_1?xLu_x)_1?yEu_y]₃Al₅O₁₂ (x = 0.1–0.5, y = 0.01–0.09) garnet solid solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong luminescence at 591 nm (the ⁵D₀ → ⁷F₁ magnetic dipole transition of Eu³⁺) upon UV excitation into the charge transfer band (CTB) at ～239 nm, with CIE chromaticity coordinates of x = 0.620 and y = 0.380 (orange-red). The quenching concentration of Eu³⁺ was estimated at ～5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd³⁺ with Lu³⁺ up to 50 at.% (x = 0.5) while keeping Eu³⁺ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and ⁵D₀ → ⁷F₁ emission bands but slightly weakens both bands owing to the higher electronegativity of Lu³⁺. The effects of processing temperature (1000–1500 °C) and Lu/Eu contents on the intensity, quantum efficiency, lifetime and asymmetry factor of luminescence were thoroughly investigated. The [(Gd_0.7Lu_0.3)_0.95Eu_0.05]₃Al₅O₁₂ phosphor processed at 1500 °C exhibits a high internal quantum efficiency of ～83.2% under 239 nm excitation, which, in combination with the high theoretical density, favors its use as a new type of photoluminescent and scintillation material.     

Optical and scintillation properties of Nd-doped complex garnet

Nd 1% doped complex garnet scintillators were prepared by Furukawa and their optical and scintillation properties were investigated on a comparison with previously reported Nd-doped YAG. Chemical compositions of newly developed complex garnets were Lu₂Y₁Al₅O₁₂, Lu₂Y₁Ga₃Al₂O₁₂, Lu₂Gd₁Al₅O₁₂, Lu₂Gd₁Ga₃Al₂O₁₂, Gd₁Y₂Al₅O₁₂, Gd₁Y₂Ga₃Al₂O₁₂, and Gd₃Ga₃Al₂O₁₂. They all showed 50–80% transmittance from ultraviolet to near infrared wavelengths with several absorption bands due to Gd³⁺ or Nd³⁺ 4f–4f transition. In X-ray induced radioluminescence spectra, all samples exhibited intense lines at 310 nm due to Gd³⁺ or 400 nm due to Nd³⁺ depending on their chemical composition. Among them, the highest scintillation light yield was achieved by Lu₂Y₁Al₅O₁₂. Typical scintillation decay times of them resulted 1.5–3 μs. Thermally stimulated glow curve after 1 Gy exposure and X-ray induced afterglow were also investigated.     

Phase equilibrium diagram for the system Gd2O3-MoO3

Phase equilibria have been established in the binary system Gd₂O₃MoO₃ including Gd₂(MoO₄)₃ with ferroelectricity and ferroelasticity below 159°C, by means of differential thermal analysis and X-ray powder diffraction techniques. It is shown that Gd₂(MoO₄)₃ has no solubility of other compounds in this system and melts congruently on its stoichiometric composition. Three distinct intermediate compounds were found. Gd₂O₃.6MoO₃ and Gd₂O₃.4MoO₃ are formed by a peritectic reaction at 730°C and 825°C, respectively. The remaining compound Gd₂O₃.MoO₃ with the structure closely related to Eu₂O₃.MoO₃ does not decompose below 1400°C.     

Synthesis and photoluminescent properties of Gd3O4Br:Er phosphors prepared by solid-state reaction method

Pure orthorhombic Gd₃O₄Br:Er³⁺ upconversion phosphors were synthesized by a solid-state reaction method and the structural properties of Gd₃O₄Br:Er³⁺ were investigated by X-ray diffraction; field emission scanning electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The results show that Gd₃O₄Br has low phonon cutoff energy, indicating that Gd₃O₄Br:Er³⁺may have high luminescent efficiency. Intense green (514–582 nm) and strong red (645–692 nm) upconverted luminescence of Gd₃O₄Br:Er³⁺ were observed under 980 nm laser excitation. The bright green emission is visible to the naked eyes even for 1 mW of the pump power (980 nm) for Gd₃O₄Br:Er³⁺ (0.1%) samples, indicating that Gd₃O₄Br:Er³⁺ may be used as upconversion phosphors.     

