EPR of Gd3+ - doped CeF3 single crystal: Linewidth variation with temperature

Detailed X-band EPR study of a Gd³⁺-doped CeF₃ single crystal has been made from 4.2 to 473 K, with particular attention to EPR linewidths. In general, it is found that there are four regions over which the log-log plot of the linewidth versus temperature is linear, implying separate power-law dependences of the linewidth. Gd³⁺ spin Hamiltonian parameters in CeF₃ have been estimated at various temperatures from the line positions. From the linewidth variation with temperature the Debye temperature has determined to be about 140 K.     

Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.     

Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor

Yb³⁺ doped phosphor of Gd₂O₃ (Gd₂O₃:Yb³⁺) have been prepared by solid state reaction method. The structure and the particle size have been determined by X-ray powder diffraction measurements. The average particle size of the phosphor is in between 35 and 50 nm. The particle size and structure of the phosphor was further confirmed by TEM analysis. The visible and NIR luminescence spectra were recorded under the 980 nm laser excitation. The visible upconversion luminescence of Yb³⁺ ion was due to cooperative luminescence and the presence of rare earth impurity ions. The cooperative upconversion and NIR luminescence spectra as a function of Yb³⁺ ion concentration were measured and the emission intensity variation with Yb³⁺ ion concentration was discussed. Yb³⁺ energy migration quenched the cooperative luminescence of Gd₂O₃:Yb³⁺ phosphor with doping level over 5%, while the NIR emission luminescence continuously increases with increasing Yb³⁺ ion concentration.     

Temperature and concentration dependences of the spin-lattice relaxation rate in four borate glasses doped with Fe2O3

Spin-lattice relaxation time (T ₁) of Fe³⁺ ions has been measured in four borate glasses doped with 0.25, 0.5, 1 and 3 wt.% Fe₂O₃ by the modulation method over the temperature range 5–250 K. In the three less concentrated samples, it was observed thatT ₁ ^?1. αT at intermediate temperatures. This is explained in terms of a relaxation mechanism involving TLS (Two Level Systems). At higher temperatures, the temperature dependence ofT ₁ ^?1 is slowed down by cross-relaxation. In the most concentrated sample, the exchange interaction plays a dominant role leading to a very different relaxation-time behaviour, described well by the Bloembergen and Wang three-reservoir model.     

Processes of the excitation energy migration and transfer in Ce-doped alkali gadolinium phosphates studied with time-resolved photoluminescence spectroscopy technique

Spectral-kinetic characteristics of Gd³⁺ and Ce³⁺ luminescence from a series of Ce³⁺-doped alkali gadolinium phosphates of MGdP₄O₁₂ type (M=Li, Na, Cs) have been studied within 4.2-300 K temperature range using time-resolved luminescence spectroscopy techniques. The processes of energy migration along the Gd³⁺ sub-lattice and energy transfer between the Gd³⁺ and Ce³⁺ ions have been investigated. Peculiarities of these processes have been compared for MGdP₄O₁₂ phosphate hosts with different alkali metal ions. A contribution of different levels from the ⁶P_j multiplet of the lowest Gd³⁺ excited state into the energy migration and transfer processes has been clarified. The phonon-assisted occupation of high-energy ⁶P_{5/2, 3/2} levels by Gd³⁺ in the excited ⁶P_j state has been revealed as a shift of Gd³⁺⁶P_j→⁸S_7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd³⁺ via phonon-assisted population of Gd³⁺⁶P_5/2 level (next higher one to the lowest excited ⁶P_7/2) is supposed to be responsible for the rise in probability of energy migration within the Gd³⁺ sub-lattice initiating the Gd³⁺→Ce³⁺ energy transfer at T<150 K, whereas further intensification of Gd³⁺→Ce³⁺ energy transfer at T>150 K is explained by the increase in probability of Gd³⁺ relaxation into the highest ⁶P_3/2 level of the ⁶P_j multiplet. An efficient reversed Ce³⁺→Gd³⁺ energy transfer has been revealed for the studied phosphates at 4.2 K.     

