Peter A. Tanner a V.V.R.K. Kumar b, C.K. Jayasankar b, M.F. Reid c ... P.A. Tanner et al. ..... [2] F.S. Richardson, M.F. Reid, J.J. Dallara and R.D. Smith, J. Chem.
Journal at
AND COM?O~JNE~$ ELSEVIER
Journal of Alloys and Compounds 225 (1995) 85-88
Comparative energy level parametrizations for lanthanide ions in octahedral symmetry environments Peter A. Tanner a V.V.R.K. Kumar b, C.K. Jayasankar b, M.F. Reid c a Department of Biology and Chemistry, City Polytechnic of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b Department of Physics, S~i Venkateswara University Postgraduate Centre, P.B. No. 50, Kurnool 51800L India c Department of Physics and Astronomy, The University of Canterbury, Christchurch, New Zealand
Abstract New experimental datasets are presented for the energy levels of tripositive lanthanide ions situated in octahedral symmetry environments, in elpasolite lattices. The freely varying atomic parameters derived from energy level parametrizations of the new datasets show empirical relations with atomic number. The magnitudes of the crystal field parameters for a given system change considerably, according to the number of energy levels employed in the fit. This fitting uncertainty blurs the trends in the parameters across the lanthanide series. The elpasolite lattice is a model system for investigating the crystal field perturbations experienced by a lanthanide ion when the chemical identity of the nth nearest neighbour in the coordination sphere is changed. Keywords: Tripositive lanthanide ions; Elpasolite lattices; Energy levels; Vibronic spectra
I. Introduction
2. Results and discussion
2.1. Electronic spectra of Ln ~+ in elpasolite crystals There have been two previous studies [1,2] of the comparative energy level (EL) parametrizations for tripositive lanthanide ions, Ln 3+, situated in elpasolitetype lattices [3]. Further EL data for Ln 3+ in Cs2MLnCI6 (M = monovalent element) has been forthcoming from the reinvestigations of the electronic spectra during the present study, using a wide-range FT spectrometer at low temperatures [4], and rather more sparse data are also now available for the analogous bromo- and fluorocompounds. The comparative parametrizations of this combined EL data for Ln 3+, based upon the fitting procedure described by Richardson [2,5], form the most important part of the present study. Due to the sparse experimental data, previous studies [1,2] utilized "mixed" EL datasets, including EL from both the neat and doped elpasolite systems. An attempt has been made to treat these systems separately in the present study, and to assess the difference in the crystal field (CF) experienced by Ln 3+ in different host lattices. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)0701.4-8
A diagram of the coordination of Ln 3+ in the elpasolite lattice is given in [3,6], together with a description of the structure. The isolated octahedral LnX~- moiety possesses six internal modes of vibration, ui (i = 1-6), labelled under the irreps of the Oh molecular point group (MPG), whereas the vibrations of the CsaNaLnX6 crystal [7] also include the lattice modes, shown in bold type in Eq. (1): F(vib, cryst)e{S1, vl(alg): $2, v2(%): S3(Tlg): 84, /25; Ss(T2g): 56, /23; 87,/24; 88; S9(rlu): 510, /26(T2u)}
(1) The pure electronic transitions between the states derived from t h e f N configuration of a given ion are electric dipole (ED) forbidden, but nevertheless are observed with oscillator strengths between 10 -6 and 10-13 when allowed via the magnetic dipole (MD) and/or electric quadrupole (EQ) mechanisms [8]. The first-order vibronic selection rules [9] permit the appearance of a spectral feature when the direct product
86
F(i) × F(vib) × F(f)
P.A. Tanner et al. / Journal of Alloys and Compounds 225 (1995) 85--88
(2)
where i and f refer to the initial and final electronic states, includes F(ED), which in the present case, is the Ta, representation of the Oh MPG. These selection rules lead us to expect that most of the vibronic intensity in the electronic spectra of Cs2NaLnC16 will arise from the moiety ~'a~ and/or ~z, modes only, depending upon the nature of the states i and f. For Ln 3+ doped into Cs2NaYCI6 at low concentration, single bands are observed for these vibronic origins, but multiple structure is observed when these ions are doped into CszNaGdCl6 (attributed to guest-host couplings: see, for example, Fig. 7 in [10]). The transverse optic (TO) and longitudinal optic (LO) splittings of vibrations give rise to multiple structure for vibronic origins involving ~-~ modes, for the neat elpasolites. Weaker features in the electronic spectra of CszNaLnC16 which have similar derived wavenumbers to those of lattice vibrations, and of gerade moiety modes, may be understood according to the space group theory of Satten [11]. The analysis of the electronic spectra of Ln 3+ in CszNaLnC16 is thus more complex than, for example, the interpretation of the predominantly ED allowed spectral transitions in LaCI3:Ln 3+ [12]. The locations of the electronic origins of many transitions (for example, describing these under MPG O: F~ ~/"1, F 1 *-~/'3,/-'1 ~ / ' 5 ) must be deduced from vibronic analysis, since the zero phonon line (ZPL) is very weak and/or masked by other structure. Although a fairly complete vibronic fingerprint may be observed, the accuracy in location of the EL is typically +_2 cm-L The selection rules for the observation of MD electronic transitions are slightly less restrictive for the cases of odd-electron systems, where only the ['6
