Luminescence spectroscopy of excitons and antisite defects in Lu3Al5O12 single crystals and single-crystal films

The nature of the intrinsic luminescence of the lutetium aluminum garnet Lu₃Al₅O₁₂ (LuAG) has been analyzed on the basis of time-resolved spectral kinetic investigations upon excitation of two model objects, LuAG single crystals and single-crystal films, by pulsed X-ray and synchrotron radiations. Due to the differences in the mechanisms and methods of crystallization, these objects are characterized by significantly different concentrations of Lu_Al antisite defects. The energy structure of luminescence centers in LuAG single crystals (self-trapped excitons (STEs), excitons localized near antisite defects, and Lu_Al antisite defects) has been established. For single-crystal LuAG films, grown by liquid-phase epitaxy from a Pb-containing flux, the energy parameters of the following luminescence centers have been determined: STEs in regular (unperturbed by the presence of antisite defects) sites of the garnet lattice and excitons localized near Pb²⁺ ions. The structure of the luminescence centers, related to the background emission of impurity Pb²⁺ ions, has also been established in the UV and visible ranges. It is suggested that, in contrast to the two-halide hole self-trapping, a self-trapped state similar to STEs in simple oxides (Al₂O₃, Y₂O₃) is formed in LuAG; this state is formed by self-trapped holes in the form of singly charged O^? ions and electrons localized at excited levels of Lu³⁺ cations.     

Nd-doped Lu3Al5O12 single-crystal scintillator for X-ray imaging

The optical and scintillation properties of Nd-doped Lu₃Al₅O₁₂ (Nd:LuAG) crystals grown by the Czochralski (Cz) method were examined under X-ray excitation. Their applicability for X-ray imaging was also inspected. The radioluminescence spectrum induced by X-rays showed a broad host emission and sharp Nd³⁺ 4f–4f emission peaks in the UV to visible wavelengths. The light output current of the Nd:LuAG was 85% of that of a standard CdWO₄ X-ray scintillator. The afterglow value measured 20 ms after X-ray irradiation was 1.5%. An X-ray radiographic image was successfully obtained using the Nd:LuAG scintillator coupled with the charge coupled device (CCD) photodetector.     

Luminescence and scintillation properties of advanced Lu3Al5O12:Pr3+ single crystal scintillators

In this paper we present the luminescence and scintillation properties of Lu₃Al₅O₁₂:Pr (LuAG:Pr) single crystals with Pr³⁺ concentration of 0.13 wt %, grown by the Czochralski method. The light yield and energy resolution were measured under 662 keV γ-ray excitation. The dominant emission band peaking at 310 nm with a shoulder at 370 nm was observed in the photoluminescence spectrum. High light yield of 24,500 ph/MeV and an energy resolution of 5.3 % were obtained for a 6 × 6 × 2 mm³ LuAG:Pr sample. Light yield dependence on sample height and shaping time was measured. The estimated photofraction in pulse height spectrum and total mass attenuation coefficient at 662 keV γ-rays were also determined and compared with the theoretical ones calculated using the WinXCom program.     

Photoluminescence of Lu3Al5O12:Bi and Y3Al5O12:Bi single crystalline films

Single crystalline films of Lu₃Al₅O₁₂:Bi and Y₃Al₅O₁₂:Bi have been studied at 4.2–450 K by the time-resolved luminescence spectroscopy method. Their emission spectrum consists of two types of bands with strongly different characteristics. The ultraviolet band consists of two components, arising from the electronic transitions which correspond to the ³P₁ → ¹S₀ and ³P₀ → ¹S₀ transitions in a free Bi³⁺ ion. At T < 80 K, mainly the lower-energy component with the decay time ～10^?3 s is observed, arising from the metastable ³P₀ level. At T > 150 K, the higher-energy component prevails, arising from the thermally populated emitting ³P₁ level. The visible emission spectrum consists of two dominant strongly overlapped broad bands with large Stokes shifts. At 4.2 K, their decay times are ～10^?5 s and ～10^?4 s and decrease with increasing temperature. Both of the visible emission bands are assumed to be of an exciton origin. The lower-energy band is ascribed to an exciton, localized near a single Bi³⁺ ion. The higher-energy band, showing a stronger intensity dependence on the Bi³⁺ content, is assumed to arise from an exciton, localized near a dimer Bi³⁺ center. The structure of the corresponding excited states is considered, and the processes, taking place in these states, are discussed.     

