Analysis of the Crystal Structure and Optical Spectra of Stoichiometric

U.P.R. 209, C.N.R.S., F-92195 Meudon, France. Przemysław Deren´ and Wiesław Stre¸k. W. Trzebiatowski Institute of Low Temperature and Structure Re...
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J. Phys. Chem. 1996, 100, 14736-14744

Analysis of the Crystal Structure and Optical Spectra of Stoichiometric SmOF Jorma Ho1 lsa1 , Eija Sa1 ilynoja,* and Pia Ylha1 Department of Chemistry, UniVersity of Turku, FIN-20014 Turku, Finland

Pierre Porcher U.P.R. 209, C.N.R.S., F-92195 Meudon, France

Przemysław Deren´ and Wiesław Stre¸ k W. Trzebiatowski Institute of Low Temperature and Structure Research, Polish Academy of Sciences, PL-50950 Wrocław, Poland ReceiVed: February 5, 1996; In Final Form: May 1, 1996X

The relationship between the crystal structure and the crystal field (CF) effect was studied in the hexagonal samarium oxyfluoride (SmOF). The crystal structure of SmOF was refined with the Rietveld method from the X-ray powder diffraction data in the range of 6.5 < 2θ < 121.4°. The Ar+ ion laser excited luminescence spectra of the Sm3+ ion (4f5 electron configuration) doped LaOF and GdOF were recorded at 77 and 300 K between 500 and 800 nm. The absorption spectra between 295 and 2055 nm were measured at selected temperatures between 9 and 300 K using the pure hexagonal SmOF. Analysis of the spectra according to the C3V symmetry for the Sm3+ site in rare earth oxyfluoride (REOF) yielded 195 Stark components out of the total 1001. For the energy level simulation, a model of 20 adjustable parameters was used including the Racah, Trees, and Judd parameters, the spin-orbit coupling constant (ζ4f), and B20, B40, B43, B60, B63, and B66 CF parameters. The experimental energy level scheme was well reproduced with a root mean square deviation equal to 17 cm-1. The CF parameter set was found to be consistent with those for other RE3+ ions in REOF matrices. Structural data were used to calculate the CF parameters by a modified point charge model. The calculated second-range contribution was too high, whereas the fourth- and sixth-rank contributions were close to the experimental ones. The experimental and calculated CF parameters indicate only slight distortion from the ideal fluorite-type cubic structure.

1. Introduction The 4f orbitals of the rare earth (RE) ions are shielded from the surroundings by the upper (5d and 6s) orbitals. The influence of the host material on the electronic transitions within the 4fn configuration is hence small. Consequently, the optical transitions yield sharp lines in the optical spectra and the decay time of the excited states is long.1 Despite of these facts, the luminescence of the RE ions is widely used in several commercial applications.2,3 The Eu3+ and Tb3+ ions are the privileged ones because of their strong luminescence. Luminescence from the Sm3+ ion is used for the immunoassay in medical applications.3 The most frequent oxidation states for the samarium ion are II and III. Sm2+ has a 4f6 configuration with an energy level scheme similar to that of the Eu3+ ion. In SmOF, the Sm3+ ion has a 4f5 electron configuration which is characterized by 198 SLJ manifolds. In the presence of the crystal field (CF), these manifolds split into a total of 1001 Stark levels.4 Despite this complication, the complicated energy level scheme of the Sm3+ ion has been reported in several inorganic and organic matrices.5-17 The most detailed spectroscopic characterization has been carried out for simple inorganic halides and oxides as LaCl3:Sm3+,5 LaF3:Sm3+,7 and Y2O3:Sm3+.10 To a certain extent, the energy level scheme of Sm3+ has also been studied for the elpasolite hosts13 Cs2NaSmCl6 and Cs2NaYCl6:Sm3+ as well as for organic hosts with complex ligands16,17 * Corresponding author. X Abstract published in AdVance ACS Abstracts, August 1, 1996.

S0022-3654(96)00348-6 CCC: $12.00

Na3[Sm(oxydiacetate)3]‚2NaClO4‚6H2O, [Sm(AP)6](ClO4)3, and [Sm(AP)6]I3 (AP ) antipyrine). The work described in this paper comprises the detailed study of the energy level scheme of the Sm3+ ion in the rhombohedral SmOF host up to 33 450 cm-1. The optical UV, visible, and near-IR absorption spectra of SmOF together with the visible luminescence spectra of the Sm3+-doped LaOF and GdOF were interpreted according to the C3V point symmetry of the Sm3+ site. The experimental energy level values were compared with the calculated ones obtained by a phenomenological model including simultaneously the electrostatic, spin-orbit, interconfigurational, and CF interactions. The evolution of the CF effect in the REOF:RE3+ series (RE3+ ) Pr, Nd, Sm, Eu, Tb, and Dy)18-22 was discussed in terms of the possible interactions. The CF parameter values obtained by this model were then compared to those given by the modified electrostatic point charge model (PCEM). The calculations were based on the structural data for SmOF derived from the simulation of the X-ray powder diffraction pattern by the Rietveld profile refinement method. The correlation of the CF parameter values with the structural distortions from the ideal cubic fluorite-type structure was also discussed. 2. Experimental Methods 2.1. Sample Preparation. Since no stoichiometric REOF single crystals can be obtained, the polycrystalline REOF samples were prepared by the solid state reaction23 between the RE sesquioxides, RE2O3, and NH4F. The reagents were heated at 985 °C for 1 h. The NH4F/Sm2O3 ratio was 2.5 in order to © 1996 American Chemical Society

