Studies of the EPR Parameters for Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Cu<Subscript>2</Subscript>O<Subscript>8</Subscript>:Er<Superscript>3+</Superscript>

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors g _x , g _y , g _z , and the hyperfine structure constants A _x , A _y , and A _z ) of the two orthorhombic Er³⁺ centers in Bi₂Sr₂ YCu₂O₈ are theoretically studied from the perturbation formulas of these parameters for a 4f¹¹ ion in orthorhombic symmetry. In these formulas, the contributions due to the admixtures of various states are taken into account, and the orthorhombic field parameters are determined from the superposition model and the local geometry of Bi³⁺ site in Bi₂Sr₂ YCu₂O₈. The calculated EPR parameters show reasonable agreement with the observed values. The anisotropy g _z >g _x (g _y ) for the g factors may be attributed to the compression of the ligand octahedra in the Er³⁺ centers.     

EPR properties of KY(WO4)2 single crystals weakly doped with Er,Yb and Nd

Well oriented KY(WO₄)₂ single crystal weakly doped simultaneously with Er, Yb and Nd have been investigated using EPR technique. Angular dependencies of the EPR lines clearly confirm C₂ symmetry of all: Er³⁺, Yb³⁺ and Nd³⁺ dopants. Basic parameters of the spin Hamiltonian, including Zeeman and hyperfine terms (g and A matrices) as well the spatial orientation between principal axes system and crystallographic axes system were determined.     

Investigations of the Spin Hamiltonian Parameters for the Cu2+ Sites in PrBa2Cu3O6+x and Pr0.5Er0.5Ba2Cu3O6+x

The spin Hamiltonian parameters (the anisotropic g factors g _∥ and g _⊥ and the hyperfine structure constants) for the Cu²⁺ sites in PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x are theoretically investigated using the high order perturbation formulas of these parameters for a 3d⁹ ion in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are determined from the superposition model and the local structures of the Cu²⁺ sites. The theoretical spin Hamiltonian parameters are in good agreement with the observed values for both systems, and the results show improvements as compared with those based on various adjustable covalency coefficients in the previous work. The larger hyperfine structure constants for PrBa₂Cu₃O_6+x than those for Pr_0.5Er_0.5Ba₂Cu₃O_6+x can be attributed to the weaker covalency due to the relatively longer Cu²⁺– O^[`2]\mathrm{O}^{\bar{2}} bonds in the former.     

Local structure and electron spin resonance of copper-doped SrTiO3 ceramics

We report X-ray diffraction and electron spin resonance (ESR) measurements of the effect of SrTiO₃ ceramics doping using Cu²⁺ ions. ESR measurements reveal two kinds of Cu²⁺ centers in weakly (0.2–0.5 mol% Cu) doped SrTiO₃. Both kinds of centers have been attributed to Cu²⁺ at octahedral Ti sites and possibly associated either with a nearest-neighboring oxygen vacancy (center #1) or some other positively charged defect (center #2). The ESR spectra of the above centers are described by the following spin Hamiltonian parameters: g _‖ = 2.263(1), g _⊥ = 2.041(1), A _‖ = 170(1) × 10^?4 cm^?1, A _⊥ = 27(1) × 10^?4 cm^?1 (center #1) and g _‖ = 2.334(1), g _⊥ = 2.059(1), A _‖ = 137(1) × 10^?4 cm^?1, A _⊥ ≈ 0(1) × 10^?4 cm^?1 (center #2). For copper concentration larger than 2 mol%, the antiferromagnetic SrCu₃Ti₄O₁₂ (SCTO) phase has been detected by both X-ray diffraction and ESR. Its volume increases with increase of Cu concentration reaching about 17 % at Cu doping of 20 mol%. The composite SrTiO₃–SCTO ceramics exhibits substantial magnetocapacitance effect, which could be enhanced by electrostriction of SrTiO₃.     

