Time-resolved spectroscopy of intrinsic luminescence of Y3Ga5O12 and (LaLu)3Lu2Ga3O12 single crystals

The nature of intrinsic luminescence of Y₃Ga₅O₁₂ (YGG) and (LaLu)₃Lu₂Ga₃O₁₂ (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y_Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y_Ga antisite defects. Due to a large (2–3%) concentration of Lu_La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu_La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.     

Analysis of Ce luminescence quenching in solid solutions between Y(3)Al(5)O(12) and Y(3)Ga(5)O(12) by temperature dependence of photoconductivity measurement

Photocurrent excitation spectra were measured to investigate the quenching in the garnet solid solutions. Intense photocurrent excitation bands attributed to the lowest 5d(1) and the second lowest 5d(2) levels were observed in the Ce-doped Y(3)Al(2)Ga(3)O(12) (Ce:YAGG) and Y(3)Ga(5)O(12) (Ce:YGG). Based on the results of temperature dependence of photoconductivity, the 5d(1) and 5d(2) levels in the Ce:YAGG are found to be located below and within the conduction band, respectively, while both levels in the Ce:YGG are found to be located within its conduction band located at lower energy levels. In addition, the threshold of photoionization from the 4f level of Ce(3+) to the conduction band in the Ce:YAGG and Ce:YGG were estimated to be 3.2, and 2.8 eV, respectively. We conclude that the main quenching process in the Ce:YAGG is caused by the thermally stimulated ionization process with activation energy of 90 meV from the 5d(1) to the conduction band, and that in the Ce:YGG is caused by the direct ionization process from the 5d levels to the conduction band.     

Structural and luminescence properties of Ho(3+)/Yb(3+)-doped Lu3Ga5O12 nano-garnets for phosphor applications

Lu3Ga5O12 nano-garnet powders doped with Ho(3+)/Yb(3+) ions have been prepared using a citrate sol-gel technique. The structural and morphological properties have been investigated by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. The materials are found to exist in single phase of cubic garnet structure with an average particle size of around 45 nm. The Ho(3+)/Yb(3+)-doped Lu3Ga5O12 nano-garnet powders give rise to an intense green and weak red emission of Ho3+ ions under 457.5 nm direct excitation. Moreover, when the Yb3+ ions are excited at 950 nm a very bright green luminescence of the Ho3+ ions is observed by the naked eyes even for such low laser power as 10 mW and the intensity of the red emission have been increased compared to that found under direct excitation of the Ho3+ ions. The power dependency and dynamics of the infrared-to-visible upconverted luminescence confirm the existence of different two-photon energy transfer processes. All these results have been compared with those obtained for other garnets doped with similar lanthanide ions which suggest that the Lu3Ga5O12 nano-garnets are potential materials for light emitting devices.     

Comparative study of the luminescence of Y3Al5O12 nanoceramics and single crystals under excitation by synchrotron radiation

Comparative investigation of the luminescent properties of Y₃Al₅O₁₂ (YAG) nanoceramics with the properties of single crystals counterpart is performed under excitation by synchrotron radiation in the exciton range of YAG host. Analysis of the luminescent properties of such different crystalline forms of YAG allows us to conclude that the behavior of YAG nanoceramics is close to the properties of single crystal analogue with large content of Y_Al antisite defects (ADs). We also have found that the relative intensity of F⁺–AD coupled centers is significantly higher in YAG nanoceramics than that for single crystal counterpart. This presupposes the strong coupling of the antisite defects and oxygen vacancy-related centers in YAG nanoceramics, mainly at the boundaries of grains.     

Kinetics and thermodynamics of intracrystalline Fe-Ga distribution in Y3(Fe,Ga)5O12

Garnets of composition Y₃Fe_5–x Ga _x O₁₂, withx=0–5, were synthesized from oxides. Samples with various Ga content were annealed at temperatures between 700–1290° C; the heating duration varied between 90 s and 1350 h. Cation distribution was measured by Mössbauer spectroscopy at room temperature. The standard free energy change for the exchange reaction Fe³⁺ (tet)+ Ga³⁺ (oct)Fe³⁺ (oct)+Ga³⁺ (tet) is about 20 kJ mol^–1, and decreases slightly with increasing Fe content. The specific rate constants for the ordering process were determined according to the Mueller model for order-disorder kinetics. The activation energies for the ordering process between 200–250 kJ mol^–1 were calculated from the temperature dependence of the specific rate constants.     

Free carrier absorption in self-activated PbWO4 and Ce-doped Y3(Al0.25Ga0.75)3O12 and Gd3Al2Ga3O12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO₄, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd³⁺ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice.     

