H.S. Murrieta

Rare earth and transition metal ion centers in LiNbO3

Abstract In this work the present state of art about the rare earth and transition metal ion centers phenomenon in LiNbO3 has been analyzed. Particular emphasis has been given to lanthanide trivalent rare earth ions (Pr3+, Nd3+, Eu3+, Dy3+, Ho3+, Er3+, Tm3+ and Yb3+) and to the Cr3+ ion, where most of information has been reported. The structure of these centers is consistent with different off-centered positions of the optically active ions inside the Li+ octahedral site. Stoichiometry and codoping effects on the center distribution are discussed. Finally, possible models to explain the origin of the different centers are discussed.

Optical absorption and fluorescence investigation of the precipitated phases of Eu2+ in KBr single crystals☆

Abstract Europium precipitation in monocrystalline KBR has been studied using optical absorption and photoluminescence techniques. The annealing of quenched samples at temperatures below 100°C produces the growth of an emission band peaking at 433 nm which appears to be related with the presence of Suzuki-type precipitates in the crystals. The aging at higher temperatures, however, produces the appearence in the emission spectrum of two other bands peaking at 427 and 459 nm. The data presented in this paper strongly suggest that the former band is associated with the stable dihalide phase EuBr2 in the host KBr, while the latter is due to a metastable precipitate whose structure appears to be quite similar to that of europium bromide. Some of the characteristics of the second phase precipitates were obtained by measuring the crystal field splitting (10 Dq) of the 4f65d configuration of the Eu2+ ions in each one of them.

Optical absorption and luminescence spectra of Eu-impurity precipitated in the lattice of NaBr and NaI crystals

Abstract The absorption, excitation, and luminescence spectra associated with the precipitated phases of divalent europium in NaBr and NaI single crystals are discussed. In both systems, three different fluorescence bands were observed in crystals which were stored for five years at room temperature without any previous heat-treatment. By analyzing the emission spectrum of the well-aged crystals as a function of the annealing temperature, it was found that the thermal dissolution of the precipitated phases of Eu2+ in these two crystals takes place in the temperature range 80–500°C. The optical absorption spectrum consists, in all cases, of two broad bands in the UV range with a crystalline field splitting 10 Dq which strongly depends on the aging and the thermal treatments given to the samples.

Red and green fluorescence of Mn2+ in NaCl

Abstract The fluorescence (emission and excitation) spectrum of Mn 2+ ion in NaCl has been investigated for freshly quenched samples (diluted Mn 2+ ions) and as grown samples (Mn 2+ ions forming aggregates and/or precipitates). Two main emission bands are generally observed in as grown samples, peaking at 505 nm and around 610 nm. Both emission bands are related to different manganese precipitates. Quenched samples show only the red emission but peaking at 605 nm and with a different excitation spectrum to that of the as grown crystals. Thus, this crystal is suitable to operate as a double color (red and green) emitting phosphor where the red and green relative intensities can be controlled by a thermal treatment.

Thermally stimulated depolarization of Eu2+-cation vacancy dipoles in alkali halide single crystals☆

Abstract Ionic thermocurrent (ITC) measurements have been performed on eight alkali halide single crystals doped with divalent europium. In all cases, the observed ITC peaks were fitted with a mono-energetic model without to appeal to any dipole-dipole interaction. Values for the reorientation parameters have been calculated. The relationship TM1nτ−1 α E previously found for I–V complexes in alkali halides has been found to be very well obeyed for the experimental data obtained in this investigation. It is also reported that the logarithm of the experimentally determined energies for free dipole reorientation shows a linear dependence on the interaction distance between the Eu2+ ion and the surrounding halogen ions in the distorted cubic site occupied by this impurity in the alkali halides.

Optical spectroscopy of Dy3+ ions in LiNbO3

Abstract A systematic spectroscopic study of Dy3+ doped LiNbO3 is presented. The energy position of the Stark levels and their symmetry character is given for most of the multiplets. Luminescence of this system has been investigated in the visible and infrared. The only emitting state in this region is the metastable 4 F 9 2 multiplet whose life time is temperature independent and with a value of 186 μs. Evidence about Dy3+ multicentres is also discussed.

Precipitation processes of divalent europium ions in NaBr1−xClx mixed crystals

Abstract The precipitation processes of Eu2+ ions in NaBr1−xClx mixed crystals have been studied by fluorescence and Electron Paramagnetic Resonance techniques. The obtained results point out that the Cl-impurity ions are located near to the europium cation vacancy complexes and that these defects can act as nucleation centers producing mixed europium dihalide phases.

X-ray induced precipitation of Eu2+ in the alkali halides

Abstract The stability of the europium second phase precipitates in NaCl, NaBr, KC1, and KBr under room temperature X-irradiation has been analyzed using the photoluminescence technique. The irradiation tends to destroy the pre-existing metastable precipitates creating complexes of a much simpler nature. In parallel with this destruction an enhanced europium precipitation into the stable dihalide phase EuX2 (X=Cl, Br) was found to occur.

Relative halide-concentration dependence of the F-centre optical absorption properties in KCl x Br(1−x):Eu2+ crystals: influence of Eu2+-cation vacancy dipoles

The influence of Eu2+-cation vacancy (I–V) dipoles on the relative halide-concentration dependence of the F-centre optical absorption properties in KCl x Br(1−x):Eu2+ crystals has been investigated. These crystals are made of a single mixed face-centred cubic (FCC)-phase whose unit-cell size a 0 obeys the Vegards rule, and their optical absorption spectrum consists of a single F-band whose spectral width depends on x, reaching 0.414±0.006 eV at x=0.5, and whose spectral frequency νm and equivalent energy ϵm satisfy the equations and ϵ m (eV)=(0.06±0.01)x 2+(0.16±0.01)x+(1.98±0.01). Comparison with the F-centre optical absorption properties reported elsewhere for pure FCC alkali-halide and KCl x Br(1−x) crystals showed that in KCl x Br(1−x):Eu2+ crystals, the alkali and halide ion sizes drive mainly the F-band spectral position, and also that this position is more sensitive to changes in a 0 in KCl x Br(1−x):Eu2+ crystals than in pure FCC alkali-halides. This is due to the crystalline electric field distortion caused locally at the F-centre by the mixed halide surroundings. This distortion affected the F-band spectral position in KCl x Br(1−x):Eu2+ crystals to a lesser extent than it did in KCl x Br(1−x) crystals. This might be due to the presence of I–V dipoles.

Bi4Ge3O12:Nd3+ and Bi12SiO20:Nd3+: A comparative spectroscopic study

Abstract A systematic and comparative study of luminescence and optical absorption properties of the trivalent rare earth neodymium ion in the Bi 4 Ge 3 O 12 and Bi 12 SiO 20 crystal lattices is presented. The incorporation of Nd 3+ ions into these crystals is more easily achieved in the first one than in the latter. In both matrices their intrinsic emission gives rise to a radiative energy transfer between the host and the impurity ion. On the other hand most of the Stark energy levels have been identified for both matrices. A calculation of the Ω 2 , Ω 4 and Ω 6 Judd–Ofelt parameters is given for each crystal.