Non-adiabatic small-polaron hopping conduction in VN–PbO–TeO2 glasses

The dc conductivity of VN–PbO–TeO₂ glasses with different mole percentages of VN, PbO and TeO₂ has been measured in the temperature range 125–450?K. The conductivity of the glasses increases with increasing VN content for a fixed mole percentage of PbO. Neither Mott's variable-range hopping (VRH) model at low temperatures (T<Θ_D/4, where Θ_D is the Debye temperature) nor Greaves’ VRH model at intermediate temperatures (Θ_D/?4<T<Θ_D/2) describe the dc conductivity data for these glasses. Multiphonon tunnelling transport of strongly coupled electrons is also unable to account for the carrier transport. However, at high temperatures (T?>?Θ_D/2), conduction is shown to be due to small-polaron hopping in the non-adiabatic regime. Alteration of the VN content causes a change in the model parameters achieved from best-fitting curves for the glasses. Modulated differential scanning calorimetry analysis shows that the glass transition temperatures T _g in this system vary from 269 to 302°C.     

Optical properties of Sm3+ and Dy3+ ions in Gd2MoB2O9 host lattice

Gd₂MoB₂O₉ doped with Sm³⁺ and Dy³⁺ were used for this study. The photoluminescence behaviors of Sm³⁺ and Dy³⁺ in this phosphor material were investigated by the excitation and emission spectra. The Sm³⁺-doped Gd₂MoB₂O₉ phosphor powders show a red luminescence, whereas the Dy³⁺-doped Gd₂MoB₂O₉ phosphor powders show a yellow luminescence. In addition, the optimum doping concentration and the time-resolved luminescence spectroscopy were also investigated.     

Spektroskopische Untersuchungen an DyAsO4

DyAsO₄ undergoes a crystallographic phase transition atT _D=11.2K which is induced by a cooperative Jahn-Teller-effect. As deduced from the optical absorption spectra the distance between the two lowest lying Kramers doublets of the Dy³⁺ ion is increased from (6.1±0.5) cm^?1 aboveT _D to (25.0±0.5) cm^?1 at 4.2 K. BelowT _D the splitting factor of the lowest doublet becomes nearly uniaxial with a maximum value ofg _b =17.5±1.0 along the crystallographicb-axis. AtT _N=2.44 K the crystals order antiferromagnetically. The absorption lines of Er³⁺ ions in DyAsO₄ show already a splitting immediately belowT _D which is explained by magnetic short range ordering of the Dy³⁺ ions in the temperature rangeT _{N D} .     

Estimation of T1 for Co2+ ions from temperature variation of the EPR linewidths for Gd3+ in Ce2Co3(NO3)12.24H2O single crystals

EPR of Gd³⁺ doped in Ce₂M″₃(NO₃)₁₂.24H₂O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd³⁺ spectra at room temperature in Ce₃Co₂(NO₃)₁₂.24H₂O has been interpreted in terms of a random modulation of the interaction between the Gd³⁺ and the Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It is found that the effective spin-lattice relaxation time T₁ ∝ T⁻ⁿ where n = 1.85 (B∥z axis) and n = 1.75 (B⊥z axis) if 103 < T < 283 K.     

Multisites Cr3+ in GGG and GSGG garnets

The emission and excitation spectra of Cr³⁺ -doped Gd₃Ga₅O₁₂ (GGG) and Gd₃Sc₂Ga₃O₁₂ (GSGG) are explained in the light of multisite effects. The situation is particularly complicated in the case of GSGG, where the different sites have²E energy levels near each other which overlap with the⁴A₂ →⁴T₂ absorption bands. The spectra obtained under selective excitations are interpreted on the multisite assumption.     

Synthesis of a new magnetic Y2Mn2/3Re4/3O7 with pyrochlore-like structure

A complex oxide of the Y₂Mn_2/3Re_4/3O₇ composition with pyrochlore-like structure and parameters of hexagonal unit cell _a=14.91(1) Å _c=17.53(1) Å was synthesized. The magnetic susceptibility and magnetization measurements showed that below 190 K this oxide possesses spontaneous magnetic moment. In the paramegnetic region, the magnetic susceptibility obeys the Curie-Weiss law χ=C/(T?Θ), with C=2.07 cm³ K mol^?1 and Θ=?160 K, and the effective magnetic moment corresponding to the cationic combination Mn²⁺-Re⁵⁺. The data obtained allow one to assume that the compound has a noncollinear antiferromagnetic structure.     

Pressure-dependent luminescence properties of Tb3+-doped K–Ba–Al fluorophosphate glass

Tb³⁺-doped fluorophosphate glass with a composition of P₂O₅+K₂O+KF+BaO+Al₂O₃+Tb₄O₇ was prepared by melt quenching technique and characterized through emission spectra and decay curves as a function of pressure up to 37.7 GPa. The ⁵D₄→⁷F₃, ⁷F₄ and ⁷F₅ emission transitions shift toward lower energy in the order of?4.7,?2.4 and?4.0 cm^?1 GPa^?1, respectively, with increasing pressure. A strong increase in the splitting of the ⁵D₄→⁷F₅ Stark levels with pressure is observed. The decay curves for the ⁵D₄ level are found to be almost perfectly single exponential in the entire pressure range studied. Lifetimes of the ⁵D₄ level are found to decrease strongly from 2564 to 886 μs when pressure is increased from ambient to 37.7 GPa. The pressure dependence of all the derived values of the present study is compared with Tb³⁺ in other glasses.     