Crystal Structure and Optical Spectra of SmOF

J. Phys. Chem., Vol. 100, No. 35, 1996 14737

obtain the stoichiometric SmOF form. The heating temperature was 1050 and 950 °C for LaOF and GdOF, respectively, and the stoichiometric NH4F/RE2O3 ratio () 2.00) was used. The LaOF and GdOF hosts were doped with 2 mol % of the Sm3+ ion for luminescence measurements. The structures were confirmed to correspond to the hexagonal (rhombohedral) REOF form by routine X-ray powder diffraction. No presence of impurity phases could be observed by optical measurements. 2.2. X-ray Powder Diffraction Measurements. The powder diffraction data of the pure stoichiometric SmOF were collected at room temperature with the Enraf-Nonius PDS120 X-ray powder diffractometer using the position sensitive INEL CPS120 detector. The monochromatic Cu KR1 radiation was used at the 2θ region between 6.5 and 121.4°. The angular resolution of the equipment was better than 0.018°. The measuring time was 1 h. The external calibration was carried out with a silicon powder 2θ/d-spacing standard (NBS 640b). 2.3. Optical Measurements. The luminescence of LaOF: Sm3+ and GdOF:Sm3+ was excited by a Carl Zeiss Jena ILA 120-1 Ar+ laser (the 457.9 nm line). The spectra were dispersed by a 1 m Czerny-Turner-type Jobin-Yvon monochromator and detected by a Hamamatsu R950 or R406 photomultiplier equipped with standard electronics. The resolution of the equipment was around 1 cm-1. The spectra were recorded at 77 and 300 K in the visible region between 500 and 800 nm. The optical absorption spectra of the pure SmOF were measured between 295 and 2055 nm by a Cary 5E UV-visnear-IR spectrometer at selected temperatures between 9 and 300 K. The instrument reproducibility was better than 2 Å, and the bandwidth used was 0.6 Å. The samples were prepared by mixing SmOF in KBr (the SmOF/KBr weight ratio was ca. 0.1) and pressing the mixture to a transparent disk. No polarized absorption or emission measurements were possible due to the polycrystalline material. 3. Theoretical Treatment 3.1. Electrostatic Point Charge Model. The simplest description of the crystal field uses the point charge concept.24 This model neglects both the finite spatial extension of the ligand charge density and the wave function overlap. In our treatment, the originality lies in the utilization of the appropriate structural data, in the modification of the basic model by the inclusion of the factors accounting for the covalence effects, and, most important, in the optimization of the charges of all ions instead of the use of the formal charges.20 As shown later, the modifications in the point charge model have resulted in considerable improvements and yielded much better results than the basic model. The ionic nature of bonding in REOF has, of course, contributed significantly to the achievements. The CF parameters Bkq can then be expressed as follows24

Bkq ) FkAkq

(1)

where Fk ) 〈rk〉 (1 - σk)τ-k. The Akq are the electrostatic host dependent lattice sum parameters20 which take into account the geometry of the charges of the lattice formed by the neighboring ions. The exact form used to calculate the lattice sum parameters is given in ref 20. The charges of the ions were optimized to the following: gSm ) +1.8, gO ) -1.2, and gF ) -0.6. When compared to the effective charges found for Y2O3 (g ) +1.51.7),10,25 our choice can be considered excellent. The Fk values are the ion dependent quantities, where 〈rk〉 values are the Dirac-Fock free ion integrals26 and σk values are the linear screening factors arising from the shielding effect of the 6s, 5d, and 5p electrons. The τ parameter is an expansion

factor accounting for the expansion of the free ion wave function when the ion is introduced into a solid. The values used for σk and τ are as follows:24 σ2 ) 0.630, σ4 ) 0.09, σ6 ) -0.04, and τ ) 0.722. The free ion radial integrals26 for the Sm3+ ion are 〈r2〉 ) 0.974, 〈r4〉 ) 2.260, and 〈r6〉 ) 10.550. 3.2. Phenomenological Simulation Model. The simulation of the energy level scheme of the 4f5 configuration was carried out by using a phenomenological model with an effective Hamiltonian which includes simultaneously the free ion and CF effects. The current analysis involves the diagonalization of two square matrices with the dimensions of 666 and 670.27 The inclusion of a complete set of wave functions and the simultaneous treatment of the free ion and CF effect should yield the most reliable and physically meaningful simulation of the 4f5 energy level scheme. The energy level structure according to the single-configuration model is determined by the free ion Hamiltonian and the interaction of the ion with the crystal field. The effective Hamiltonian is given by the following equation4,28

Ek(nf,nf)ek + ζ4fASO + RL(L + 1) + ∑ k)0-3 βG(G2) + γG(R7) + ∑ Tktk + ∑BkqCkq(i) k)2-4,6-8 k,q,i

H ) HO -

(2)

where HO is the one-electron part which treats the RE ions as spherically symmetric. The Racah parameters Ek (k ) 0-3) are for the radial dependence of the electrostatic interaction while ek values are the angular part. The radial dependence of the spin-orbit interaction is parametrized by ζ4f, whereas the angular part is described by ASO. The angular parts of the electrostatic and spin-orbit interactions can be calculated exactly.4 The two- and three-body configuration interactions were also included in the model. The parameters R, β, and γ were used to characterize the spin-independent two-body interactions with L as the total orbital angular momentum of levels involved. G(G2) and G(R7) are the eigenvalues of Casimir’s operators for the group G2 and R7 used to classify the states of the fn configuration.4 Tk (Judd parameters) describe the three-body electrostatic configuration interaction parameters while tk (k ) 2-4 and 6-8) are the corresponding operators.28 The standard one-electron CF Hamiltonian HCF is given by a sum of the products between the CF parameters Bkq(i) and the tensors Ckq(i) of rank k. The latter are related to the spherical harmonics as follows4,29

Ckq )

(2k4π+ 1)

1/2

Ykq(θi,φi)

(3)

with summation i over all 4f electrons of the electron configuration. The values of k and q are limited by the point symmetry of the RE3+ site. For the C3V point symmetry, the CF Hamiltonian has the following form (eq 4) which is the same as adopted in refs 4 and 29. 4 - C43) + HCF ) B20C20 + B40C40 + B43(C-3 6 - C63) + B66(C66 + C66) (4) B60C60 + B63(C-3

The best fit set of the free ion and CF parameters was obtained through minimizing the root mean square (rms) function between the observed and calculated energy levels by the leastsquares refinement.30 4. Results and Discussion 4.1. Crystal Structure Refinement. The X-ray powder diffraction data for SmOF were treated with the Rietveld method

14738 J. Phys. Chem., Vol. 100, No. 35, 1996

Ho¨lsa¨ et al. TABLE 2: Selected Structural Data for SmOFa SmOF zRE zO zF RE-Fb (Å) (1×) RE-F′ (Å) (3×) RE-O (Å) (1×) RE-O′ (Å) (3×) F′-RE-F′ (deg) F-RE-F′ (deg) O′-RE-O′ (deg) O-RE-O′ (deg)

0.2418(2) 0.1157(12) 0.3704(10) 2.517(22) 2.507(5) 2.468(22) 2.318(3) 103.25(2) 64.86(1) 115.96(2) 78.25(1)

ideal29 0.250 0.125 0.375

109.47 70.52 109.47 70.52

a All atoms reside in position with x ) y ) 0. b O(F) atoms lie on the 3-fold symmetry axis.