Studies of the EPR and optical spectra for Cr3+-doped KAl(MoO4)2 crystal

The EPR parameters (zero-field splitting D and g factors g₆, Δg = g₆ − g_⊥) and the first excited-state splitting δ(²E) for Cr³⁺-doped KAl(MoO₄)₂ crystal are calculated from the high-order perturbation formulas. In the calculations, the parameters are obtained from the optical spectra and structural data of the studied system. The results show good agreement with the observed values.     

Luminescence of GaS:Er<Superscript>3+</Superscript> and GaS:Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript> layered crystals

Data are presented on the 300-K photoluminescence in GaS crystals doped with Er³⁺ or codoped with Er³⁺ and Yb³⁺. IR excitation (λ_ex = 976 nm) gives rise to anti-Stokes luminescence in GaS:Er³⁺ (0.1 at %) and GaS:Er³⁺,Yb³⁺ (0.1 + 0.1 at %) and leads to an increased intensity of the emission due to the ⁴ I _11/2 → ⁴ I _15/2 transitions. The anti-Stokes luminescence is shown to result from consecutive absorption of two photons by one Er³⁺ ion, and the increased intensity of Er³⁺ luminescence in GaS: Er³⁺,Yb³⁺ is due to energy transfer from Yb³⁺ to Er³⁺.     

Electron paramagnetic resonance (EPR) investigations of the local environment around Co2+ ions doped in PbMoO4 single crystals – Correlation with optical studies

The electron paramagnetic resonance (EPR) measurements of PbMoO₄ single crystals doped with Co²⁺ ions (0.5 wt.%) carried out at temperature range 10–12 K are reported. The observed spectra reveal evidently two major groups of EPR spectra, each group consists of eight hyperfine structure components associated with the Co nuclear spin I = 7/2. Additionally, EPR spectra of the unintentional impurities have been observed, which have been assigned to Nd³⁺ ions in PbMoO₄. The analysis of the various characteristic features of the EPR spectra as well as our earlier optical absorption studies of Co²⁺:PbMoO₄ indicate that the Co²⁺ ions are located at two crystallographically distinct complexes, denoted Co²⁺(α) and Co²⁺(β). The fictitious ‘spin’ S′ = ½ has been assigned to the observed ground Kramers doublet state of each of the two Co²⁺ complexes tentatively identified as located at the Mo sites in PbMoO₄. The spin Hamiltonian parameters for Co²⁺ (S′ = ½) ions in PbMoO₄, including the components of the Zeeman and hyperfine structure tensors g_ij and A_ij, respectively, are experimentally determined for the first time. Comparison of our results with the values of g_ij and A_ij for Co²⁺ ions in other structurally similar compounds further strengthens this conclusion. Correlation of the present results with those obtained earlier from optical studies is discussed. Alternative assignments of the ground states and options for the origin of the ‘spin’ S′ = ½ for Co²⁺ ions in PbMoO₄ will be considered in a subsequent paper.     

Absorption spectra of bulk aluminum nitride crystals doped with Er<Superscript>3+</Superscript> ions

This Letter presents results of analysis of the absorption spectra of AlN:Er³⁺ bulk crystals. In the spectral of 370–700 nm, absorption lines responsible for intraconfiguration electron transition from the ground state ⁴I_15/2 to the excited states of Er³⁺ ions are found. Transitions to the levels of the ⁴F_9/2, ²H_11/2, and ⁴G_11/2 states at 2 K are studied in detail. The number of observed lines for these transitions fully agrees with that theoretically possible for f–f electron transitions in Er³⁺ ions found in a noncubic crystal field. The small width of the observed lines and their number indicate that erbium ions displace mostly one regular crystal position. Most probably, Er³⁺ occupies the position of Al. Energy positions of excited states for the considered transitions are determined.     

