Electroluminescence of alkali halide crystals activated with europium ions

The prebreakdown electroluminescence of NaCl-Eu, KCl-Eu, and KBr-Eu crystals has been studied. The activator concentration in the crystals was on the order of 10¹⁸ ions per cubic centimeter. The emission spectra, the brightness wave during pulsed excitation, and the concentration dependence of the quantum yield were measured. The experimental results support the suggestion of an impact mechanism for the excitation of the electroluminescence of alkali halide crystals. Recombination and exciton mechanisms for energy migration play an insignificant role. There is no significant contribution from impurity-vacancy dipoles, which act as emission centers, in electric fields up to 10⁶ V/cm.Tomsk Academy of Control and Electronic Systems. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 105–109, February, 1994.     

X-band ESR studies of forbidden transitions of europium ions in CaF2: Eu crystals

The characteristics of allowed and forbidden transitions of Eu²⁺ ions in CaF₂ single crystals have been investigated at room temperature. The Angular dependences of these transition intensities were theoretically calculated with the use of perturbation theory, and compared with the experimental values. The experimental results can be explained by taking into account crystal field effects $\text{(}\text{1}\text{60}\text{) b}{}_4\text{[O}{}^0{}_4\text{+ 5O}{}^4{}_4\text{]}$, and a hyper fine interaction A_iS · I_i in the spin Hamiltonian.     

Absorption bands of lead in alkali chlorides in the presence of hydroxyl ions

Absorption studies of divalent lead in crystals and solutions of alkali chlorides with an admixture of hydroxyl ions showed that the absorption band lying at 250 nm can be attributed to hydroxochlorocomplexes of lead.It is the author's agreeable duty to thank Dr. A. Bohun for having drawn his attention to this problem and V. Piskáková for help in the absorption measurements.     

ESR study of europium ions in hydrolyzed CaF2: Eu crystals

Oxidation and reduction processes of europium ions in CaF₂ were studied by optical and ESR methods. Europium ions can be readily oxidized from the divalent to the trivalent states by heat-treatment above 600°C in a moist air atmosphere. The conversion can be well approximated by first order kinetics with an activation energy of 0.75 eV.

The reverse conversion from the trivalent to the divalent states can be accomplished by exposure to ionizing X-ray radiation. However, those reduced europium ions are thermally unstable and bleached completely by heating to 200°C. The decay process by low temperature annealing also follows first order kinetics with an activation energy of 0.60 eV. The conversions between the two states are explained by electron transfer.     

Absorption of Ni-centres in alkali chlorides

Absorption spectra of Ni²⁺ doped NaCl, KCl, and RbCl were measured in the spectral range from 55,000 to 5,000 cm^?1. The bands in the UV region are ascribed to the transition 3t _1u(σ, π)→3e _g(σ) of NiCl₆ complex ion. The connection of the intensity of charge transfer andd-d transitions has been discussed.     

Conduction bands and invariant energy gaps in alkali bromides

Electronic structure calculations of the alkali bromides LiBr, NaBr, KBr, RbBr and CsBr are reported. It is shown that the conduction band has primarily bromine character. The size of the band gaps of bromides and alkali halides in general is reinterpreted. Received: 26 January 1998 / Revised: 16 March 1998 / Accepted: 23 March 1998     

The hydration of alkali chlorides in aqueous solutions

Summary The temperature dependence of the adiabatic compressibility of solution of lithium chloride in water has been determined for different solute concentrations from 4 to 8 wt%. A simple model has been employed to interpret the data in terms of the size and compressibility of the hydration shells. The data have been compared with previously obtained compressibility data for NaCl, KCl and CsCl aqueous solutions. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Luminescence of europium ions in lanthanum-magnesium hexaaluminate

The photoluminescence of Eu³⁺ ions in hexagonal lanthanum-magnesium aluminate is studied for the first time. The concentration and temperature dependences of the luminescence intensity and the structure of the spectra of La_1–xEu_xMg·Al₁₁O₁₉ for X = 0.01–1.0 were studied, the existence of several types of luminescence centers (LC) was established. Under excitation, predominantly high-symmetry LC (D_3h) are manifested in the charge-transfer band in the luminescence spectra. A group-theoretical analysis of the structure of the luminescence spectra of europium for possible LC in hexaaluminate with the structural type of magnetoplumbite (space group P6₃/mmc) is carried out.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 45–49, October, 1985.We thank our co-workers at the Laboratory of Vibrations, Institute of General Physics of the USSR Academy of Sciences, for assistance in the experiments on selective laser spectroscopy.     

Electric quadrupole interactions of europium in alkali europium tungstates by Mössbauer and laser spectroscopies

¹⁵¹Eu Mössbauer spectroscopy and optical-r.f. double resonance spectroscopy measurements of the quadrupole interaction parameters in KEu(WO₄)₂ give good agreement and are of comparable accuracy, while laser heterodyne spectroscopy measurements provided improved accuracy by an order of magnitude. However in the crystallographically disordered compound NaEu(WO₄)₂, Mössbauer spectroscopy was more suited to producing fittable spectra. The difference in the hyperfine field of the two compounds in a 9 T applied field is attributed to a transferred hyperfine interaction.     