Figure 1. The observed, calculated, and difference X-ray powder diffraction patterns for SmOF measured at room temperature.

TABLE 1: Information Concerning the Data Collection and Crystal Structure Based on the X-ray Diffraction Measurements of Polycrystalline SmOF Data Collection compound name samarium oxyfluoride formula SmOF formula weight (g mol-1) 185.4 scan type 2θ 2θ range (deg) 6.5 < 2θ < 121.4 scan step (2θ) (deg) 0.029 radiation (Å) 1.5406 h,k,l range (max) 3,3,21 number of observed reflections 69 crystal system space group Z a (Å) c (Å) V (Å3) 2θ range (deg) zero point (deg) profile parameters Rp Re Rwp S

Rietveld Refinement hexagonal R3hm - D53d (No. 166)33 6 3.9312(2) 19.5666(6) 261.88(1) 20 < 2θ < 121.4 -0.09 U ) 0.860(59) V ) -0.498(42) W ) 0.112(7) 0.029 0.015 0.039 2.7

to refine the crystal structure of the stoichiometric SmOF. The Rietveld profile refinement program used was DBWS-9006PC.31 In this case, there were 17 parameters to be varied independently. The refinement was carried out by varying the parameters in the following order: (1) scale factor together with background parameters; (2) unit cell parameters; (3) profile and asymmetry parameters; (4) atomic positional parameters; (5) isotropic thermal parameters. Most refinement cycles included step 1 as well. After constant values were achieved for all other parameters, step 5 was executed. The 2θ data range between 20.0 and 121.4° was simulated by using the Gaussian profile shape function (Figure 1). The starting values for the unit cell and atomic positional parameters were those for YOF.32 The refinement was carried out to a constant Rwp value of 3.9% (Table 1). The stoichiometric SmOF has a hexagonal (rhombohedral) structure distorted and ordered from a cubic fluorite.34 All atoms lie in a 6-fold special position 6c with x ) y ) 0. Details of the refinement and final results are shown in Tables 1 and 2. The small deviation of the positional parameters from the values of the ideal cubic structure indicates only slight trigonal distortion from the fluorite structure. The Sm3+ ion is coordinated to four oxygens and four fluorines which form a trigonal antiprism with two capping

atoms along the C3 axis (Figure 2). The point symmetry of the Sm3+ site is C3V. The Sm-F distances are longer than the corresponding Sm-O distances. The rather large uncertainty of the z values for the light elements (O and F) is due to the large X-ray scattering factor for Sm compared to that of O or F which induces considerably error in the Sm-O(F) distances. The O-Sm-O angles are wider and the F-Sm-F angles narrower than the corresponding angles in the ideal fluorite structure (Table 2), but differ significally from 180° which is most advantageous to magnetic superexchange interactions.22 4.2. Interpretation of Spectroscopic Data. The majority of the electronic transitions between the free ion 2S+1LJ levels of Sm3+ originates from the electric dipole (ED) interactions with the ∆J e 6 selection rule. Contribution from the magnetic dipole (MD) interaction with the ∆J ) 0, (1 selection rule35 cannot be neglected, however. The J-mixing by the CF effect lifts partially the restrictions imposed by the free ion selection rules. For the C3V point symmetry of the Sm3+ site in REOF, the following types of Stark levels exist: the D1/2 doublet and the Kramers conjugate level (S1, S3). According to the group theoretical selection rules, transitions between all CF levels are allowed as both the ED and MD transitions.35 4.2.1. Luminescence Spectra. The Ar+ ion laser line at 457.9 nm excites effectively the 4G5/2 state of the Sm3+ ion. The energy gap between the 4G5/2 state and the next lower 6F11/2 one is ca. 7000 cm-1, and thus the nonradiative relaxation is negligible in RE oxyfluorides where the energy of the highest energy lattice vibration is low,36 around 500 cm-1. According to absorption measurements, the energy difference between the two lowest Stark levels of the 4G5/2 state is 280 cm-1, indicating that only the lowest Stark level is significantly occupied in 77 K, which was confirmed experimentally by the simple emission spectrum. The luminescence to the 6HJ (J ) 5/2-11/2) states was observed in the visible wavelength region from 550 to 750 nm in both the LaOF and GdOF matrices (Figure 3). The intensity of the transitions was lower in LaOF than in GdOF. The most intense transition of the group was 4G5/2 f 6H7/2. The transitions in the LaOF matrix appeared at a shorter wavelength than those in GdOF. This is due to the nephelauxetic effect observed for the RE3+ ions.37 The replacement of the host cation by Sm3+ (smaller than La3+ but larger than Gd3+) causes the expansion or shrinking of the Sm3+ electron cloud as well as the 4f intraelectron energy level scheme in the LaOF or GdOF hosts, respectively. The total splitting of the 6H5/2 ground state is 105 and 111 cm-1 in LaOF and GdOF, respectively. Similar differences were observed for the other states, too, which suggest a stronger CF effect in the GdOF host. The energy level scheme of the GdOF:Sm3+ consisting of 14 Stark levels for the 6H3/2-11/2 states was used in the calculations in order to minimize the host effect.

Crystal Structure and Optical Spectra of SmOF

J. Phys. Chem., Vol. 100, No. 35, 1996 14739

Figure 2. A stereoscopic ORTEP view of the layered (perpendicular to the c-axis) REOF structure. Small circles: RE cations. Open circles: oxygens. Circles with ellipsoids: fluorides. The size of the atoms is not drawn in the actual thermal parameter scale.