Single crystals growth and magneto-thermal properties of Dy3Ga5O12 garnet

Single crystals of Dy₃Ga₅O₁₂ garnet have been grown by the Czochralski method and the anisotropy of magnetic entropy evaluated for use as a magnetic material for a magnetic refrigeration in place of Gd₃Ga₅O₁₂ single crystals. As the size of the single crystal is large enough, and the anistropy is small, this crystal is suited to use for magnetic refrigeration in the temperature range of 1.8 to 15 K.     

Comparison of fluorescence spectra of Yb:Y3Al5O12 and Yb:YAlO3 single crystals

Yb:Y₃Al₅O₁₂ (Yb:YAG) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 25 at.%, 50 at.%, 100 at.% and Yb:YAlO₃ (Yb:YAP) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 30 at.% were grown by the Czochralski process. The fluorescence spectra of these crystals and the effects of self-absorption on the shape of the fluorescence spectra were studied. Through comparing the fluorescence spectra of Yb:YAG and Yb:YAP, all results indicate that the effects of self-absorption on the fluorescence spectra of Yb:YAP are remarkably stronger than that of Yb:YAG at the same Yb concentration.     

Time-resolved spectroscopy of Bi3+ centers in Y4Al2O9

Steady-state and time-resolved emission and excitation spectra as well as luminescence decay kinetics are studied at 4.2–400 K under excitation in the 3–6 eV energy range for Bi³⁺ ions substituting for Y³⁺ ions in four inequivalent crystal lattice sites of Y₄Al₂O₉:Bi ceramics. Luminescence characteristics of Bi³⁺ centers of all the four types are identified and are shown to arise from the radiative decay of the triplet relaxed excited state (RES) of Bi³⁺ ions. The parameters of the triplet RES, namely, probabilities of the radiative and nonradiative transitions from the metastable and emitting levels as well as the energy distance between these levels, are determined. The influence of the nearest surroundings of Bi³⁺ ions on the luminescence characteristics and the parameters of the triplet RES of Bi³⁺ centers is discussed.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.     

Piezoelectric properties of Sr3Ga2Ge4O14 single crystals

A new piezoelectric single crystal, Sr₃Ga₂Ge₄O₁₄ (SGG), has been grown successfully by the vertical Bridgman method with crucible-sealing technique. SGG crystal up to 2″ in diameter has been obtained. The relative dielectric constants, the piezoelectric strain constants, elastic compliance constants and electromechanical coupling factors have been determined with resonance and anti-resonance frequencies method by using the impedance analyzer (Agilent 4294A). The results show that the piezoelectric strain constants and electromechanical coupling factors of SGG single crystal are higher than those of LGS single crystals making it a potential substrate material for surface-acoustic wave applications.     

Two-gap structure in Yb(Y)Ba2Cu3O7−x single crystals

Tunneling effects in YbBa₂Cu₃O_7?x (YbBCO) and YbBa₂Cu₃O_7?x (YBCO) single crystals have been investigated using an improved technique of break junction preparation. A two-gap structure with BCS-like density of states has been observed. The value of the “large” gap is in the range of 24–28 meV for YBCO (T _c =85–90 K) and 29–30 meV for YbBCO (T _c =87–90 K). A variation of the value of the “small” gap between 0.5 and 11 meV has been detected, even for the same crystal. As a possible explanation, two-band superconductivity, surface properties, or strong gap anisotropy will be discussed.     

Mechanical relaxation of 4 wt% Y2O3-ZrO2 tetragonal single crystals

The mechanical relaxation was examined by a resonance piezoelectric method for a 3.5 wt % Y₂O₃-ZrO₂ polycrystal and 4 wt% Y₂O₃-ZrO₂ tetragonal single crystals with orientations in the 100 and 111 directions. The relaxation was observed for the 111-oriented crystal, but not for the 100-crystal. The results indicated that the relaxation was active only for the elastic compliance, S₄₄, and inactive for (s ₁₁–s ₁₂). The amplitude of the anelastic relaxation measured for the polycrystalline and the 111-crystal body was 4.2 × 10^–12 m²N^–1 and 6.7×10^–12 m² N^–1, respectively. An observed broad relaxation peak suggested that complex processes exist even in single crystals.     

Effect of annealing on the magnetic properties of Y3Fe5O12 and Y2.95La0.05Fe4.7Ga0.3O12 films

We have studied the effect of high-temperature (1273–1673 K) and low-temperature (523–873 K) anneals in air, O₂, and N₂ on the magnetic properties of Y₃Fe₅O₁₂ and Y_2.95La_0.05Fe_4.7Ga_0.3O₁₂ garnet ferrite films. The films, 5–15 μm in thickness, were grown on (111)-oriented gadolinium gallium garnet (Gd₃Ga₅O₁₂) substrates by liquid phase epitaxy. High-temperature annealing was shown to change the saturation magnetization and increase the ferromagnetic resonance linewidth ΔH of the films. Low-temperature annealing at 723 K in flowing dry oxygen for 20 h reduces the ΔH of the ferromagnetic resonance by 18–25% and makes it insensitive to changes in temperature and ambient humidity.     

