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.     

Effect of Yb concentration on optical properties of Yb:CaF2 transparent ceramics

In Yb:CaF₂, the coordination of Yb³⁺ in the CaF₂ lattice determines the spectroscopic properties that make Yb:CaF₂ a good candidate for high power laser applications. In this work, we measure the optical absorption, emission, and fluorescence lifetime of 0.1, 1, 5, and 10 at% Yb:CaF₂ ceramics to determine whether Yb³⁺ substitutes as hexamer clusters giving rise to the tenability and long fluorescent lifetime observed in Yb:CaF₂ single crystals. Absorption and emission spectra show that the concentration of Yb³⁺ present in hexamer clusters, as opposed to isolated ions, increases with increasing Yb³⁺ content. Fluorescence lifetime also increases with increasing Yb³⁺ content. Laser testing on a 1 at% Yb:CaF₂ transparent ceramic demonstrates that these materials are viable laser gain media.     

Preparation,spectroscopic properties and enhanced luminescence of Tb<Superscript>3+</Superscript>-doped LuAG phosphors and transparent ceramics by introduction of Sc<Superscript>3+</Superscript>

Tb-doped LuAG(lutetium aluminum garnet) and LuSAG(lutetium scandium-aluminum garnet) precursors were synthesized through a co-precipitation process, using ammonium hydrogen carbonate as precipitator. Single-phase cubic LuAG/Tb and LuSAG/Tb phosphors were obtained after calcination at 1000 and 1200 °C, respectively. These powders could be easily sintered into corresponding transparent LuAG/Tb and LuSAG/Tb ceramics in H₂ atmosphere at 1850 °C. The PL excitation and emission spectra were recorded for both phosphors and ceramics. Emission spectra of all materials were found to be typical for Tb³⁺, resulting from radiative relaxation of D level. Both the Tb-doped LuSAG phosphors and ceramics show higher efficient luminescence than LuAG , especially the transparent Tb-doped LuSAG ceramic shows about 150% higher luminescence intensity than transparent Tb-doped LuAG ceramic.     

Recent progress in quantum cutting phosphors

Luminescent materials with the quantum efficiency (QE) higher than unity could be playing a significant role in the progress of lighting industry and certain electronic display systems. The recent demonstration of an efficient visible quantum cutting (QC) in vacuum ultraviolet (VUV)-excited LiGdF₄:Eu phosphors [Wegh RT, Donker H, Oskam KD, Meijerink A. Visible quantum cutting in LiGdF₄:Eu³⁺ through downconversion. Science 1999; 283: 663-6] has provided an exciting and interesting trends in the development of several potentially important luminescent materials and devices. The possibility of the higher QE depends on the principle of QC in phosphors which could generate two or more low-energy photons for every incident high-energy photon that is being absorbed by phosphors. Investigation on QC systems has started on single ions doped-fluorides capable of a cascade emission from ions such as Pr³⁺, Tm³⁺, Er³⁺ and Gd³⁺. The focus has now been shifted to the combination of two ions, where the energy of the donor ion could be transferred stepwise to two acceptor ions via a downconversion. A well-known example is the Gd³⁺-Eu³⁺ dual ions. QC via downconversion has now been widely witnessed in many rare earths (RE)-based phosphors, the interesting and appreciable QE in the visible spectral region has earlier been reported from LiGdF₄:Eu (190%) and BaF₂:Gd,Eu (194%) phosphors. QC materials could also be used in solar cells, if conversion of one UV-visible photon into two near-infrared (NIR) photons is realized, and energy loss due to thermalization of electron-hole pairs is minimized. The present article reviews on the recent progress made on: (a) materials and developments in the fields of UV-visible QC phosphors and the mechanism involved, including QC in single RE ion activated fluorides- and oxides-based phosphors, energy transfer and downconversion, QC in dual/ternary ions activated phosphors; and (b) NIR QC in RE³⁺-Yb³⁺ (RE = Tb, Tm, and Pr) dual ions doped phosphors via cooperative energy transfer. Appropriate discussions have been made on materials, materials synthesis and characterization, the structural and luminescence properties of various QC luminescent materials via different synthesis techniques. In addition, applications, challenge and future advances of the visible- and NIR-QC phosphors have also been dealt with.     

