R. Alcalá

Optical properties and cross relaxation among Sm3+ ions in fluorzincate glasses

Optical absorption and emission spectra are presented for Sm3+ ions in ZnF2−CdF2-based glasses (0.1, 1.5 and 2.5 mol% of Sm3+). The measured oscillator strengths and branching ratios for several transitions are compared with values obtained with The Judd-Ofelt theory. Energy transfer among Sm3+ ions has been investigated. The decay curves are nonexponential when the concentration is 1.5 or 2.5 mol%. It can be explained by cross relaxation between ions of Sm3+. Energy migration does not play a relevant role in these decay processes.

Optical properties of Gd3+ in fluorozirconate glasses

Abstract Optical absorption and emission spectra of Gd 3+ ions in fluorozirconate glasses with two different compositions (ZBLA and ZBLALi) have been measured. Using the Judd-Ofelt (JO) theory and the experimental oscillator strengths of the absorption bands, JO parameters have been obtained for Gd 3+ which result to be the same for both hosts. These parameters are compared with those for other trivalent rare earth ions in the same glasses. Emission from 6 D, 6 I and 6 P terms have been detected and the lifetime of the 6 P 7 2 and 6 I 7 2 levels have been measured. These results, together with the radiative emission probabilities calculated from the JO parameters, allow us to give an estimate of the multiphonon relaxation rates which are discussed on the basis of the phenomenological gap law previously derived from Ho 3+ and Er 3+ data.

Spectroscopy of chromium (III) in yttrium-stabilized ZrO2

Abstract Chromium-doped yttria-stabilized zirconia (YSZ) single crystals were investigated using EPR, optical absorption, photoemission and emission lifetime techniques at temperatures between 8 and 300 K. A trigonal EPR signal is associated with the Cr 3+ ions. The hyperfine splitting with 53 Cr is about 44 MHz. The lineshapes and their dependence on the magnetic field orientation are explained by the presence of a Gaussian distribution of rhombic distortions. The optical properties are typical of a Cr 3+ ion in a six-fold coordination. The emission is dominated by the broad 4 T 2 fluorescence but at 8 K the R -emission is detected. The emission bands are inhomogeneously broadened and their intensities and lifetimes decrease as the temperature increases. We propose that Cr 3+ enters the lattice in a cation substitutional position whose environment has been distorted by a static Jahn-Teller effect. The local disorder of the YSZ lattice is responsible for the inhomogeneous broadening of the resonance lines.

Optical properties of ZnF2CdF2 glasses doped with 4f ions

Abstract Glasses with composition (in mol %), 32 ZnF 2 , 28 CdF 2 , 20 BaF 2 , 11 LiF, 5 AlF 3 , 4 LaF 3 have been prepared and doped with some 4f ions (Ho 3+ , Tm 3+ , Er 3+ ). The energy levels and oscillator strengths of the electronic transitions have been obtained. Using the Judd-Ofelt theory, the radiative transition probabilities between the different levels were calculated. We have measured the emission bands and lifetimes and determined the non-radiative transition rates, the luminescence efficiencies and the emission cross-sections of some “laser transitions”. The optical properties of this new material are slightly improved with respect to those of the fluorozirconate glasses.

EPR study of Ni3+ in RbCaF3 below the cubic-tetragonal phase transition

Abstract The EPR spectra of Ni3+ ions created by X-irradiation of RbCaF3 at 300 K have been observed at temperatures lower than 60 K. They can be described by an axial spin Hamiltonian with an effective spin S = 1 2 and gz.dfnc; = 3.833 and g⊥ = 4.370. An axial superhyperfine interaction with six neighbour fluorine ions has also been observed with A‖ = 397 MHz and A⊥ = 50 MHz for two of them and A‖ = 446 MHz and A⊥ = 53 MHZ for the other four. The superhyperfine tensors of these four fluorines have been found to be titled from the cubic 〈100〉 directions by an angle of ϑ = 5.2°. This tiliting as well as the non equivalence of the F superhyperfine interactions and the g tensor anisotropy are associated with the 195 K phase transition of RbCaF3. Except for these facts related with the phase transition, the Ni3+ centres are similar to those reported in KMgF3.

EPR study of Ni3+ centers in CsCaF3

Abstract The EPR spectra of Ni3+ ions created by RTX-irradiation of CsCaF3 have been measured at 15 K. They can be described by an isotropic Zeeman term with an effective S = 1 2 and g=4.183 and the superhyperfine structure corresponding to the interaction with the six nearest neighbor fluorines. The values of the superhyperfine parameters are A|| = 141×10−4 cm−1 and AtT=18×10−4 cm−1. Fro the isotropic part of these superhyperfine parameters we have estimated the distance between the central ion and the ligands. The value obtained has been compared with the results for Ni3+ in other isostructural fluoroperovskites.

Optical absorption spectrum of X-irradiated CaF2:Cr and SrF2:Cr

Abstract The optical absorption spectra of X-irradiated CaF2:Cr and SrF2:Cr are studied. X-irradiation induces the formation of a prominent band in the u.v. region (centered at 277 nm in CaF2 and 285 nm in SrF2) as well as a weak absorption in the visible region. Combined EPR and optical absorption measurements together with some optical bleaching experiments suggest that the u.v. absorption bands are related with either Cr+ or Cr3+ ions. Several arguments are presented in order to tentatively associate these bands with an interconfigurational transition of Cr+ in a cubal environment.

Time resolved spectroscopy of Ni2+ doped fluoroclorozirconate glasses

Abstract Optical absorption, time resolved luminescence and fluorescence decay measurements of Ni2+-doped fluoroclorozirconate glasses are reported. Time resolved luminescence measurements were taken exciting the samples at different wavelengths. The position of the orange emission band (1T2g(D) → 3A2g(F)) moves to the blue when the excitation is changed to higher energies. For a fixed excitation wavelength, the emission moves also to the blue when the time of measurement after the laser pulse increases. The fluorescence decays are non-exponential and depend of the detection wavelength. They are slower for the shorter detection wavelengths. The results show a distribution of Ni2+ sites with different ‘crystal field’ environments.

Endor study of Ni2+ in KMgF3

Abstract An ENDOR study of cubic Ni2+ ions in KMgF3 is presented. The superhyperfine (SHF) interaction with second shell fluorine ions is found to have axial symmetry, the values of the isotropic and anisotropic interaction constants being a = 0.36 ± 0.03 MHz and b = 1.02 ± 0.03 MHz. A calculation of these SHF parameters has been performed assuming that the lattice is undistorted around Ni2+ and using a simple overlap model. The calculated values agree within 15% with the experimental ones.

Spectroscopic properties of Mn2+ ions in mixed fluoroperovskites

The spectroscopic properties of Mn 2+ in RbCd x Ca 1-x F 3 and Cs x Rb 1-x F 3 have been investigated by EPR and photoluminescence. Only the transitions between the M s = ±1/2 Zeeman levels are observed by EPR. Forbidden transitions associated with changes in the M I of one of the surrounding fluorines are detected. The superhyperfine interaction with the fluorines decreases when the lattice constant increases. The decay of the luminescence emission corresponding to the 4 T 1g ( 4 G)→ 6 A 1g ( 6 S) transition of cubic Mn 2+ has been measured in RbCa x Cd 1-x F 3 crystals. A continuous change in the lifetime has been found with a minimum at x 0.3. These results indicate that there is a distribution of environment for Mn 2+ in the mixed crystals.