Growth, spectroscopic characteristics and laser potential of Yb3+:Ca3La2(BO3)4 crystal

Crystal of Yb³⁺-doped Ca₃La₂(BO₃)₄ has been grown by the Czochralski technique. The room temperature absorption and fluorescence spectra of the crystal have been investigated. The result showed that this crystal exhibits broad absorption and emission with the FWHM of 11 nm at 978 nm and 66 nm FWHM at 1025 nm, respectively. The stimulated emission cross-section of Yb³⁺ ions were calculated using the reciprocity method and Fuchtbauer-Ladenburg method, respectively. The room temperature fluorescence decay curves of ² F _5/2 manifold of Yb³⁺ ions were recorded for both crystal and powder samples. The effect of radiation tapping on the spectroscopic properties is discussed. The result that the lifetime of the powder sample is shorter than that of the bulk sample demonstrates the existence of radiation trapping effect. The laser potentiality was also evaluated and the results show that this crystal is a good candidate for tunable and ultrashort pulse lasers.     

Laser spectroscopy of spinel microcrystallites in a Cr3+ doped silicate glass

Laser spectroscopy of Cr³⁺ ions makes it possible to follow the crystallization process in a cordiente glass (52 SiO₂, 347 Al₂O₃, 12.5 MgO and 08 Cr₂O₃) Absorption and fluorescence are interpreted by structural considerations showing the variation of Cr³⁺ environment during heat treatment Fluorescence line narrowing is performed at 4 2 K giving information on the detailed crystal structure in MgAl₂O₄ spinel microcrystallites formed during heat treatment The splitting of the ²E level is close to 70 cm^?1 and the ¦2D¦ parameter ranges between 0 95 and 1 35 cm^?1 This wide distribution is associated with the well known disordered distribution of Mg²⁺ and Al³⁺ cations in MgAl₂O₄ spinels.     

Measurement of optical dephasing of Eu3+ and Pr3+ doped silicate glasses by spectral holeburning

Long-lived spectral holeburning has been observed in Eu³⁺ and Pr³⁺ doped silicate glasses, and used to measure homogeneous linewidths (φ_h). In the case of Eu³⁺, holes are burned by optical pumping of nuclear quadrupole levels, and our low temperature measurement of φ_h shows that the previously determined T^1.8 temperature dependence holds down to at least 1.6 K. On the other hand, in Pr³⁺ doped glass, holes are burned by optically induced local lattice rearrangements. In contrast to previously studied inorganic glasses, we find a linear temperature dependence of φ_h for the ³H₄ ? ¹D₂ transition from 1.6 K to 20 K. This was confirmed by picosecond accumulated grating photon echoes. Current theories of dephasing in glasses do not predict these temperature dependences. Holeburning has also been observed in Pr³⁺ doped BeF₂ and phosphate glasses and Nd³⁺ doped silicate glass and thus appears to be a rather general phenomenon in inorganic glasses, as has been found for molecules in organic glasses.     

Electron spin-lattice relaxation in noncommon metals YBa2Cu3Ox and Rb1C60

Using an original modulation technique, the electron spin-lattice relaxation have been investigated in two noncommon metals: YBa₂Cu₃O_x, high-T_c material doped with 1% Gd, and Rb₁C₆₀, linear polymer phase fulleride. In the first case, the Korringa-like temperature dependence of the Gd³⁺ longitudinal relaxation time T₁, is found forx = 6.59 in a wide temperature range 4.2 <T < 200 K, both above and below T_c = 56 K. Atx = 6.95 (T_c = 90 K), the T₁ behavior within 50 <T < 200 K is evidently affected by spin gap opening with the gap value of about 240 K. At 200 K, an unexpected acceleration of the relaxation rate takes place, suggesting some change in the relaxation mechanism. The data are discussed in terms of the Barnes-Plefke theory with allowance made for microscopic separation of the normal and superconducting phases. In Rb₁C₆₀, the evolution of the ESR line and relaxation rates have been studied accurately in the range of the metal-insulator transition (below 50 K). Interpretation is suggested which takes into account breaking down the relaxation bottleneck due to opening of the energy gap near the Fermi surface. The gap value of about 100 cm^?1 is estimated from the analysis of relaxation rates, lineshape and spin susceptibilities.     