Characteristics and synthesis mechanism of Gd2O2S:Tb phosphors prepared by vacuum firing method

Characteristics and synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing were investigated by photoluminescence (PL) spectra, X-ray diffraction (XRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). The mixtures of raw materials were tested by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The firing temperature was reduced to about 725 °C by the vacuum firing method. The particle size decreased. Meanwhile the particle size distribution and morphology were improved and the luminescence of Gd₂O₂S:Tb phosphors was also enhanced as the degree of vacuum decreases. With the decrease of the degree of vacuum, the intensity of the excitation spectrum was strengthened and the band was widened being the particle size of the host lattice decreased to nano scales. The peak with high intensity around 272 nm in the excitation spectra (λ_Em = 545 nm) of Gd₂O₂S:Tb nanophosphor may be attributed to the 4d–5f transitions of Gd atoms, which may play a significant role in the energy transfer between Tb³⁺ and Gd³⁺ ions.⁵D₄–⁷F_J transitions of Tb³⁺ ion were mainly concentrated in the narrow green emission spectrum (535–555 nm) with its sharp peak at 545 nm. The synthesis mechanism of Gd₂O₂S:Tb phosphors prepared by vacuum firing was also studied.     

Engineering of luminescence from Gd2O3:Eu nanophosphors by pulsed laser deposition

Nanostructured Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The dependence of structural, morphological and optical properties of these films on photoluminescence was systematically studied by varying the annealing temperature, Eu³⁺ incorporation concentration and laser fluence. The intensity of the XRD peak from (2 2 2) crystal plane was found to increase with annealing temperature in the range 973–1173 K. Films annealed at 1173 K show a preferential growth along (2 2 2) crystal plane of the cubic Gd₂O₃ and enhanced photoluminescence at 612 nm. XRD and Micro-Raman spectra and lattice strain investigations suggest that Eu³⁺ incorporation introduce a strong lattice distortion in Gd₂O₃ matrix. Morphological investigations using atomic force microscopy indicate a strong influence of the annealing process on the surface roughness and particle size. This kind of transparent thin film phosphors may promise for applications in flat-panel displays and X-ray imaging systems.     

The VUV–vis spectroscopic properties of phosphors Ca3Gd2(1−x)Ln2x(BO3)4 (Ln3+ = Ce,Sm, Eu,Tb)

The spectroscopic properties in VUV–vis range for phosphors calcium and gadolinium double borate Ca₃Gd₂(BO₃)₄ doped with rare-earth ions Ce³⁺, Sm³⁺, Eu³⁺ and Tb³⁺ were investigated. The host-related absorption, the f–d transitions of Ce³⁺ and Tb³⁺, as well as the charge transfer transitions of Sm³⁺ and Eu³⁺ in the host lattice are assigned and discussed. The CIE chromaticity coordinates for Eu³⁺- and Tb³⁺-activated phosphors are calculated.     

Synthesis and photoluminescence study of narrow-band UVB-emitting LiSr<Subscript>4</Subscript>(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Gd<Superscript>3+</Superscript>, Pr<Superscript>3+</Superscript> phosphor

A series of Pr³⁺, Gd³⁺ and Pr³⁺–Gd³⁺-doped inorganic borate phosphors LiSr₄(BO₃)₃ were successfully synthesized by a modified solid-state diffusion method. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. Surface morphology of the sample was studied by scanning electronic microscopy (SEM). The optimal concentrations of dopant Gd³⁺ ions in compound LiSr₄(BO₃)₃ were determined through the measurements of photoluminescence (PL) spectra of phosphors. Gd³⁺-doped phosphors LiSr₄(BO₃)₃ show strong band absorption in UV spectral region and narrow-band UVB emission under the excitation of 276 nm was only due to ⁶P _J → ⁸S_7/2 transition of Gd³⁺ ions. The effect of Pr³⁺ ion on excitation of LiSr₄(BO₃)₃:Gd³⁺ was also studied. The excitation of LiSr₄(BO₃)₃:Gd³⁺, Pr³⁺ gives a broad-band spectra, which show very good overlap with the Hg 253.7 nm line. The photoluminescence spectra of LiSr₄(BO₃)₃ with different doping concentrations Pr³⁺ and keeping the concentration of Gd³⁺ constant at 0.03 mol have also been studied. The emission intensity of LiSr₄(BO₃)₃:Pr³⁺–Gd³⁺ phosphors increases with increasing Pr³⁺ doping concentration and reaches a maximum at 0.01 mol. From the photoluminescence study of LiSr₄(BO₃)₃:Gd³⁺, Pr³⁺ we conclude that there was efficient energy transfer from Pr³⁺→ Gd³⁺ ions in LiSr_4?x?y Pr _x Gd _y (BO₃)₃ phosphors.     

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.     