Optical absorption and fluorescence properties of Er3+ in sodium borate glass

Spectroscopic properties of Er³⁺ ions in sodium borate glass have been studied. The indirect and direct optical band gaps (E_opt) and energy level parameters (RacahE ¹, E² and E³), spin-orbit (ξ_4f) and configurational interaction (α)) are evaluated. Spectral intensities for various absorption bands of Er₃₊ doped sodium borate glass are calculated. Using Judd-Ofelt intensity parametersΩ ₂, Ω₄, Ω₆, radiative transition probabilities (A), branching ratios (β) and integrated absorption cross sections (Σ) are reported for certain transitions. The radiative lifetimes (τ_R) for different excited states are estimated. From the fluorescence spectra, the emission cross section (σ_P) for the transition,⁴I_13/2 → ⁴I_15/2 is reported.     

Theoretical studies on the defect structure for Mn in KTaO3

The defect structure for Mn²⁺ in KTaO₃ is theoretically studied by using perturbation formulas of the spin Hamiltonian (SH) parameters for 3d⁵ ions in tetragonal symmetry based on the strong-field scheme. By analyzing the electron paramagnetic resonance (EPR) data of the studied system, we suggest that the impurity Mn²⁺ ion occupy the dodecahedral K⁺ site, rather than the octahedral Ta⁵⁺ site. Based on the studies, it is found that the Mn²⁺ impurity undergoes an off-center displacement away from the ideal K⁺ site by about 0.60 Å along the C₄ axis. The above displacement is qualitatively consistent with the recent result based on the generalized gradient approximation (GGA) and that obtained from EPR and dielectric spectroscopy studies.     

Enhanced emission of 2.7 μm from Er3+/Nd3+-codoped LiYF4 single crystals

An Er³⁺/Nd³⁺ co-doped LiYF₄ single crystal of ～Φ 12 mm × 95 mm size with high quality was grown by a Bridgman method. The luminescent properties of the crystals with different Er³⁺ and Nd³⁺ concentrations were studied. Compared with the Er³⁺ single-doped LiYF₄ crystal extremely enhanced emission at 2.7 μm from the Er³⁺/Nd³⁺ co-doped LiYF₄ was observed upon excitation of an 800 nm laser diode. Meanwhile, the green up-conversion emission and near infrared emission at 1.5 μm from Er³⁺ in the co-doped crystals were effectively restricted. The luminescent mechanisms for the Er³⁺/Nd³⁺ co-doped crystals were analyzed and the possible energy transfer processes were proposed. The energy transfer efficiencies for (Er³⁺:⁴I_13/2, Nd³⁺:⁴I_9/2) → (Er³⁺:⁴I_15/2, Nd³⁺:⁴I_15/2) and (Nd³⁺:⁴F_3/2, Er³⁺:⁴I_15/2) → (Nd³⁺:⁴I_9/2, Er³⁺:⁴I_9/2) were calculated. It was found that Er³⁺/Nd³⁺ co-doped single crystal may be a potential host for 2.7 μm lasers.     

Absorption spectra of Er3+ ions in Li6Y(BO3)3 crystals

The optical absorption spectra of Li₆Y(BO₃)₃:Er³⁺ crystals have been studied. The absorption lines corresponding to intraconfiguration electronic transitions from the ⁴I_15/2 ground state to the levels of excited states of Er³⁺ ions have been found in the spectral range of 370–700 nm. The transitions to the ⁴F_9/2, ⁴S_3/2, ²H_11/2, ⁴F_7/2, ⁴F_5/2, ⁴F_3/2, ²H_9/2, and ⁴G_11/2 levels have been investigated in detail at 2 K. The number of lines observed for these transitions corresponds to the theoretical maximum for the f-f electronic transitions in Er³⁺ ions located in the noncubic crystal field. The narrowness of the lines observed and their number indicate that erbium occupies one regular position (specifically, the yttrium position). The energy levels of the excited states have been determined for the transitions under study.     