Infrared spectroscopy and lattice dynamical calculations of Gd3Al5O12, Tb3Al5O12 and Lu3Al5O12 single crystals

The infrared (IR) reflectance spectra of Gd₃Al₅O₁₂, Tb₃Al₅O₁₂ and Lu₃Al₅O₁₂ single crystals are studied for the first time. At room temperature, 15 infrared active modes for Lu₃Al₅O₁₂ and 14 for Gd₃Al₅O₁₂, Tb₃Al₅O₁₂, out of the 17 theoretically predicted, have been experimentally recorded. The complex dielectric function the refractive index, the absorption coefficient as well as the longitudinal (ω_LO) and transverse (ω_TO) frequencies of the long-wavelength T_1u modes are determined by the Kramers-Kronig transformation of the reflectance spectra. The experimental data are compared and discussed with the theoretical results obtained by the rigid ion model. Our theoretical analysis reveals that the bonds in the tetrahedra exhibit a covalent character while those in the dodecahedra almost ionic character, which is in accordance with the results for other materials of this crystal family.     

Distortion of the Ligand Environment of Gd3+ and Eu2+ Ions in Garnets: Y3Al5O12, Lu3Al5O12, and Y3Ga5O12

Physics of the Solid State - Using experimental second-rank&nbsp;parameters of the spin Hamiltonian of the rhombic centers Gd3+ and Eu2+ in three garnets and the superposition approximation...     

Studies of the spin-Hamiltonian parameters and defect structures for the trigonal Mo 3+ centers in Y 3Al 5O 12 and Lu 3Al 5O 12 crystals

The complete diagonalization (of energy matrix) method (CDM) and the perturbation theory method (PTM) are applied to calculate the spin-Hamiltonian (SH) parameters (electron paramagnetic resonance g factors g _//, g _⊥ and zero-field splitting D) of the trigonal Mo ³⁺ centers in Y ₃Al ₅O ₁₂ and Lu ₃Al ₅O ₁₂ crystals. Both methods are based on the cluster approach in which the covalence effect due to the admixture between the d orbitals of central d ⁿ ion and p orbitals of ligands is considered. The g factors calculated by both methods are close to each other and agree with the experimental values. However, the calculated zero-field splittings D from PTM for both crystals are about 84% those from CDM. The reasons that the CDM is preferable to the PTM in the calculations of SH parameters are discussed. The angular distortions of Mo ³⁺ centers in both garnet crystals are predicted.     

Defect states in Lu 3 Al 5 O 12 :Ce crystals

Wavelength-resolved TSL measurements after x-ray irradiation at room temperature were performed on undoped, Ce- and (Ce, Zr)-doped Lu 3 Al 5 O 12 crystals. Several glow peaks were observed in the 20-450 v C temperature region, whose intensities and emission spectra are influenced by doping. Namely, the TSL spectra are governed by defect and trace impurity-related emissions in the undoped crystal, while only the Ce 3+ emission is detected in the doped crystals. Moreover, EPR measurements performed at 12 v K revealed typical spectra of Ce 3+ (4f 1 , S =1/2) occupying Lu 3+ substitutional positions. In situ light irradiation (250-330 v nm) of the crystals results in a step-like decrease of the Ce 3+ EPR intensity. Such a behaviour can reflect a creation of Ce 4+ ions under UV irradiation or an increase of the Ce 3+ population in the excited (5d) state.     