Investigation on photoluminescence properties and defect chemistry of GdAlO3:Dy3+ Ba2+ phosphors

GdAlO₃:Dy³⁺ Ba²⁺ phosphors are synthesized by citrate-based sol-gel method. Photoluminescence and positron annihilation studies are used to investigate the emission and defect chemistry of the phosphors respectively. The strong yellow (Dy³⁺) emission properties of phosphors are discussed for various concentrations of Dy³⁺ ions. Upon the addition of Ba²⁺ ion, an enhancement in emission intensity is observed due to the lattice distortions around Dy³⁺ ion. The positron studies indicate the presence of defects at crystallite boundaries, vacancy clusters and large voids in the materials. The influence of Ba²⁺ ion on the photoluminescence and lattice distortion around Dy³⁺ is also explored.     

Energy transfer phenomena in GdMgB5O10

The luminescence of Mn²⁺, Bi³⁺, Ce³⁺ and Tb³⁺ in GdMgB₅O₁₀ and some codoped materials is reported. Energy transfer rates are derived from the experiments. The Bi³⁺ → Gd³⁺ and Gd³⁺ → Bi³⁺ transfer rates are about equal at room temperature. The excitation energy is able to migrate among the Gd³⁺ sublattice. The Ce³⁺ ion is a good sensitizer for this sublattice. By diluting the Gd³⁺ sublattice with La³⁺ ions, the energy migration in Gd³⁺ zig-zag chains was blocked and interchain energy transfer occurred. Very efficient phosphors can be obtained by using Ce³⁺ as a sensitizer, the Gd³⁺ sublattice as an intermediary and Tb³⁺ as an activator.     

Optical investigation of Eu:PbF2 ceramics and transparent glass-ceramics

This paper reports an optical investigation of glass-ceramics formed by annealing glasses with compositions 50 GeO₂-40 PbO-10 PbF₂-x EuF₃, x = [0.5; 1; 1.5; 2] and polycrystalline ceramics with composition 100 PbF₂, y EuF₃, with y = 5, 10, 15 and 20. For each material, the photoluminescence spectrum and the photoluminescence lifetimes of the ⁵D₀, ⁵D₁ and ⁵D₂ Eu³⁺ levels are measured. Occurrence of Eu³⁺:β-PbF₂ nanocrystallites in the glass-ceramics is confirmed and total ceramisation requires more than 10% of EuF₃ with respect to PbF₂ in the starting glass.In the Eu³⁺:β-PbF₂ ceramics and glass-ceramics, Eu³⁺ ions replace Pb²⁺ in their regular cubic site, but they interact together to form dimers and higher nuclearity clusters. These two species are easily distinguished according to their photoluminescence decay rate. For the EuF₃ rates investigated here, there are no isolated Eu³⁺ ions in the PbF₂ lattice.A preliminary investigation of the optical properties of co-doped Gd³⁺:Eu³⁺:β-PbF₂ ceramics was also performed. It shows that mixed Gd³⁺-Eu³⁺ dimers and clusters are formed, and that efficient Gd³⁺ → Eu³⁺ energy transfer occurs in these ceramics. The Pb²⁺ ions of the lattice may also be involved in the energy transfer process.     

Effect of Calcium on Nd:GdVO4 single crystal

The Ca_0.08Gd_0.92VO₄ crystal and Nd-doped Ca_0.08Gd_0.92VO₄ crystals were grown by the Czochralski method; the lattice parameters of the crystals were determined by XRD. The results showed that substituting Nd³⁺ caused an increase in lattice parameter; the segregation coefficient of Nd³⁺ ions in the crystal is lower than that of Nd:GdVO₄ crystal; the absorption spectra, fluorescence spectra, absorption cross-section and the emission cross-section of the Nd:Ca_0.08Gd_0.92VO₄ crystal were investigated at room temperature; furthermore, the Raman performance of Ca_0.08Gd_0.92VO₄ crystal was improved.     

EPR spectra of V (IV) in zirconolite-rich ceramics

EPR spectra of V⁴⁺ in zirconolite-rich ceramics with nominal composition Ca_1.00Gd_0.01Zr_0.99Ti_1.99V_0.01O_7.00 were studied. It was found that these spectra contain the lines of Gd³⁺ and V⁴⁺ ions. Computer calculation of the spectrum of V⁴⁺ with hyperfine structure showed that it is a superposition of two spectra of V⁴⁺ ions due to V⁴⁺ ions substituting for Ti⁴⁺ ions in two non-equivalent positions of these ions in zirconolite lattice. The analysis of linewidths of hyperfine components of the V⁴⁺ spectrum indicates the correlated positions of Gd and V in the zirconolite lattice with the distance between them r_v-Gd1 nm. It is assumed that V⁵⁺ ions also substitute for Ti⁴⁺ ions and compensate the electric charge of Gd³⁺ ions, which replace Zr⁴⁺ in the same unit cell of zirconolite crystal lattice.     