Preparation and phase transition of Gd4(Al1 − xGax)2O9 solid solutions

The solid-solution compounds of Gd₄(Al_{1 – x} Ga _x )₂O₉ (x = 0.0–1.0) were prepared at 1600°C for 5 h in air. The unit cell volume of the compounds increased from 0.853 to 0.878 nm³ with x. Phase transitions having a temperature hysteresis were observed from 1100° to 1400°C by calorimetry and dilatometry. The transition temperature increased with x. The volume of the high-temperature phase was 0.5% smaller than that of the low-temperature phase at the transition temperature. The volume changes were independent of x. The hysteresis width observed by the dilatometry was about 300°C for the Gd₄Al₂O₉ ceramics (grain size: about 1 m) and decreased to 50°C for the Gd₄(Al_0.2Ga_0.8)₂O₉ ceramics (grain size: over 10 m). Gd₄Ga₂O₉ was unstable at low temperature and decomposed to Gd₃GaO₆ and Gd₃Ga₅O₁₂ during the thermal analyses.     

Epitaxial growth experiments on SmLu- and EuLu-garnet films for the generation of magnetic bubble domains

Epitaxial growth of (SmLu)₃(FeAl)₅O₁₂ and (EuLu)₃(FeAl)₅O₁₂ films on the (111) plane of Gd₃Ga₅O₁₂ substrate has been studied with respect to relationships between melt composition and magnetic properties. A composition for bubble domains (2.5–8μm) has been defined experimentally, principally attained by changing the Al content in the melt. A lattice parameter for the mixed garnet can be expressed by a subtraction of 0.071 x (x = Al content in the film) from the interpolated lattice parameter for the garnet of end member. Growth induced uniaxial anisotropic energy (K_G) for SmLu-garnet (= 8022 erg/cm³) and for EuLu-garnet (= 10240 erg/cm³) exists in the films which have no lattice mismatch with the substrate. K_G decreases with an increasing of Al content (0.67 < x < 1.08 in atomic formula), whereas it is almost constant for SmLu-garnet.     

Green and blue magneto-optical photonic crystals

A series of one-dimensional heteroepitaxial all-garnet magneto-optical photonic crystals (MOPCs) were pulsed laser deposited to operate at 550 and 470 nm wavelength. We explored the concept of blue shift of the optical absorption edge of ferric ions by substituting Fe with Ga on the tetrahedral sites as well as Bi and Y, respectively, with Ca and Ce at the dodecahedral coordinated positions. 17-layered [Y₂Ce₁Fe₅O₁₂/Gd₃Ga₅O₁₂] MOPC with a total thickness of 968 nm demonstrates superior magneto-optical performance: Faraday rotation Θ_Fmax = + 2.0° and transmittance as high as 0.35 at the resonance wavelength of 470 nm.     

UV and VUV characteristics of (YGd)2O3:Eu phosphor particles prepared by spray pyrolysis from polymeric precursors

Red-emitting (YGd)₂O₃:Eu phosphor particles, with high luminescence efficiency under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation, were prepared by a large-scale spray pyrolysis process. To control the morphology of phosphor particles under severe preparation conditions, spray solution with polymeric precursors were introduced in spray pyrolysis. The prepared (YGd)₂O₃:Eu phosphor particles had spherical shape and filled morphology even after post-treatment irrespective of Gd/Y ratio. In the case of solution with polymeric precursors, long polymeric chains formed by esterification reaction in a hot tubular reactor; the droplets turned into viscous gel, which retarded the precipitation of nitrate salts and promoted the volume precipitation of droplets. The brightness of (YGd)₂O₃:Eu phosphor particles increased with increasing gadolinium content, and the Gd₂O₃:Eu phosphor had the highest luminescence intensity under UV and VUV excitation. The maximum peak intensity of Gd₂O₃:Eu phosphor particles under UV and VUV were 118 and 110% of the commercial Y₂O₃:Eu phosphor particles, respectively.     