Synthesis and luminescence properties of Yb3+–Er3+ co-doped LaOCl nanostructures

LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers were prepared by electrospinning combined with a double-crucible chlorination technique using NH₄Cl as chlorinating agent. X-ray powder diffraction analysis indicated that LaOCl:Yb³⁺, Er³⁺ nanostructures were tetragonal with space group P4/nmm. Scanning electron microscope analysis and histograms revealed that diameters of LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers, respectively, were 117.87 ± 15.48 and 141.09 ± 17.10 nm under the 95 % confidence level. Up-conversion (UC) emission spectra analysis manifested that LaOCl:Yb³⁺, Er³⁺ nanostructures exhibited strong green and red UC emission centering at 526, 548, and 671 nm, respectively, attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_l5/2 transitions of Er³⁺ ions under the excitation of a 980-nm diode laser. It was found that the relative intensities of green and red emissions vary obviously with the addition of Yb³⁺ ions, and the optimized molar ratio of Yb³⁺ to Er³⁺ was 10:1 in the as-prepared nanofibers. Moreover, the near-infrared characteristic emissions of LaOCl:Yb³⁺, Er³⁺ nanostructures were achieved under the excitation of a 532-nm laser. CIE analysis demonstrated that color-tuned luminescence can be obtained by changing doping concentration of Yb³⁺ (and/or Er³⁺) ions and morphologies of nanomaterials, which could be applied in the fields of optical telecommunication and optoelectronic devices. The UC luminescent mechanism and the formation mechanisms of LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers were also proposed.     

Quadrature frequency resolved spectroscopy of upconversion photoluminescence in GeGaS:Er<Superscript>3+</Superscript>; II. elucidating excitation mechanisms of red emission besides green emission

The paper addresses the quadrature frequency resolved spectroscopy (QFRS) on the red (⁴ F _9/2 → ⁴ I _15/2) upconverted photoluminescence (UCPL) as well as the green (⁴ S _3/2 → ⁴ I _15/2) UCPL of Er³⁺ in GeGaS chalcogenide glasses at the pump wavelength 975 nm. On the basis of the perturbed rate equations for the generally accepted 5-level model of the green and red UCPLs of Er³⁺, we have derived the formulas for the QFRS on the green and red UCPLs of the two differently Er-doped samples. Adopting the respective formulas as model functions for the nonlinear regressions on the QFRS data of both the UCPLs, we have determined the relevant parameters such as relaxation rates at the four excited levels, the excited absorption cross section and energy transfer parameters. Thereby the differences of UCPL dynamics between the green and red UCPLs as well as between the two differently Er-doped Ge₂₅Ga₁₀S₆₅ samples are interpreted in terms of the determined parameters.     

Theoretical Investigations of EPR g Factors in Tetrahedral Compounds

Complete high-order perturbation formulas are established in ZnSe:Ti²⁺, CdTe:Ti²⁺, and ZnSe:V³⁺ crystals, based not only on the contributions of the spin-orbit coupling effect of the central ion and the ligands, but also on that of the charge-transfer levels. The electron paramagnetic resonance (EPR) g-factors are calculated from the formulas and the calculated values of the EPR g-factors agree well with the experimental ones. The contribution rate of the charge-transfer levels (|Δg ^T /Δg ^F |) are 10.1 %, 7.6 %, and 24.9 % for ZnSe:Ti²⁺, CdTe:Ti²⁺, and ZnSe:V³⁺ crystals, respectively.     

Judd–Ofelt analysis and energy transfer processes of Er3+ and Nd3+ doped fluoroaluminate glasses with low phosphate content

Spectroscopic property and energy transfer processes of singly doped and codoped Er³⁺ and Nd³⁺ fluoroaluminate glasses with low phosphate content are systematically analyzed. The absorption spectra of these glasses are tested, and the Judd–Ofelt (J–O) and radiative parameters are discussed based on J–O theory and the parameters changes substantially because of the other codoping ions. As for Nd³⁺: the main emission bands at 0.9 and 1.05 μm decrease in the codoped sample under the excitation of an 800 nm laser diode from the emission spectra because the Er³⁺: ⁴I_11/2 level reduces the Nd³⁺: ⁴F_3/2 level effectively through the energy transfer process Nd³⁺: ⁴F_3/2 → Er³⁺: ⁴I_11/2. For Er³⁺, the emission at 1.5 μm is restrained by codoping with Nd³⁺ ions from the energy transfer process Er³⁺: ⁴I_13/2 → Nd³⁺: ⁴I_15/2. The emission at 2.7 μm is enhanced because the Nd³⁺ ions deplete the lower level and exert a positive effect on the upper laser level. The microparameters of the energy transfer between the Er³⁺ and Nd³⁺ ions are calculated and discussed using Forster–Dexter theory. The energy transfer efficiencies of the Nd³⁺: ⁴F_3/2 to the Er³⁺: ⁴I_11/2 and the Er³⁺: ⁴I_13/2 to the Nd³⁺: ⁴I_15/2 are 28.8% and 74.5%, respectively. These results indicate that Nd³⁺ can be an efficient sensitizer for Er³⁺ to obtain Mid-infrared (Mid-IR) emission and the codoped Er³⁺/Nd³⁺ fluoroaluminate glass with low phosphate content is suitable to be used as the fiber optical gain media for 2.7 μm laser generation.     

