F. J. López

Optical spectroscopy of Mn2+ ions in solid solution and Suzuki phase in NaCl single crystals

The optical spectroscopy of Mn2+ ions both in dipolar form and forming Suzuki‐phase precipitates has been studied in NaCl by photostimulated luminescence at room and liquid nitrogen temperatures. An analysis of the positions of the crystal field transitions shows that the 10 Dq value is slightly higher (about 7%) in the Suzuki than in the dipolar phase while the covalency is the same in both cases. Such analysis allows us to estimate the value fσ−fπ=4.8% from the optical data which is in agreement with the EPR data for [MnCl6]4− clusters. Moreover, an intense band peaking at 231 nm which disappears when the dipolar phase is formed has been ascertained to be associated with the Suzuki phase. This band has been assigned to the double excitation 6A1(S) +6A1(S) → 4T1(G)+[4A1(G), 4E(G)]; that is consistent with the existence of Mn2+ ions in close positions in the Suzuki phase.

Thermoluminescence and electron spin resonance after room-temperature X-ray irradiation of NaCl:Mn2+

Abstract A parallel investigation of thermoluminescence (TL) and electron spin resonance (ESR) spectra on room-temperature (RT) X-irradiated NaCl:Mn 2+ has been performed. The TL spectra in the range 20–300°C consist of five glow peaks, numbered from I to V. Temperatures at maximum height are 41°, 68°, 118°, 152° and 216°C, respectively. Peaks I, II and IV obey first-order kinetics, whereas peaks III and V fit second-order behavior. The wavelength spectrum for all glow peaks consists of two bands centered at 595 and 400 nm. The 595 nm emission is attributed to hole capture by Mn + and subsequent deexcitation of Mn 2+ . The 400 nm emission is produced as a consequence of hole-F center recombination. The correlation of TL glow peaks to various defects has been investigated. Peak II is clearly related to manganese-vacancy dipoles and peak I can be roughly associated to free cation vacancies. Peak IV appears to relate to large Mn-aggregates, whereas peak V is intrinsic and not related to impurities. On the other hand, ESR data indicate that each glow peak in the 595 nm emission is associated to the annihilation of a given Mn-center; Peak I to Mn 0  C , peak II to Mn 0  C and Mn + , peak III to Mn + and peaks IV and V to Mn 0 - D .

Cr3+ in pure and Mg-doped LiNbO3: analysis of the EPR and optical spectra

The electron paramagnetic resonance spectra of Cr3+ in LiNbO3 and heavily Mg-doped LiNbO3, have been measured and analysed according to the superposition model. The axial spectrum of Cr3+ in LiNbO3 is consistent with Nb substitution. On the other hand, the dominant isotropic spectrum found in Mg-doped LiNbO3 can be accounted for if an appropriate ( approximately=0.1 AA) relaxation of the Cr3+ along the c-axis is allowed. Moreover, a point-ion calculation shows that the axial shift of the impurity ion is also qualitatively consistent with recent observations of an enhanced pi -polarization character of the optical transitions in LiNbO3:Mg, Cr.

Relationship between thermoluminescence and X-ray induced luminescence in alkali halides☆

Abstract The wavelength spectra of thermoluminescence and X-ray induced luminescence in pure and divalent cation doped alkali halides, in the temperature range LNT—RT has been studied. The more important conclusion is that the wavelength spectra in both cases are very similar. This allows a new point of view to be presented on thermoluminescence mechanisms.

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.

A new EPR centre of Er3+ in MgO or ZnO co-doped LiNbO3 single crystals

Abstract Electron Paramagnetic Resonance (EPR) experiments have been carried out on congruent crystals of LiNbO 3 heavily doped with Mg or Zn and co-doped with Er. In addition to the EPR spectrum previously observed in crystals of LiNbO 3 :Er 3+ an erbium spectrum not reported before is detected in samples co-doped with Mg or Zn. The new spectrum has the following g values: g ∥ =4.3±0.2, g ⊥ =7.6±0.3 for the Mg-doped samples and g ∥ =4.26±0.05, g ⊥ =7.8±0.1 for the Zn-doped ones, coinciding within experimental uncertainty. This spectrum is associated to a new Er 3+ centre and is compared to additional centres observed for some trivalent transition metal ions (particularly Cr 3+ ) in LiNbO 3 heavily doped with Mg or Zn. On these grounds the new centre can be attributed to Er 3+ located at the Nb 5+ site of LiNbO 3 .

Photochromic behaviour of doped Bi4Ge3O12 single crystal scintillators

The changes induced by ultraviolet (UV) illumination on the optical absorption and electron paramagnetic resonance (EPR) spectra of Bi4Ge3O12 single crystals, doped either with Fe (and Gd) or Mn, have been followed at room temperature (RT). In both crystals several overlapping optical absorption bands develop under UV illumination, covering from ≈ 0.7 eV up to the band edge of the matrix. The optical damage can be bleached by heating the samples above RT or by illumination with visible light. Although these optical changes temporarily correlate with the variation of the Gd3+ and Mn2+ concentrations, it has been concluded that other defects are present and partially responsible for the optical damage.

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.

Characterization of chromium impurities in Bi4Ge3O12 single crystals

Abstract The charge state of chromium ions and their position in the Bi4Ge3O12 lattice have been investigated by means of X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). It has been concluded that the Cr ions enter in the Bi4Ge3O12 lattice with valence 4+ in the Ge-site.

EPR study of Nd3+ ions in Bi4Ge3O12 single crystals

Abstract An electron paramagnetic resonance study of trivalent neodymium in Bi4Ge3O12 has been carried out at 5 K. The ions are substituting for Bi3+ in a crystal field of trigonal symmetry, which agrees with previous crystal-field analysis of the optical spectra. A detailed study of the measured g-factor values indicates that corrections to the existing crystal-field parameters B m n should be performed. It is suggested that corrections to the sixth-rank parameters are particularly important.