Analysis of the energy level scheme of Nd in NdOBr

INTRODUCTlON

The rare earth oxybromides, RJZOBr, usually doped with the trivalent thulium and terbium ions, have been used as commercial phosphors since mid 1970's. Despite the extensive commercial use the interactions affecting the energies and intensities of the electronic transitions have not yet been studied in detail. In this work, the experimental energy level scheme of the neodymium oxybromide was deduced from the optical absorption spectra and simulated with a phenomenological model which accounts simultaneously for the free ion and crystal field (cf.) interactions. The results were compared to those for NdOF (1) and NdOCl (2) and the evolution of the interactions in the NdOX series was discussed. Reuriuts R-J. Lummiumtiki, University of Turku, Department of Chemistry, FIN-20014 Turku, FINLAND.

EXPERIMENTAL DETAILS

The polycrystalline NdOBr was prepared by the solid state reaction between NdzOs and mBr. A NH4Br/NdzOs ratio of 2.5 and a reaction temperature of 900 "C in static N2 atmosphere were used. The routine X-ray powder diffraction (XRD) analysis revealed no impurity phases.

The optical absorption spectra of the NdOBr sample were measured by using a Cary 5E UVvis-NIR spectrometer at selected temperatures between 9 and 300 K from 170 to 3300 nm. The instrument reproducibility was better than 0.2 A and the band width was 0.6 A.

RESULTS and DISCUSSION

Theoretical Background

The simulation of the energy level scheme of the 4f3 configuration was carried out by using a phenomenological model which treated simultaneously the free ion and cf. effects. The effective free ion Hamiltonian included the Slater integrals, F k (k = 0, 2, 4, and 6) accounting for the Coulombic repulsion between the 4f electrons, I;lf representing the spin-orbit interaction and the Trees (01, p, and y) and Judd Tk (k = 2, 3, 4, 6, 7, and 8) parameters describing the two-and three-body configuration interactions (3,4).

The c.f. part of the Hamiltonian was represented by the Bl c.f. parameters the number of which was defined by the site symmetry of the RE3' ion (5). The RE oxybromides, as well as the lighter RE oxychlorides (RE = La -Ho), have a tetragonal PbFCl-type structure (6) (space group P4 / nmm ; NC 129, Z = 2). The point symmetry of the RE site is Cd". The number of the non-zero c.f. parameters is then restricted to five, i.e. Bi , B," , B," , B," , and B," . The free ion and c.f. parameters were obtained by a least-squares fitting procedure between the experimental and calculated energy level values by minimizing the mean square deviation cr (7). The c.f. interaction was evaluated by using the strength S (8) and individual Sk (k = 2, 4, and 6) (9) parameters. These parameters can be used as a quantitative measure of the crystal field in a particular host (8, 10) but also provide a measure of the cf. strength between different matrices. The S2, S4, and S6 parameters called as the long-, mid-, and short-range cf. strength parameters respectively, give information about the spatial extension of the c.f. effect.

Internretation of Snectrosconic Data

The 4f3 configuration of the Nd3' ion consists of 182 doubly degenerate Stark levels 2 0.5 (Kramer doublets). The number of the Stark levels for a free ion 2s+lLJ state is J + l/2 for 8 o.4 $ any symmetry lower than cubic. According to the group theoretical selection rules, transitions between all Stark levels are allowed as both electric and magnetic dipole transitions (5). Figure. Hot band absorption spectra to the 2P,n state of energies of five Stark levels (0, 9.1, 151, 275, Nd'+ ion in NdOBr at 9,30,77, and 300 K.

and 359 cm-') were confiied (figure). A total of 93 Stark levels were resolved from the absorption spectra.

Energy Level Simulation

The observed energy levels give a basic set sufficient enough for a reliable energy level simulation. A model with 19 param9ters including 14 free ion (Slater integrals, Trees and Judd pammeters, and spin-orbit coupling constant) and five c.f. parameters was used for NdOBr. The Judd parameters were left with fixed values, however. The simulations 19 cm". Since the barycenters of the c.f. multiplets, i.e. the 2s+ f ave a satisfactory rms deviation of LJ states, do not change from one host lattice to another, the free ion parameters assumed similar values for the three NdOX matrices (table). However, with increasing covalence of the bobd to halide, the interaction between the 4f electrons is reduced since the electrons are delocalized and thus the Slater integrals decrease. F* is the one most affected by the environment and provides the best measure of covalence (11).

If one or all of the Slater integrals decrease, the barycenters of the individual 2s+lL J states will lower. The 2Pin state of the Nd3+ ion at 23000 cm-' is isolated and observed as one line in the spectra. In the NdOX series, the energy of the *Pi/z state decreases as a function of the increasing covalence of the bond with halide which agrees with the conclusions made on the Slater integrals.

The spin-orbit interaction is constant in the NdOX series whereas the interconfigurational terms, i.e. the Trees and Judd parameters, show no clear trends. In general, all parameters are well defined with low estimated standard deviations o, but the simulation of the energy level scheme of NdOBr cannot be considered as a final one. Table. The free ion and c.f. parameters (cm-') for the NdOX series (X = F (l), Cl (2), and Br). The cf. parameter values are affected by the distances, bonding angles, and the nature of ligands which characterize the crystallographic site of the RE3' ion (11). For the NdOX series, the value of Bt describing the long-range c.f. interaction differs the most and is much larger for the tetragonal NdOCl and NdOBr matrices than for the hexagonal NdOF (table). The fourth-and sixthrank parameters are best treated in terms of the relative cf. strength Sk and the overall c.f. strength S parameters. The evolutions of the mid-and long-range cf. strength parameters are reverse to that of Bi but similar to the overall cf. parameter.

Parameter

For the tetragonal oxyhalides (NdOCI and NdOBr), the significance of the long-range c.f. effect increases and S4 as well as S6 decrease when compared to NdOF. The electrostatic interaction we&ens and the other effects, ie. covalence, gain more importance in agreement with the reduction observed in the F2 parameter value.

CONCLUSIONS

The complete energy level scheme of the Nd3+ ion in the oxybromide matrice was simulated according to the C.+" site symmetry by using the basis set of energy level values 6btained from the absorption spectra which yielded 93 Stark levels. By using a phenomenological model of 19 parameters a rms deviation of 19 cm-' was achieved. The value of the Slater integral F2, which is the most sensitive to the environment, indicates the weakest Coulombic interaction in the oxybromide matrix. The c.f. effect, measured as the c.f. strength parameter, is almost twice as strong in the hexagonal NdOF than in the tetragonal NdOCl and NdOBr. The decrease in the c.f. strength and in the energy of the ~S+*LJ levels together with the diminishing electron repulsion indicates increasing nephelauxetic effect and covalence from NdOF to NdOBr.