Influence of Ce on the luminescence properties of Er/Yb:YVO4 crystals

YVO₄ single crystals doped with Ce³⁺, Er³⁺ and Yb³⁺ ions were grown by the Czochralsski technology. The luminescence properties of Er³⁺/Yb³⁺:YVO₄ single crystals with different concentration of Ce³⁺ were studied, and the energy transfer mechanism between Er³⁺, Yb³⁺ and Ce³⁺ was discussed based on their energy level properties. The branching ratios of the ⁴I_11/2 → ⁴I_13/2 transition in different samples were calculated. The results indicate that codopants of Ce³⁺ greatly enhance the population rate of the ⁴I_13/2 level due to the fast resonant energy transfer between Er³⁺ and Ce³⁺, i.e., ⁴I_11/2(Er³⁺) + ²F_7/2(Ce³⁺) → ⁴I_13/2(Er³⁺) + ²F_5/2(Ce³⁺).     

ESR of vanadyl ions in borate glasses A. YADAV,V. P. SETH

The electron spin resonance (ESR) of VO²⁺ in Li₂O-RO-B₂O₃ (where R = Sr, Zn) glasses hasbeen studied. Spin Hamiltonian parametersg _\tT,g\t],A _\tT andA\t], dipolar hyperfine coupling parameterP and Fermi contact interaction parameter K have been calculated. It is found that with a decrease in the percentage of B₂O₃ there is an improvement in the octahedral symmetry of the V⁴⁺ site. The theoretical optical basicity A_th, of the glasses has also been calculated.     

Synthesis and luminescence properties of Yb3+–Er3+ co-doped LaOCl nanobelts via electrospinning combined with chlorination technique

LaOCl:Yb³⁺, Er³⁺ nanobelts were prepared by electrospinning combined with a double-crucible chlorination technique using NH₄Cl as chlorinating agent. X-ray powder diffraction analysis indicated that LaOCl:Yb³⁺, Er³⁺ nanobelts were tetragonal with space group P4/nmm. Scanning electron microscope analysis and histograms revealed that width of LaOCl:Yb³⁺, Er³⁺ nanobelts was 6.12 ± 0.18 μm under the 95% confidence level, and the thickness was 113 nm. Transmission electron microscope observation showed that as-obtained LaOCl:Yb³⁺, Er³⁺ nanobelts were composed of nanoparticles. LaOCl:Yb³⁺, Er³⁺ nanobelts emitted strong green and red up-conversion emission centring at 523, 551 and 667 nm, respectively, attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_l5/2 transitions of Er³⁺ under the excitation of a 980-nm diode laser (DL) excitation. Moreover, the near-infrared characteristic emission of LaOCl:Yb³⁺, Er³⁺ nanobelts was achieved under the excitation of a 532-nm laser. Commission Internationale de L'Eclairage analysis demonstrated that colour-tuned luminescence can be obtained by changing doping concentration of Yb³⁺ and Er³⁺, which could be applied in the fields of optical telecommunication and optoelectronic devices. The up-conversion luminescent mechanism and the formation mechanism of LaOCl:Yb³⁺, Er³⁺ nanobelts were also proposed.