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.     

Flame synthesis and effects of host materials on Yb/Er co-doped upconversion nanophosphors

The upconversion nanophosphors (UCNPs) of Yb³⁺/Er³⁺ co-doped into Y₂O₃, La₂O₃, and Gd₂O₃ were synthesized via the combustion method and characterized by powder X-ray diffractometer (XRD), scanning electron microscopy (SEM) and upconversion fluorescence spectroscopy. The characterization results showed that at the same flame temperature (2705 K) and precursor concentration (0.1 M), pure monoclinic and cubic-phase phosphors were achieved on Gd₂O₃ and Y₂O₃ hosted UCNPs, respectively; while the mixed phases were observed on La₂O₃ hosted UCNPs. Further annealing process at 850 °C produced pure cubic-phase La₂O₃:Yb³⁺,Er³⁺ UCNPs; while there was no phase transition observed on Gd₂O₃:Yb³⁺,Er³⁺ UCNPs. The dependence of upconversion luminescence on precursor concentrations and host materials was then examined. The La₂O₃ and Gd₂O₃ hosts were shown to be the promising alternates for the commonly used Y₂O₃ hosts for rare-earth doped phosphors.     

Mise en evidence d'un transfert d'energie Gd3+ → Eu2+ dans les phases RbGd3F10 dopees a l'europium divalent

The α and β Rb_1?xEu_xGd₃F_{10+ x} phases are efficient U.V. phosphors. Emission consists of narrow f-f lines. Energy transfer between Gd³⁺ and Eu²⁺ ions is reported.     

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.     

Preparation of Gd2O2S:Pr Scintillation Ceramics by Pressureless Reaction Sintering Method

Fabrication of Gd₂O₂S:Pr scintillation ceramics by pressureless reaction sintering was investigated. The 2Gd₂O _3˙(Gd,Pr)2(SO ₄)_3˙mH₂O precursor was made by hydrothermal reaction using commercially available Gd₂O₃, Pr₆O₁₁ and H₂SO₄ as the starting materials. Then single phase Gd₂O₂SO₄:Pr powder was obtained by calcining the precursor at 750°C for 2 h. The Gd₂O₂SO₄:Pr powder compacts can be sintered to single phase Gd₂O₂S:Pr ceramics with a relative density of 99% and mean grain size of 30 μm at 1750°C for 2 h in flowing hydrogen atmosphere. Densification and microstructural development of the Gd₂O₂S:Pr ceramics were examined. Luminescence spectra of the Gd₂O₂S:Pr ceramic under 309 nm UV excitation and X-ray excitation show a green emission at 511 nm as the most prominent peak, which corresponds to the ³P₀-³H₄ transition of Pr³⁺ ions.     

Green emission enhancement of Ba2SiO4:Eu2+ phosphor via Gd co-doping and charge compensation

A series of new green-emitting Ba_2?x?2ySiO₄:xEu²⁺, yGd³⁺, yR⁺ (R = Li, Na or K) phosphors were synthesized by the solid-reaction method. X-ray diffraction (XRD) and fluorescence spectrophotometer are utilized to characterize the crystal structure and luminescence properties of the as-synthesized phosphors, respectively. The XRD patterns reveal that the doping of Gd³⁺, Eu²⁺ and R⁺ ions have no significant influence on the Ba₂SiO₄ phase. The green emission of Eu²⁺ ion associated with 4f⁶5d¹ → 4f⁷ can be obtained by 396 nm UV excitation source, which match well with the emission wavelength of UV-LEDs chip (380–420 nm). Moreover, the effect of charge compensator ions (Li⁺, Na⁺ or K⁺) on the luminescence intensity of (Ba, Gd)₂SiO₄:Eu²⁺ phosphors were also investigated. When introducing the Li⁺ ions into the (Ba, Gd)₂SiO₄ host lattices, the as-prepared phosphors show the strongest emission. The emission intensity of Ba_1.95SiO₄:0.04Eu²⁺, 0.005Gd³⁺, 0.005Li⁺ is about 1.39 times than that of Ba_1.96SiO₄:0.04Eu²⁺. Furthermore, the mechanism of energy transfer and concentration quenching of Ba_1.982?xSiO₄:xEu²⁺, 0.009Gd³⁺, 0.009Li⁺ phosphors are also discussed.