Luminescence of europium and ytterbium ions in strontium hexaborate

We have studied the luminescent properties of Eu^2+/3+ and Yb²⁺ ions in strontium hexaborate SrB₆O₁₀ for excitation in the 120–400 nm region. The luminescence spectra of Ln²⁺ ions in SrB₆O₁₀ consist of overlapping bands in the 370–520 nm region, due to 5d → 4f transitions at several nonequivalent centers. In the excitation spectra, besides the bands associated with 4f → 5d transitions in the Ln²⁺ ions, we also observe a band in the 135–160 nm region due to the transitions O(2p) → B(2s,2p) within the borate anions. The luminescence of the Eu³⁺ ions is excited most efficiently in the region of the Eu³⁺ charge transfer band (λ_max = 226 nm). The results obtained are compared with data for Ln in other strontium borates. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 770–774, November–December, 2006.     

Isothermal decay of colour centres in microcrystalline powders of alkali bromides

The bleaching of F-centres on storing in darkness of electrolytically coloured single crystals and microcrystalline powders of solid solutions of KBr with other alkali bromides is reported. The colouration of the microcrystalline powders decays isothermally while that of single crystals is comparatively stable. The decay proceeds slower in the powders of mixed crystals than in the pure materials. A tentative explanation is suggested.     

Mössbauer studies of 151Eu in europium oxalate, europium bissalen ammonium and europium benzoate

Although a number of europium water insoluble chelates have been prepared for several decades, the covalent nature of these compounds has never been established in any quantitative fashion. Shifts in the I.R. bands and conductivity measurements of these salts were hitherto used to qualitatively compare their molecular nature. In this communique we have used temperature coefficients of ¹⁵¹Eu Mössbauer spectra to determine the Debye temperatures (θ _D) of three europium chelates: namely europium oxalate, europium bissalen ammonium (recently reported) and europium benzoate and compared their θ _D with the measured θ _D of the known ionic EuF₃. Additionally, the mean square amplitude (<x ²) of these four compounds was computed and plotted as a function of temperature giving in each case a reasonable linear plot. It was interesting to note that the θ _D of the most ionic EuF₃ was greatest (283 ± 10 K) followed by the oxalate (166 ± 15 K), then the europium bissalen ammonium (133 ± 5 K) and lastly the europium benzoate with a θ _D of (105 ± 5 K).     

Pressure shifts of F-center hyperfine interaction parameters in alkali chlorides

We report here calculated values of the pressure shift in the shell I hyperfine interaction constants for F-centers in KCl, NaCl and LiCl. The calculation is based on a modified version of the pseudopotential approach of Bartram et al. with electronic polarization included by means of an r-dependent polarization potential. The results agree reasonably well with the available experimental data, and the strong dependence of the pressure shift on ion size ratio is shown to be mainly due to changes in the indirect-overlap contributions to the spin density. These terms result from the requirement of mutual orthogonality of core states on neighbouring ions, and their dependence on the lattice spacing is stronger than that of other contributions to the spin density. Our calculations further indicate that although the predicted value of the pressure shift is sensitive to errors in the pseudopotential and polarization parameters, the relative importance of the indirect overlap contributions is evident in spite of such errors.     

Spontaneous emission of europium ions embedded in dielectric nanospheres

We measure fluorescence lifetimes of emitters embedded in isolated single dielectric nanospheres. By varying the diameters of the spheres from 100 nm to 2 microm and by modifying their dielectric surrounding, we demonstrate a systematic change of paradigm in the spontaneous emission rate, as we cross the border from the superwavelength regime of Mie resonances to the nanoscopic realm of Rayleigh scattering. Our data show inhibition of the spontaneous emission up to 3 times and are in excellent agreement with the results of analytical calculations.     

Photodetachment measurements of alkali negative ions

Cross sections of alkali negative ions from 0.45 eV up to about 2.5 eV photon energy were measured. Electron affinities of the corresponding atoms are derived from the low energy onsets: EA(Li)=0.611 eV, EA(Na)=0.539 eV, EA(K)=0.497 eV, EA(Rb)=0.490 eV and EA(Cs)=0.470 eV with an estimated uncertainty of ±0.020 eV.     

ESR studies of sepiolites

The electron spin resonance (ESR) spectra for each beige and brown sepiolites from Polatli-Ankara, Turkey, a clay mineral of Mg₈Si₁₂O₃₀(OH)₄(OH₂)₄(H₂O)₈, at 77 K have their own single characteristic peak atg ≈ 1.99, which begins to anneal from 437 and 536°C, respectively. The sextet lines of Mn²⁺ were also observed in addition to two peaks (g = 4.4 and 5.0) associated with Fe³⁺. A high-quality sepiolite called “meerschaum” from Sepetci-Eskisehir, Turkey, has two characteristic peaks atg ≈ 1.99, which begins to anneal from 419°C, and atg ≈ 2.03. ESR intensities of the signal atg ≈ 1.99 for beige, brown sepiolites and meerschaum are enhanced by γ-ray irradiation to give equivalent dosesD _E = 11.8 ± 3.4, 4.7 ± 2.3 and 4.6 ± 1.2 kGy, respectively. The ESR ages obtained by assessing the annual dose rate from the content of²³⁸U,²³²Th and⁴⁰K determined by γ-ray spectroscopy are 1.8 ± 0.8, 0.7 ± 0.5 and 0.9 ± 0.2 Ma, respectively, falling into the Pliocene Epoch in geological time scale in agreement with the stratigraphy.