4.2.2. Absorption Spectra. The absorption spectra of the pure stoichiometric SmOF were measure between 295 and 2055 nm (33 900 and 4865 cm-1) at selected temperatures between 9 and 300 K. The absorption spectra at 9 K consists of 181 lines originating from the absorption from the ground state 6H5/2 to 6H 6 4 4 4 4 13/5,15/2, F1/2-11/2, G45/2-11/2, F33/2-9/2, I39/2-13/2, M15/2-21/2, 6P 4 4 4 4 4 3/2-7/2, D31/2-3/2, H17/2-13/2, L13/2-19/2, K111/2-15/2, G27/2-13/2, 2L3 4P2 2K5 , , and (Table 3). The energy difference 15/2 3/2,5/2 13/2 between the two lowest Stark levels (6H5/2,3/2 and 6H5/2-5/2) of the ground state is only 18 cm-1. Accordingly, at 9 K the second lowest Stark level is partly occupied and additional lines were observed, which makes the interpretation of the absorption spectra more difficult. By increasing the measuring temperature, the absorption from the second lowest Stark level was easy to recognize. In the near-IR region, 32 Stark levels out of the theoretical 36 for the 6H13/2,15/2 and 6F1/2-11/2 states were observed. All seven Stark levels for the first excited state 6H13/2 were observed at the wavelength range from 1900 to 2060 nm by absorption. From 900 to 1700 nm, the absorption appears to the partially overlapping states 6H15/2 and 6F1/2-11/2 (Figure 4). Transitions to the other states of the 6F term are well-resolved. In the visible and UV regions below 510 nm, the density of the absorption lines increases as shown in a part of the absorption spectrum recorded at 9 K (Figure 5). The final wellresolved lines, at a wavelength range of 532 to 568 nm, correspond to the 6H5/2 f 4G45/2 and 6H5/2 f 4F33/2 transitions, beyond which the J-mixing of states becomes high and the unambiguous assignment of levels is difficult. The level assignments were thus made principally on the basis of line position only. The last occasional absorption lines corresponding to the transitions from the ground state to 2K513/2 were found at 299 nm. Below 299 nm a large band due to the activatorhost lattice interaction is predominant. The total number of the CF levels observed from the luminescence and absorption spectra was 195 out of the 1001 theoretical Kramers doublets (Table 3). On the other hand, the levels observed include the most representative ones belonging to the 12 lowest SLJ manifolds (6HJ and 6FJ). The energy level set spans 46 SLJ manifolds in total. 4.2.3. Vibronic Transitions. Vibronic transitions were observed in both the luminescence and absorption spectra. These transitions occur as sidebands accompanying the zerophonon transitions when the sharp transitions of the static crystal field are frequency modulated by the lattice host. In contrast

to the zero-phonon transitions which can occur by the MD and ED processes, vibronic transitions are believed to be predominantly of the ED nature and are associated mainly with the oddparity vibrations. Coupling of the electronic CF wave function to the IR-active vibrational modes is thus predicted.38,39 In the luminescence spectra, vibronic transitions were observed at the lower energy side of the 4G5/2 f 6H5/2 transition. In the absorption spectra, they occur at the higher energy side of the zero-phonon line, most clearly observed with the most intense transitions i.e., 6H5/2 f 6F5/2 and 6H5/2 f 6F7/2) (Figure 4). The IR-active vibrations involved were confirmed by the IR spectra of pure SmOF.36,40 Lines corresponding to the frequencies 130, 230, and 300 cm-1 were usually found. The vibronic transitions observed were mainly due to the coupling with the RE-O vibrations because of the increasing polarizability and covalency of the ligand from F- to O2-,41 but coupling with the RE-F vibrations could not be excluded either. Moreover, the line shapes and analysis of the line intensities as a function of a temperature supported the identification of the vibronic lines. 4.3. Energy Level Scheme Simulation. The analysis of the absorption and luminescence spectra provided 195 Stark levels which enabled the variation of the 14 free ion and the six CF parameters. The fitting procedure used has been described earlier.22 The simulation was carried out to a satisfactory rms deviation of 17 cm-1 (Table 4), and the uncertainty of each parameter value was low. All parameter values are thus well-determined, and the overall simulation gives a good agreement between the observed and calculated energy levels (Table 3). Only the 6F11/2 state remains poorly fitted, mainly because of the experimental uncertainty due to the change of detector and radiation source between 850 and 900 nm. 4.3.1. Free Ion Parameters. As expected, due to the shielding of 4f electrons by 5s and 5p shells, the free ion parameters of Sm3+ in the SmOF matrix (Table 4) are similar to those found for the Sm3+ ion doped lanthanum trifluoride7 and trichloride5 matrices. The values of E0-3 and the spinorbit coupling constant ζ4f fit well with those found for the REOF:RE3+ series (RE3+ ) Pr, Nd, Sm, Eu, Tb, and Dy)18-22 with an increase in these values from Pr3+ to Dy3+ as expected owing the increasing number of the 4f electrons. The trends in the Trees and Judd parameterssthe interconfiguration interaction termssare not clear. Only R decreases smoothly when the parameter values for the pure REOF are accounted for. A

14740 J. Phys. Chem., Vol. 100, No. 35, 1996

Ho¨lsa¨ et al.

TABLE 3: Observed and Calculated Energy Levels of SmOF in cm-1a main component 6H 5/2,3/2 -5/2 1/2 6H

7/2-5/2 7/2 -5/2

3/2 6H 9/2,7/2 9/2 1/2 3/2 -5/2 6

H11/2-9/2 7/2 -11/2 -3/2 1/2

-5/2 6H 13/2-11/2 9/2 7/2 13/2 -5/2 3/2 1/2 6

H15/2,9/2 7/2 -11/2 6F 1/2,1/2 6F 3/2,3/2 1/2

6H

15/2-5/2 -15/2 13/2 3/2 1/2

6F 5/2-5/2 1/2 -3/2 6F 7/2,1/2 7/2 3/2 -5/2 6F 9/2,1/2 3/2 7/2 9/2 -5/2 6F 11/2,7/2 9/2 6F 11/2-5/2 3/2 1/2 -11/2 4

G45/2-5/2 1/2 3/2 4F3 3/2,3/2 1/2 4

G47/2,7/2 1/2 3/2 -5/2 4I3 9/2,7/2

4M

15/2,9/2 4I3 9/2-5/2 -5/2 -3/2

4M

15/2-11/2 13/2 9/2 1/2 -5/2

3/2 4I3 9/2,1/2 4

I311/2-5/2

Eobsd S1, S3 D1/2 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2

0 20 111 978 1108 1259 1270 2203 2320 2465 2473 3535 3715 3811 4870 4880 4957 5136 5179 5194 5246 6170 6250 6428 6629 6635 6731 6781 6862 7143 7227 7243 7997 8015 8027 8122 9144 9169 9189 9222 9240 10 507 10 523 10 533 10 599 10 630 17 593 17 867 18 035 18 779 18 792 19 814 19 958 19 979 20 237 20 289 20 322 20 335 20 381 20 444 20 555 20 641 20 675 20 712 20 789 20 816 20 837