The magnetic domain structure in low anisotropy epitaxial garnet films

Single-crystal films of (YCa)₃(FeGe)₅O₁₂ garnets were prepared by the liquid phase epitaxial method on the (111) plane of Gd₃Ga₅O₁₂ substrates. These magnetic bubble domain films have comparable uniaxial anisotropy energy, cubic anisotropy energy and demagnetizing energy. A photographic technique was developed to measure the angles of inclination of the domain magnetization vectors with respect to the normal to the sample surface. Without an applied magnetic field these inclination angles were 58° and 122°. A simple stripe domain model was used to provide an explanation of the experimental results. This model can easily be extended for other types of mixed anisotropy cases.     

Conversion of green emission into white light in Gd2O3 nanophosphors

Gd₂O₃ nanophosphors were prepared by combustion synthesis with and without doping of Dy³⁺ ions. The X-ray powder diffraction patterns indicate that as-prepared Gd₂O₃ and 0.1 mol% Dy₂O₃ doped Gd₂O₃ nanophosphors have monoclinic structures. The transmission electron microscope (TEM) studies revealed that the as-prepared phosphors had an average crystallite sizes around 37 nm. The excitation and emission properties have been investigated for Dy³⁺ doped and undoped Gd₂O₃ nanophosphors. New emission bands were observed in the visible region for Gd₂O₃ nanophosphors without any rare earth ion doping under different excitations. A tentative mechanism for the origin of luminescence from Gd₂O₃ host was discussed. Emission properties also measured for 0.1 mol% Dy³⁺ doped Gd₂O₃ nanophosphors and found the characteristic Dy³⁺ visible emissions at 489 and 580 nm due to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively. The chromaticity coordinates were calculated based on the emission spectra of Dy³⁺ doped and undoped Gd₂O₃ nanophosphors and analyzed with Commission Internationale de l'Eclairage (CIE) chromaticity diagram. These nanophosphors exhibit green color in undoped Gd₂O₃ and white color after adding 0.1 mol% Dy₂O₃ to Gd₂O₃ nanophosphors under UV excitation. These phosphors could be a promising phosphor for applications in flat panel displays.     

Defect formation in Gd3Fe5O12-based garnets: a Mössbauer spectroscopy study

The Mössbauer spectroscopy of Gd_2.2Pr_0.8Fe₅O₁₂ garnet, which exhibits higher electronic conduction with respect to Gd₃Fe₅O₁₂ due to the presence of Pr⁴⁺ cations, showed that praseodymium doping decreases the coordination of Fe³⁺ in octahedral sites. Penta-coordinated Fe³⁺ ions, in combination with small quantities of Fe⁴⁺, are also formed in the lattice of Gd_2.5Ca_0.5Fe₅O₁₂ where the variations of ionic and electronic transport properties indicate charge compensation via generation of oxygen vacancies and electron holes. The mechanisms of garnet lattice disorder, induced by acceptor- and donor-type doping, appear thus quite similar; in all cases, the ionic defect formation requires substantial structural reconstruction, probably associated with direct linking of iron–oxygen tetrahedra. Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd₃Fe₅O₁₂-based garnets.     

Recent progress in advanced optical materials based on gadolinium aluminate garnet (Gd3Al5O12)

Abstract

This review article summarizes the recent achievements in stabilization of the metastable lattice of gadolinium aluminate garnet (Gd₃Al₅O₁₂, GAG) and the related developments of advanced optical materials, including down-conversion phosphors, up-conversion phosphors, transparent ceramics, and single crystals. Whenever possible, the materials are compared with their better known YAG and LuAG counterparts to demonstrate the merits of the GAG host. It is shown that novel emission features and significantly improved luminescence can be attained for a number of phosphor systems with the more covalent GAG lattice and the efficient energy transfer from Gd³⁺ to the activator. Ce³⁺ doped GAG-based single crystals and transparent ceramics are also shown to simultaneously possess the advantages of high theoretical density, fast scintillation decay, and high light yields, and hold great potential as scintillators for a wide range of applications. The unresolved issues are also pointed out.