Jorma Hölsä

Crystal field strength in C-type cubic rare earth oxides

Abstract Luminescence of the Eu 3+ -doped cubic C-type rare earth (RE) oxides, RE 2 O 3 :Eu 3+ (RE=Eu, Gd, Lu, Y, In and Sc) powder samples at 77 and 298 K was investigated under UV, argon ion and dye laser excitation. Intense transitions from the 5 D 0 to 7 F 0–4 levels of the ground 7 F multiplet were observed between 575 and 720 nm. The complete lifting of the 7 F J level degeneracy as well as the absence of transition selection rules imposed by the group theory are consistent with the C 2 point symmetry of the RE 3+ site. Only a few lines originating from Eu 3+ ions in the high symmetry S 6 site were observed. The crystal field (c.f.) analysis conducted on the basis of the 18–21 7 F 0–4 c.f. levels yielded satisfactory results despite the high number (14) of the B k q and S k q parameters. The strength of the c.f. effect increases in the RE series with decreasing ionic radius of the RE 3+ host cation. This results from the increased electrostatic effect of the host lattice on the Eu 3+ ion.

Simulation of the energy level scheme of Nd3+ and Eu3+ ions in rare-earth orthovanadates and phosphates

The absorption spectra of the Nd3+ ion in the tetragonal xenotime-type NdVO4 and in the monoclinic monazite-type LaVO4, LaPO4 and NdPO4 as well as the luminescence of the Eu3+ ion in LaVO4 and LaPO4 were measured and the spectra were analysed according to the appropriate symmetry of the RE site. In addition to pure zero-phonon electronic lines the absorption spectra of the Nd3+ ion showed extensive vibronic side bands which were identified with the aid of the measured Raman scattering spectra of LaVO4:Nd3+ and NdVO4. Based on the energy level schemes derived from the absorption and luminescence spectra the parametrization of the free-ion and crystal-field effects were carried out with good results as indicated by the low RMS deviation values. The parameter sets for Nd3+ and Eu3+ ions in the same matrices are consistent with each other but no correlation was found between sets for matrices of different structures.

Crystal fields in Pr3+ doped rare earth gallium garnets, RE3Ga5O12:Pr3+

The optical absorption, site selective excitation and luminescence spectra of the trivalent praseodymium ion in three rare earth gallium garnet matrices, RE3Ga5O12 (REGG; RE=Y, Gd, and Pr) were detected at 4, 77 and 300 K. The 4f2 energy level schemes comprising 56 to 68 crystal field (CF) sublevels were simulated according to a phenomenological model taking into account both the free ion and CF effects. The model consisting of 17 parameters with 8 free ion and 9 CF parameters appropriate to the D2 point symmetry of the RE3+ site reproduced the experimental energy level schemes in a satisfactory manner with RMS deviations between 17 and 23 cm-1. The CF parameters vary only slightly as a function of the host matrix. The Bqk parameter sets for PrGG and YGG:Pr3+ are consistent with those obtained for the other REGG and YGG:RE3+, respectively.

Interplay between crystal structure and magnetic susceptibility of tetragonal ROBr

The average magnetic susceptibilities for trivalent rare earth (R3+) ions in polycrystalline tetragonal rare earth oxybromides (ROBr, R = Ce–Nd, Sm, Eu, and Tb–Yb) were measured between 2 and 298 K. The susceptibilities of all ROBr follow the paramagnetic Curie–Weiss behaviour down to low temperatures except for SmOBr and EuOBr where the effect of crystal field was observed already above 200 K. The observed effective magnetic moments μeff(RT) were generally higher than the free ion values. For the heavier R3+ ions (Ho3+ and Er3+) μeff was found to be less than the free ion value due to crystal field mixing in the opposite way. The Weiss constant θ was found to be negative for all ROBr, suggesting antiferromagnetic (AFM) ordering at lower temperatures but only SmOBr (SmOCl) and DyOBr (DyOCl) show such ordering at 5(8) and 8(11) K, respectively. The increasing interlayer distance between adjacent (RO)n+n layers from ROCl to ROBr and within the ROBr series lowers the Neel temperature. AFM ordering thus depends on the 3D interactions while 2D interactions have negligible effect. The temperature dependence of the experimental paramagnetic susceptibility for each ROBr was simulated with the aid of the van Vleck formalism based on the wave functions and energy level values obtained from spectroscopic data. Lattice expansion and consequent modifications in crystal field caused discrepancies at higher temperatures.

Simulation of the Gd3+ energy level scheme in GdOCl

Abstract The luminescence materials of the new generation containing gadolinium have received considerable attention because of the quantum cutting and down-conversion exhibited. The prerequisite information for studies of these phenomena include a detailed analysis of the UV energy level scheme of the Gd 3+ ion with the 4f 7 electron configuration. In this paper such a study was carried out for the tetragonal GdOCl matrix. The UV absorption spectra of GdOCl were measured at 9 K between 200 and 315 nm. The experimental energy level scheme derived from the spectra was interpreted according to the C 4 v site symmetry of the Gd 3+ site in the oxychloride matrix. This level scheme of 24 c.f. components representing seven 2 S +1 L J ( 8 S 7/2 , 6 P J , 6 I J , 6 D J ) levels was then successfully simulated by a phenomenological model using 14 free ion and five real c.f. parameters. A satisfactory match with an rms deviation of 21 cm −1 between the experimental and calculated energy level schemes was obtained despite the high number of parameters and the sparse set of energy levels. The present results obtained for GdOCl were found consistent with the previous ones for other RE 3+ ions in the REOCl hosts.

Anti-Stokes luminescence of europium(III)-doped lanthanum oxychloride

Abstract The anti-Stokes luminescence from the 5D1,2,3 levels of Eu3+-doped LaOCl was observed at 300 and 77 K under dye-laser excitation to the 5D0 level. A two-photon absorption from the 7F0 ground level to charge-transfer states via the 5D0 level was concluded to be the mechanism involved. The absorption of the first photon through the forbidden 5D0→7F0 transition determines the upconversion efficiency. The two-photon absorption seems to occur as an intra-ion process without interionic energy transfer.

Synchrotron Radiation Study of the M2MgSi2O7:Eu2+ Persistent Luminescence Materials

The synchrotron radiation luminescence and excitation spectra of the Eu doped M2MgSi2O7 (M = Ca, Sr, Ba) materials were investigated at the SUPERLUMI station of HASYLAB at DESY, Germany. The measurement of the band gap (Eg) energy of all three host lattices using the excitation spectra of Eu gave values around 7 eV. The measured band gap energies were very similar but slightly higher than those calculated with the DFT-methods. The effect of R co-doping was studied at different temperatures between 10 K and room temperature. A significant increase in the band gap energy was observed with decreasing temperature. In contrast, no noteworthy change in this value was observed as a function of the R co-doping ion.

Optical properties of Eu3+-doped tetragonal lutetium oxyhydroxide

Abstract The UV and dye-laser-excited luminescence spectra of the Eu 3+ -doped tetragonal high-pressure form of LuOOH were measured at 77 and 300 K. The analysis of the emission spectra yielded a complete crystal-field (c.f.) energy-level scheme for the 7 F 0–4 levels. The simulation of c.f. splittings was conducted in the 25 Stark-level basis set in C 2 v and C 2 symmetries. A satisfactory simulation was obtained only with the C 2 c.f. parameter set. A comparison with the results of a c.f. analysis of the monoclinic form of LuOOH was also carried out.