Ecalcd -23 10 134 963 1094 1247 1253 2198 2272 2332 2442 2468 3528 3543 3662 3722 3736 3813 4874 4901 4975 5150 5181 5196 5233 6189 6212 6270 6455 6651 6651 6688 6744 6769 6835 6866 7139 7237 7268 7978 7997 8028 8137 9126 9145 9181 9212 9230 10 465 10 480 10 484 10 590 10 595 10 663 17 623 17 903 18 029 18 789 18 813 19 785 19 944 19 998 20 047 20 242 20 246 20 316 20 326 20 391 20 433 20 609 20 630 20 694 20 736 20 788 20 806 20 835

main component 4M

15/2,15/2

4

I311/2,7/2 3/2 -11/2

4 M15/2,1/2 4I3 11/2-9/2 4I3 13/2,7/2 -11/2 1/2 -9/2 5/2 3/2 4

I311/2,1/2 4F3 5/2,3/2 -5/2 1/2

4M

17/2,13/2 9/2 13/2 -15/2 1/2 -5/2 -9/2 7/2

4

G49/2-9/2

7/2 4I3 15/2-11/2 4M 17/2,3/2 1/2 4G4 9/2,7/2 4I3 15/2,9/2 7/2 4

M17/2-17/2 4 I315/2,13/2 5/2 -3/2 1/2 15/2 4M

19/2,13/2 15/2

-11/2 6P 5/2-5/2 3/2 1/2 -3/2 4

M19/2,13/2 -17/2 -5/2 3/2 1/2 19/2

4

L13/2,13/2 1/2 -3/2 1/2 4L 13/2,7/2 -5/2 -9/2

4F3

7/2-5/2 3/2 7/2

6

P3/2,1/2

3/2 4F3 7/2,1/2 4K1 11/2-11/2 9/2 1/2 4

M21/2-17/2 -15/2 -17/2 4

K111/2,7/2 3/2 7/2

4M

21/2-3/2 -11/2

4

M21/2,1/2

S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2

Eobsd

Ecalcd

20 890 21 030

20 876 21 020 21 044 21 076 21 357 21 375 21 462 21 524 21 561 21 563 21 581 21 625 21 634 21 951 21 965 22 153 22 234 22 261 22 263 22 298 22 299 22 479 22 589 22 645 22 704 22 723 22 749 22 786 22 809 22 843 22 854 22 860 22 892 22 951 23 049 23 084 23 112 23 155 23 764 23 775 23 834 23 897 23 926 23 955 23 973 23 988 24 019 24 084 24 121 24 149 24 249 24 351 24 369 24 374 24 389 24 539 24 586 24 616 24 717 24 737 24 756 24 800 24 816 24 904 24 981 24 994 25 003 25 048 25 051 25 082 25 128 25 135 25 176 25 222 25 231 25 248

21 336 21 442 21 503 21 552 21 568 21 581 21 626 21 643 21 977 21 995 22 150 22 212 22 257

22 472 22 608 22 626 22 697 22 740 22 756 22 775 22 799 22 835 22 853 22 865 22 867 22 958 23 065 23 089 23 115 23 146 23 761 23 808 23 828 23 924

24 366 24 372 24 385 24 551 24 609 24 625 24 733 24 756 24 771 24 806 24 842 24 901 24 973 24 996 25 059 25 145 25 169

Crystal Structure and Optical Spectra of SmOF

J. Phys. Chem., Vol. 100, No. 35, 1996 14741

TABLE 3. (Continued) main component 19/2 4

L15/2-15/2 1/2 3/2 -5/2 13/2

4G4 11/2-5/2 7/2 4M

21/2,7/2 -3/2 7/2

4

G411/2,3/2 M21/2-5/2 4G4 11/2-9/2 4L 15/2-11/2 4

-5/2 -9/2 4

G411/2,1/2

4M 4

21/2-21/2

D31/2,1/2 6P 7/2,3/2 -5/2 1/2 7/2

4

L17/2,1/2 -3/2 -17/2 -17/2 13/2 -15/2 7/2

4K1

13/2-11/2 9/2 9/2 1/2 -5/2 3/2

4L

17/2-11/2 4K1 13/2,7/2 4F3 9/2,7/2 7/2 -5/2 3/2 1/2 -9/2 6

P5/2-5/2 4 F39/2,1/2 4D2 3/2,3/2 1/2 6

P5/2-3/2 4H1 7/2,1/2 -5/2 3/2 -5/2 4

K115/2,15/2 13/2 -11/2 1/2 -5/2

a

Eobsd D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2

25 434 25 445

25 574 25 595 25 615 25 661 25 742

26 311 26 386 26 402 26 426 26 537 26 561 26 579 26 617 26 697 26 784 26 797 26 809 26 839 26 934 27 001 27 181 27 288 27 366 27 500 27 519 27 525 27 547 27 998 28 096 28 152 28 171 28 544 28 554

Ecalcd 25 339 25 453 25 460 25 463 25 470 25 482 25 575 25 596 25 605 25 606 25 616 25 632 25 669 25 706 25 742 25 755 25 758 25 769 25 837 26 358 26 393 26 397 26 400 26 454 26 535 26 551 26 560 26 587 26 598 26 604 26 678 26 739 26 779 26 794 26 797 26 820 26 885 26 895 26 955 26 991 27 113 27 180 27 195 27 311 27 373 27 412 27 506 27 517 27 532 27 556 27 989 28 071 28 152 28 167 28 510 28 518 28 544 28 551 28 564

main component 3/2 -9/2 7/2 4H1 9/2,7/2 -3/2 4H1 13/2,13/2 9/2 1/2 4L 4

19/2-11/2

H113/2-3/2 5/2 4

L19/2,9/2

13/2 4H1 13/2,7/2 4G2 7/2-5/2 1/2 3/2 7/2 4G2 5/2,3/2 4 G29/2,1/2 4G2 7/2,7/2 4G2 9/2,7/2 4G2 9/2-9/2 4G2 5/2,1/2 -5/2 -3/2 2L3 15/2-11/2 -9/2 13/2 4P2 1/2,1/2 4G2 11/2-5/2 1/2 -3/2 2 L315/2-13/2 4G2 11/2-11/2 7/2 9/2 2L3 15/2-5/2 -3/2 4P2 3/2,1/2 -3/2 2L3 15/2,1/2 15/2 4P2 5/2,1/2 -5/2 -3/2 1/2 2K5 13/2-11/2 9/2 -11/2 4F2 9/2-7/2 7/2 -5/2 -3/2 2

K513/2,3/2 -5/2 4

F29/2,1/2

-9/2 2K5 13/2,1/2

S1, S3 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 S1, S3 D1/2 D1/2 D1/2 D1/2 S1, S3 S1, S3 D1/2 D1/2 S1, S3 D1/2