Luminescence of La and Sc impurity centers in YAlO3 single-crystalline films

The luminescence of La_Y³⁺ and Sc_Y³⁺ and Sc_Al³⁺ centers created by lanthanum and scandium ions at Y³⁺ and Al³⁺ cation sites of YAlO₃ perovskite lattice was investigated. The features of emission of excitons localized at the mentioned centers in YAlO₃:La and YAlO₃:Sc single-crystalline films were analyzed by means of time-resolved emission spectroscopy and luminescence decay kinetics measurements under excitation by synchrotron radiation at 9 and 300 K.     

Comparative study of Tm-doped and Tm-Sc co-doped Lu3Al5O12 scintillator

The crystals of Tm doped and Tm-Sc co-doped Lu₃Al₅O₁₂ (LuAG) grown by the floating zone (FZ) method were examined for their optical and scintillation properties. In transmittance spectra, strong absorption lines due to Tm³⁺ 4f–4f transitions were observed. X-ray excited radioluminescence spectra were measured and broad and sharp emission peaks were detected. The former one was attributed to Sc³⁺ and the latter one was due to Tm³⁺ 4f–4f transitions. Scintillation yield enhancement due to Sc co-doping was observed by means of ¹³⁷Cs pulse height spectra. Scintillation decay times were several tens of μs under pulse X-ray excitation.     

Luminescence centers in Sc2O3

Sc₂O₃ luminescence spectra are studied. The spectra are separated into elementary bands by the Alentsev–Fock method. It is established that the luminescence spectra consist of a number of overlapping bands with maxima at 3.5; 3.05; 2.65; 2.35, and 2.05 eV. The band at 3.5 eV is interpreted as emission of self-localized excitons, and the other bands, as defect-center recombination. L’vov State University, 50, Dragomanov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 776–778, November–December, 1997.     

Enhanced superhyperfine interaction on Ru3+ in Tm3Al5O12 and on Ru3+ and Yb3+ in Tm3Ga5O12

Measurements of the enhanced superhyperfine interaction linking the electron spin of an impurity EPR ion and the nuclear moments of Van Vleck host lattice ions are reported for Ru³⁺ in Tm₃Al₅O₁₂ and for Ru³⁺ and Yb³⁺ in Tm₃Ga₅O₁₂. The Ru³⁺ results appear to be the first reported where this interaction is large enough to give rise to a well resolved superhyperfine structure. For Yb³⁺ the observed temperature dependent line narrowing evidences the role of Tm spin fluctuations.     

Luminescence of Pr3+ ions in Y3Al5−xGaxO12 solid solutions

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 5, pp. 774–778, November, 1991.     

Absorption and luminescence of Nd3+:Y3Al5O12

In this paper there are briefly summarized the results of our experimental measurements of the fluorescence spectra, originating in levels of the⁴ F _3/2 term and terminating on the first two terms of the⁴I multipet, and the results of the absorption spectra at room temperature (RT) and at liquid-nitrogen temperature (NT) from which Nd³⁺: Y₃Al₅O₁₂ energy-level diagram was determined. Further, using the splitting of the⁴F_3/2,⁴F_5/2 and⁴F_7/2 terms the crystal-field parameters were calculated.     

Synthesis and luminescence properties of Er3+-doped Lu3Ga5O12 nanocrystals

The concentration-dependent luminescence properties of sol–gel-derived nanocrystalline Lu_3(1?x)Er_3xGa₅O₁₂ powders (where x=0.01, 0.05 and 0.1) have been studied. Laser-excited luminescence spectra, emission decays and upconversion luminescence of Er³⁺-doped Lu₃Ga₅O₁₂ nanocrystalline samples have been measured. The decay curve of the (²H_11/2,⁴S_3/2) emission exhibits a non-exponential behavior presumably due to cross-relaxation process. Moreover, near-infrared to visible upconversion luminescence has been observed in the green region for 1.0 mol% Er³⁺ ions in Lu₃Ga₅O₁₂ nanocrystals upon 815 nm excitation. The power dependence of the anti-Stokes luminescence suggests that upconversion is probably achieved through the sequential absorption of two photons. To the best of our knowledge, this is the first report on the preparation and optical properties of Er³⁺-doped Lu₃Ga₅O₁₂ in the form of nanocrystalline powders.