(Y3-x-yREx)Al5O12:Cey(RE=Tb,Gd)荧光粉晶体结构与光致发光

采用化学共沉淀法合成具有Tb、Gd稀土掺杂的Y3Al5O12:Ce3 荧光粉,通过XRD和荧光光谱研究其晶体结构和光致发光.结果表明,(Y2.95-xTbx)Al5O12:Ce3 和(Y2.95-xGdx)Al5O12:Ce3 系列荧光粉具有与Y3Al5O12:Ce3 相同的石榴石晶体结构,但晶胞参数随Tb、Gd取代Y而略有增加;随Tb、Gd掺杂量增加,荧光粉发射波长逐渐红移,当x=2.95时,发射波长λmax分别从532nm红移至551和542nm;根据晶体场理论解释了荧光粉发射波长红移现象.这种发射波长红移的荧光粉能够显著地增强蓝光LED芯片与荧光粉组合形成的白光中红光成分,进而改善白光LED光源质量.     

Luminescent and scintillation properties of Lu3Al5O12:Sc single crystal and single crystalline films

The work is dedicated to growth by the liquid phase epitaxy method and study of the luminescence and scintillation properties of Sc³⁺ doped single crystalline films (SCF) of Lu₃Al₅O₁₂ (LuAG) garnet. The scintillation properties of SCF are compared with single crystal (SC) analogues grown by the Horizontal Direct Crystallization and Czochralski methods. We consider the dependence of intensity of the Sc³⁺ emission in LuAG host on the activator concentration and influence of flux contamination on the light yield (LY) of the Sc³⁺ luminescence in LuAG:Sc SCF with respect to their SC counterparts and the reference YAP:Ce scintillator. From the NMR investigations of LuAG:Sc SCF we confirm the substitution by Sc³⁺ ions both the octahedral and dodecahedral positions of LuAG host and formation of the Sc_Al and Sc_Lu related emission centers, respectively. We also show that the luminescence spectrum in the UV range and decay kinetics of LuAG:Sc SCF can be effectively tuned by changing the scandium content.     

Anti-stokes luminescence of Y2O3:Er

Diffuse reflectance and spontaneous photoluminescence excitation spectra of the Y₂O₃:Er (10 at % Er) compound have been studied under varied optical pumping conditions. The results demonstrate that the anti-Stokes luminescence of erbium-doped yttrium oxide has high intensity when two different photon energies are used for infrared illumination. The resonance wavelengths of IR photons for two-photon excitation of visible emission in the Y₂O₃:Er phosphor have been determined, and the corresponding electron transitions in erbium-related emission centers have been identified for the Stokes and anti-Stokes luminescence.     

Electro-mechanical behaviour of single domain single crystals of bismuth titanate (Bi4Ti3O12)

The piezoelectric constantsd ₁₁,d ₂₂ andd ₃₃ for a single domain single crystal of Bi₄Ti₃O₁₂ have been measured at room temperature using appropriate crystal cuts on a Berlincourtd ₃₃ meter. From the known temperature dependence of the spontaneous polarization, lattice strain and dielectric permittivity, the equivalent piezoelectric constants have been calculated assuming a simple proper ferroelectric in which,P _s, is the order parameter. Constants in the plane of the perovskite-like sheets in the Bi₄Ti₃O₁₂ structure (d ₁₁,d ₂₂) show good agreement with experimental values. The calculated value of the constantd ₃₃ is more than an order of magnitude larger than the experimentally-measured value and gives clear indication of the indirect coupling to the strain in the tetrad axial direction and the need for a more sophisticated phenomenology.     

Effect of doping with Pb, Ba, and Si on the properties of Y3Fe5O12 single crystals

In doped single-crystal yttrium iron garnet, Y₃Fe₅O₁₂, photoinduced changes in the cubic magnetostriction constant λ₁₁₁ and coefficient (λ_σ) that characterizes extra stress in the material are determined by the sublattice the dopant is incorporated into. The incorporation of dopant cations into a particular crystallographic site can be controlled by adjusting the crystal growth process. Varying the process for the growth of doped Y₃Fe₅O₁₂ single crystals with semiconducting properties, one can tune the type and magnitude of their photoinduced response. Since an ideal sample cannot be created in practice, and even the lowest densities of growth defects in a single crystal lead to photomagnetism, the present results suggest that, at sufficiently low temperatures, photomagnetic changes occur in almost all Y₃Fe₅O₁₂ single crystals. The behavior of their photomagnetic response depends on the type of defect.