Eobsd

Ecalcd

28 579

28 570 28 617 28 631 28 730 28 734 29 518 29 5429 550 29 590 29 601 29 606 29 618 29 623 29 678 29 753 29 878 29 892 29 909 29 959 29 973 29 983 30 034 30 060 30 110 30 310 30 336 31 039 31 083 31 096 31 110 31 172 31 178 31 200 31 243 31 261 31 282 31 300 31 391 31 457 31 550 31 551 31 603 31 616 32 492 32 560 32 589 33 154 33 369 33 386 33 442 33 604 33 619 33 665 33 675 33 710 33 734 33 778 33 787 33 840

28 631 28 708 28 730 29 498 29 562

29 632 29 764 29 903 29 962 29 974 30 041 30 058 30 102 30 303 30 316 31 037

31 230 31 299 31 309 31 368 31 560 31 590 31 676 32 510 32 605 33 178 33 390

Due to the strong J-mixing, the same SLJM combination may be encounterd more than once.

similar decrease has been found for the LaF3:RE3+ and LaCl3: RE3+ series.5,7 4.3.2. Crystal Field Parameters. The B20 value is quite close to 0, but the fourth- and sixth-rank CF parameters assumed high values (Table 4). According to the theoretical calculations for cubic symmetry,42 the values of the CF parameters should have the following ratios: B40/B43 ) -0.837, B60/B63 ) 1.656, and B60/B66 ) 1.579 as well as B20 ) 0. For SmOF (Table 4), the B40/B43 ratio is significantly greater (65%) than the cubic one, but the B60/B63 and B60/B66 ratios are the same within the experimental uncertainty. The crystal structure can be considered as only slightly distorted from the ideal cubic one.

Evolution of the individual CF parameters in the REOF:RE3+ series (RE3+ ) Pr, Nd, Sm, Eu, Tb, and Dy)18-22 is shown in Table 4. The value of the B20 parameter describing the longrange CF interaction is low but also quite constant over the whole REOF series. The other axial CF parameters, B40 and B60, have trends that are less clear. The absolute values of B43, B63, and B66 decrease as a function of the increasing number of the 4f electrons. The values of the B43 parameters diminish throughout the whole REOF series, but the values of the shortrange CF parameters B63 and B66 diminish up to the Tb3+ ion, beyond which they increase. It should be noted, however, that

14742 J. Phys. Chem., Vol. 100, No. 35, 1996

Ho¨lsa¨ et al. increased nuclear charge which pulls the electron orbitals closer to the nucleus, thus reducing the CF effect. The change should be the greatest in the beginning of the series where the relative decrease in the ionic radius is the greatest. Such an evolution in the CF effect was observed in the value of the CF strength parameter10 S up to the Tb3+ ion, beyond which the CF seems to get stronger again (Table 4). The relative CF strength of the short-range (k ) 6), midrange (k ) 4), and long-range (k ) 2) CF interactions43,44 were calculated according to the following equation:

Sk ) [(Bk0)2 + 2∑(Bkq)2]1/2

(5)

q>0

Figure 3. The Ar+ laser (λex ) 476.5 nm) excited luminescence spectrum of the hexagonal (rhombohedral) GdOF:Sm3+ at 77 K between 550 and 750 nm.

Figure 4. Part of the near-IR absorption spectrum of SmOF at 9 K between 910 and 1700 nm. The vibronic transitions marked with V were confirmed by IR spectroscopy.40

Figure 5. Part of the UV and visible absorption spectrum of SmOF at 9 K from 350 to 575 nm. Above 500 nm the J-mixing of states becomes high and the assignment of levels is difficult.

for Pr3+, Eu3+, and Tb3+ the host cation in REOF is different from the ion studied and for Eu3+ and Tb3+ only the ground term 7F has been employed in the simulations. The host lattice may modify considerably the strength of the crystal field effect in the case on Pr3+ where the data in the YOF host only are available. For Eu3+ and Tb3+, the size difference to the Gd3+ ion is not large and has no significant influence to the values of the crystal field parameters. The maximum difference in the CF parameters deduced from the REOF:Eu3+ (RE ) La, Gd, and Y) series20 was 7%, which is close to the experimental uncertainty of parameters. One should expect a decrease in the magnitude of the CF parameters over the REOF series from Pr3+ to Dy3+ due to the

The short-range strength parameter shows evolution similar to that of the S parameter as a function of the number of the 4f electrons. The Sk values have a clear discontinuity in the middle of the series (Figure 6). The two-electron CF effect may cause the slight increase and the discontinuity of the CF effect in the REOF series, because it is likely that the one-electron operators Uk would change the sign at the center of the series but the two-electron operators would not.4 If that contribution is not included in the oneelectron CF parameters, there would be a break when crossing the center of the series.7 The addition of the two-electron interactions seems to improve the simulation of the CF splitting for some individual states,45,46 but it increases the number of parameters. The low C3V symmetry of the RE3+ site in REOF does not allow the additional parametrization. Other effects may also cancel the influence of the nuclear charge on the evolution of the CF parameter values. The electrostatic point charge model gives the Bkq parameters as the products of the lattice sum Akq parameters and the radial integrals (section 3.1). For the lighter RE3+ ions, the drastic decrease in the radial integrals26 is not compensated by the slight increase in the Akq values for REOF.20 For the latter half of the RE3+ series, the change in the radial integrals is smoothed out, but the lattice sums Akq continue increasing. This might cause the increase in the CF effect. The energy of the 5s and 5d orbitals may also have additional influence on the CF parameter values. In principle, the lowering of these orbital energies should strengthen the CF effect by increasing the mixing of the opposite parity terms to the 4f wave functions.47 4.4. Electrostatic Point Charge Model Calculations. The theoretical calculations were performed by using the modified PCEM model. The structural information for the calculations was obtained from the Rietveld profile analysis carried out for the pure SmOF. The lattice sums were calculated as described earlier.20 The calculated CF parameters have the following values: B20 ) -734, B40 ) 1812, B43 ) -1436, B60 ) 744, B63 ) 799, and B66 ) 1044. According to the calculations carried out for the REOF:Eu3+ series (RE ) La, Gd, and Y), the effective charge and the deviations of the atomic parameters, especially zO, from the ideal cubic structure have an important effect on the Bkq values.20 The atomic parameters affect mostly the axial Bk0 parameters and thereby the Bkq ratios by increasing the distortion from the ideal fluorite structure. The calculated CF parameter ratios are B40/B43 ) 1.262, B60/B63 ) 0.932, and B60/B66 ) 0.711. The B40/B43 ratio of PCEM is closer to the cubic value than to the experimental one (Table 4). The B60/B63 and B60/B66 values obtained by PCEM differ more from the ideal values than do the experimental ones. The PCEM calculations seem to exaggerate the structural distortions. The PCEM model gives an adequate description of the fourthand sixth-rank parameters since REOF has an ionic lattice. The