Electron paramagnetic resonance of Gd<Superscript>3+</Superscript> aqua complexes in vitrified aqueous solutions and Gd<Superscript>3+</Superscript> aqua ions adsorbed on the capillary surface

Electron paramagnetic resonance (EPR) spectra of Gd³⁺ aqua complexes are measured in dilute aqueous solutions of Gd(NO₃)₃ (C<0.2 M) at room temperature. A partial resolution of the fine structure observed in the spectra is characteristic of solid disordered systems and results in an increase in the effective width of the EPR line with a decrease in the Gd³⁺ concentration. This phenomenon is explained in terms of adsorption of Gd³⁺ aqua ions on the surface of the measuring capillaries. The fine structure is revealed in the EPR spectra of Gd(NO₃)₃ aqueous solutions, namely, the Gd(NO₃)₃ solutions vitrified at a temperature of 77 K (with an addition of 10–15 vol % glycerol) and Gd(NO₃)₃ solutions quasi-vitrified at 298 K (with an addition of 70–90 vol % glycerol). Analysis of the EPR spectra demonstrates that these solutions contain two types of aqua complexes with fine structure parameters D₁=180 G and D₂=580 G. Reasoning from a comparison with x-ray diffraction data, the fine structure parameters D₁ and D₂ are assigned to higher symmetric eight-coordinate and lower symmetric nine-coordinate Gd³⁺ aqua complexes, respectively.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Fabrication and luminescence behavior of phosphate glass ceramics co-doped with Er3+ and Yb3+

Transparent phosphate glass ceramics co-doped with Er³⁺ and Yb³⁺ in the system P₂O₅Li₂OCaF₂TiO₂ were successfully synthesized by melt-quenching and subsequent heating. Formation of the nanocrystals was confirmed by X-ray powder diffraction. Judd–Ofelt analyses of Er³⁺ ions in the precursor glasses and glass ceramics were performed to evaluate the intensity parameters Ω_2,4,6. Under 975 nm excitation, intense upconversion (UC) and infrared emission (1545 nm) were observed in the glass ceramics by efficient energy transfer from Yb³⁺ to Er³⁺. The luminescence processes were explained and the emission cross section was calculated by Fuchtbauer–Ladenburg (F–L) formula. The results confirm the potential applications of Er³⁺/Yb³⁺ co-doped glass ceramics as laser and fiber amplifier media.     

Comparative structural and photoluminescent study of Eu-doped La2O3 and La(OH)3 nanocrystalline powders

Eu³⁺-doped La₂O₃ nanocrystalline powder was prepared by polymer complex solution method and further used for preparation of Eu³⁺-doped La(OH)₃. Structural and optical characterization was carried out by powder X-ray diffraction and photoluminescent spectroscopy. XRD measurements confirmed the formation of hexagonal La₂O₃ and its recrystallization into La(OH)₃ in a humid atmosphere. Excitation spectra show redshift of host lattice and charge transfer emission bands in La(OH)₃ while bands that correspond to Eu³⁺f–f transitions are placed at same wavelengths in both samples. Photoluminescence spectra recorded over the temperature range from 10 K to 300 K show that intensities of emission lines in Eu³⁺-doped La₂O₃ do not depend on temperature as much as in La(OH)₃ sample. Observed dominant ⁵D₀→⁷F₂ and markedly visible ⁵D₀→⁷F₀ emissions in doped La₂O₃ indicate that Eu³⁺ ion is located in a structural site without an inversion center. On the other hand, in Eu³⁺-doped La(OH)₃⁵D₀→⁷F₀ transition is barely visible while ⁵D₀→⁷F₂ is not prominent, and with temperature drop three ⁵D₀→⁷F_J (J=1, 2, 4) transitions become almost of the same intensity. In both La₂O₃ and La(OH)₃ structures Eu³⁺ ion replaces La³⁺ in non-centrosymmetric C_3v and C_3h crystallographic sites, respectively, and difference in symmetry of the crystal field around europium ion is explained by comparing shape and volume of these sites. Decay times of the ⁵D₀- level recorded over the temperature range 10−300 K revealed that emission lifetime values in La₂O₃ (~0.7 ms) are almost two times higher than in La(OH)₃ (~0.4 ms), and unlike in La₂O₃, lifetime in La(OH)₃ is temperature dependent.