Crystal Structure and Optical Spectra of SmOF

J. Phys. Chem., Vol. 100, No. 35, 1996 14743

TABLE 4: Evolution of the Free Ion and CF Parameters in the REOF Seriesa parameter

YOF:Pr3+

NdOF

SmOF

E0 E1 E2 E3 R β γ T2 T3 T4 T6 T7 T8 ζ4f B20 B40 B40/B3c 3 B43 B60 B60/B6c 3 B63 B60/B6c 6 B66 Sd S2 S4 S6 no. of levels σ

9766 4506 21.14 457 20.12 -604 1480

23429 4689 23.17 478 21.62 -625 1880 424 52 55 -282 379 312 870 -144 1839 -1.12 -1643 1005 1.16 866 1.24 812 650 144 2324 1957 122/182e 17

46306(1) 5190(0.2) 25.45(1) 518(0.1) 19.24(2) -562(2) 1841(1) 306(1) [33]b [94]b -248(8) 314(5) 278(4) 1147(1) -200(22) 1786(29) -1.35 -1308(21) 1139(35) 1.54 739(31) 1.41 810(31) 580 200 2405 1924 195/1001e 17

742 -124 1612 -0.71 -2276 1237 1.13 1096 1.31 745 780 124 2379 2245 58/91 15

GdOF:Eu3+

-3 1230 -0.77 -1588 1029 2.39 431 1.31 786 556 3 1905 1633 21/49f 11

GdOF:Tb3+

DyOF

-27 1522 -1.02 -1494 1059 2.01 527 2.16 490 554 27 2135 1469 19/49f 6

55250 6132 30.40 623 18.11 -603 1600 350 [78]b [41]b -360 350 354 1915 57 1717 -1.17 -1272 1204 1.02 556 0.865 640 543 57 2418 1699 153/1001e 17

a The data were obtained for Pr3+, Nd3+, Sm3+, Eu3+, Tb3+, and Dy3+ in YOF,18 NdOF,19 SmOF (this work), GdOF,20 GdOF,21 and DyOF22 matrices, respectively. All values in cm-1. b The T3 and T4 parameter values in brackets were not varied freely but fixed close to earlier values.7 c Ideal cubic ratios: B4/B4 ) -0.837, B6/B6 ) 1.656, and B6/B6 ) 1.579.42 d The CF strength parameter S was defined as follows10 0 6 0 3 0 3

S)

(

1

1

∑ 2k + 1[(B ) 3

k 2 0

k

∑((B )

+2

k 2 q

q>0

)

+ (Sqk)2)]

1/2

where Bkq denotes the real part and Skq denotes the imaginary part of the CF parameters. e Kramers doublets. f Only the 7F term was used for the energy level simulation.

calculated value of the B20 parameter is too large and remains the main problem. In order to resolve this, more sophisticated models may have to be used. All in all, good correlation with phenomenological parameters was achieved, much better than usual. This is a unique example where PCEM works quite well. 5. Conclusions The structure of the stoichiometric SmOF was refined from the X-ray powder diffraction data by the Rietveld refinement method. Good correlation was achieved with a weighted Rwp value of 3.9%. The small deviation of the positional parameters from the ideal ones for the fluorite-type cubic structure indicates that the structure has only slight trigonal distortion. The optical absorption and visible luminescence spectra of the Sm3+ ion in the REOF matrices were measured at selected temperatures between 9 K and room temperature. 195 Stark levels representing 46 SLJ states of the 4f5 configuration were resolved. The energy level scheme was successfully simulated according to the C3V site symmetry by using a phenomenological model involving 14 free ion and six CF parameters which were refined simultaneously. The final rms deviation between the experimental and calculated energy level schemes was 17 cm-1. The CF parameters observed for Sm3+ resemble those obtained for the other RE3+ ions in the REOF:RE3+ system (RE3+ ) Pr, Nd, Eu, Tb, and Dy). The B20 value, close to 0, and the CF parameter ratios B40/B43 and B60/B6q (q ) 3 and 6),

Figure 6. The evolution of the relative strength Sk parameters of the short-range (k ) 6), midrange (k ) 4), and long-range (k ) 2) CF interaction in the REOF series (RE3+ ) Pr, Nd, Sm, Eu, Tb, and Dy).18-22

found close to the ideal cubic values, reflect the cubic pseudosymmetry of the REOF structure. Similar results (except for B20) were obtained by the modified electrostatic point charge (PCEM) calculations using the refined structural data. The CF parameter set for Sm3+ respects those obtained in the previous studies for Pr3+, Nd3+, Eu3+, Tb3+, and Dy3+ in the RE oxyfluoride matrices. The CF effect decreases from Pr3+ to Tb3+ due to the increasing nuclear charge, while beyond Tb3+ the CF effect increases again.

14744 J. Phys. Chem., Vol. 100, No. 35, 1996 Further work is currently in progress with the heavier RE oxyfluorides. Both spectroscopic and structural investigations are to be carried out to obtain more information about the correlation between the crystal structure and CF effect. In order to obtain more exact approximations to the CF effect, more elaborated models may have to be used instead of the PCEM method. In the phenomenological model, the two-electron interaction may have to be taken into account by employing, for example, the orbitally- and/or spin-correlated crystal field scheme. Acknowledgment. The authors (E.S. and J.H.) are indebted to Prof. Jussi Valkonen for the use of the X-ray powder diffraction equipment. E.S. and J.H. are also grateful to the Academy of Finland (project 4966) for financial support. References and Notes (1) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer: Berlin, Germany, 1994; p 25. (2) Tecotzky, M. Luminescence Applications. In 1787-1987 Two Hundred Years of Rare Earths; Gschreidner, K. A., Jr., Capellen, J., Eds.; Rare Earth Information Center: Iowa State University, Ames, IA, 1987; p 17. (3) Hemmila¨, I. A. Applications of Fluorescence in Immunoassay; John Wiley: New York, 1991; p 140. (4) Wybourne, B. G. Spectroscopic Properties of Rare Earths; Interscience: New York, 1965. (5) Jayasankar, C. K.; Richardson, F. S.; Reid, M. F. J. Less-Common Met. 1989, 148, 289. (6) Magno, M. S.; Dieke, G. H. J. Chem. Phys. 1962, 37, 2354. (7) Carnall, W. T.; Goodman, G. L.; Rajnak, K.; Rana, R. S. J. Chem. Phys. 1989, 90, 3443. (8) Lammers, M. J. J.; Blasse, G. Phys. Status Solidi B 1985, 127, 663. (9) Nara, H.; Schlesinger, M. Phys. ReV. B 1971, 3, 58. (10) Chang, N. C.; Gruber, J. B.; Leavitt, R. P.; Morrison, C. A. J. Chem. Phys. 1982, 76, 3877. (11) Veyssie, M.; Dreyfus, B. J. Phys. Chem. Solids 1967, 28, 499. (12) Gru¨nberg, P. Z. Phys. 1969, 225, 376. (13) Reid, M. F.; Richardson, F. S. J. Chem. Phys. 1985, 83, 3831. (14) Babkina, T. V.; Gaiduk, M. I.; Zorina, L. N.; Soshchin, N. P. Opt. Spectrosc. 1974, 37, 401. (15) Pillai, S. M.; Vallabhan, C. P. G. Phys. Status Solidi B 1986, 134, 383. (16) May, P. S.; Reid, M. F.; Richardson, F. S. Mol. Phys. 1987, 61, 1455. (17) Berry, M. T.; Richardson, F. S. Mol. Phys. 1989, 66, 703. (18) Antic-Fidancev, E.; Ho¨lsa¨, J.; Krupa, J.-C.; Lemaitre-Blaise, M.; Porcher, P. Proc. 4th Int. Conf. Solid State Chem.; Dresden, FRG, Sept. 7-9, 1992; p 157. (19) Ho¨lsa¨, J.; Sa¨ilynoja, E.; Ylha¨, P.; Antic-Fidancev, A.; LemaitreBlaise, M.; Porcher, P. Unpublished work. (20) Ho¨lsa¨, J.; Kestila¨, E. J. Chem. Soc., Faraday Trans. 1995, 91, 1503. (21) Antic-Fidancev, E.; Ho¨lsa¨, J.; Krupa, J.-C.; Lemaitre-Blaise, M.; Porcher, P. Int. Conf. Lumin. (ICL’93); Storrs, CT, Aug. 9-13, 1993; p Th4-33.

Ho¨lsa¨ et al. (22) Ho¨lsa¨, J.; Kestila¨, E.; Ylha¨, P.; Sa´ez-Puche, R.; Deren´, P.; Stre¸ k, W.; Porcher, P. J. Phys.: Condens. Matter 1996, 8, 1575. (23) Ho¨lsa¨, J.; Niinisto¨, L. Thermochim. Acta 1980, 37, 155. (24) Leavitt, R. P.; Morrison, C. A.; Wortman, D. E. Report TR-1673, Harry Diamond Laboratories, Adelphi, MD, 1975. (25) Jollet, F.; Noguera, C.; Thormat, N.; Gautier, M.; Duraud, P. Phys. ReV. B 1990, 42, 7587. (26) Freeman, A. J.; Desclaux, J. P. J. Magn. Magn. Mater. 1979, 12, 11. (27) Porcher, P. Computer Programs Reel and Image for the Simulation of dN and fN Configurations InVolVing the Real and Complex Crystal Field Parameters; C.N.R.S.: Meudon, France, 1989. (28) Crosswhite, H.; Crosswhite, H. M.; Judd, B. R. Phys. ReV. 1968, 169, 130. (29) Morrison, C. A.; Leavitt, R. P. Spectroscopic Properties of Triply Ionized Lanthanides in Transparent Host Crystals. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Eds.; North Holland Publishing Company: Amsterdam, The Netherlands, 1982; p 461. (30) Antic-Fidancev, E.; Lemaitre-Blaise, M.; Porcher, P.; Ho¨lsa¨, J. Phys. Status Solidi B 1992, 130, K147. (31) Sakthivel, A.; Young, R. A. Program DBWS-9006PC for RietVeld Analysis of X-ray and Neutron Powder Diffraction Patterns; Georgia Institute of Technology: Atlanta, GA, 1991. (32) Mann, A. W.; Bevan, D. J. Acta Crystallogr. Sect. B 1970, 26, 2129. (33) Henry, N. F. M., Lonsdale, K., Eds. International Tables for Crystallography; Kynoch Press: Birmingham, U.K., 1969; Vol. 1, p 273. (34) Zachariasen, W. H. Acta Crystallogr. 1951, 4, 231. (35) Prather, J. L. Monograph 19, U.S., National Bureau of Standards, Washington, DC, 1961. (36) Ho¨lsa¨, J.; Piriou, B.; Ra¨sa¨nen, M. Spectrochim. Acta A 1993, 49, 465. (37) Antic-Fidancev, E.; Lemaitre-Blaise, M.; Caro, P. New J. Chem. 1987, 11, 467. (38) Imbusch, G. F. Advances in the Characterization of Excited States of Luminescent ions in Solids. In Optical Properties of Excited States in Solids; Di Bartolo, B., Ed.; NATO ASI Series, Ser. B: Physics, Vol. 301; Plenum Press: London, U.K., 1989; p 207. (39) Meijerink, A.; de Mello Donega´, C.; Ellens, A.; Sytsma, J.; Blasse, G. J. Lumin. 1994, 58, 26. (40) Ho¨lsa¨, J.; Sa¨ilynoja, E. Unpublished work. (41) Stewart, B. The Ligand Polarisation Model for d-d and f-f Intensities. In Vibronic Processes in Inorganic Chemistry; Flint, C. D., Ed.; NATO ASI Series, Ser. C: Mathematical and Physical Series, Vol. 288; Plenum Press: London, U.K., 1989; p 327. (42) Caro, P. Structure Electronique des Ele´ ments de Transition; PUF: Paris, France, 1976; p 150. (43) Metcalf, D. H.; Hopkins, T. A.; Richardson, F. S. Inorg. Chem. 1995, 34, 4868-4878. (44) Hopkins, T. A.; Metcalf, D. H.; Richardson, F. S. Inorg. Chem. 1995, 34, 4879-4887. (45) Yeung, Y. T.; Newman, D. J. J. Chem. Phys. 1987, 86, 6717. (46) Reid, M. F. J. Chem. Phys. 1987, 87, 2875. (47) Malta, O. 1994. Private communication.

JP960348+