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Concentration enhancement of the vibronic transitions of the Pr3+ ion
1. Phys. Chum Solids Vol. 54. No. 3, pp. 293400, Printed in Great Britain.
1993
0%?2-3697/93 $6.00 + 0.00 0 1993 Fwgamon F’ress Ltd
CONCENTRATION ENHANCEMENT OF THE VIBRONIC TRANSITIONS OF THE Pr3+ ION C. DE MELLO DONEG.& A. ELLENS, A. MEIJERINK and G. BLASSE Debye Research Institute, University of Utrecht, P.O. Box 8OCO0,3508TA Utrecht, The Netherlands (Received 21 September 1992; accepted 1 November 1992)
Abstract-The effects of the Pr3+ concentration on the excitation and emission spectra of Pr3+ in (La, _Jr.J203, (RE, _XPrX),02S (RE = Y, La) (x Q 0.2), and LiY, _,Pry4 (x 6 0.1) are investigated. Concentration enhancement of vibronic transitions and of certain zero-phonon transitions is observed for the excitation spectrum of Pr3+ m (La, _XPrX),03 and (RE, _XPrr)202S. These henomena can be ascribed to superexchange interaction between Pr 3+ ions over distances of about 10 1. Keywordr : Pr3+ spectroscopy, vibronics, concentration enhancement, superexchange interaction.
1. INTRODUCTION Recently one of us has reviewed the studies on vibronic transitions in the rare earth ions [l]. For several rare earth ions (Eu3+ [2], Gd3+ [3], Pti+ [4]) the intensity of these transitions has been shown to depend strongly on the host lattice. This host lattice dependence can be explained in terms of existing theories [5-71. The increase of the vibronic coupling strength is attributed to an increase of several related effects: covalency, the polarisability of the ligands, and the opposite-parity configuration admixing. Moreover, the vibronic intensity has been observed to depend on the concentration of the rare earth ions [l]. Hoshina et al. [8,9] observed concentration enhancement of the vibronic intensity of excitation transitions on the Eu3+ ion in the system (Y, _ xEu,), 0, S. Auzel et al. [lo] reported a doubling of the vibronic coupling strength going from x = 0.01 tox = l.Oin thesystem[(C,H,)&Y,_,Eu,(NCS),. Van Vliet and Blasse [l l] observed an increase of a factor four of the vibronic intensities in the excitation spectrum of Eu3+ in Na, Gd, _ xEu,(WO,), from x = 0.01 to x = 1.0. The concentration enhancement of the vibronic coupling was also observed for the 6P7,z + 8S7,2 emission transition of Gd3+ in (Y,_,Gd,)rOrS [12]. For Pr’+ such effects were reported by Strek and Galczynski [13] in LiLa,_,Pr,P,On and by Donega and Blasse [14] in (La,-,Pr,)@,. Our previous results on (La, _ xPrx)2 0, [ 141suggest that the concentration enhancement of the vibronic intensity is related to a high degree of covalency and the presence of opposite-parity configurations at low energies. In order to obtain insight in mechanisms responsible for the concentration dependence of
the vibronics, we have investigated the systems LiY, _xPr,F, and (La, _+Pr,),O, over a wide concentration range. These two systems were chosen because of the large differences in the degree of covalency, the position of the 4f 5d configuration and the vibronic coupling strength [4]. For comparison a few samples of (RE, _zPr,),02S (RE = Y, La) were also investigated. 2. EXPERIMENTAL
Powder samples of (La,_,Pr,),O, (x = 1 x 10m4-0.2) were prepared using procedures described in [3]. Firing was carried out in a N,:H2 (3: 1) atmosphere. The (RE,_,Pr,)202S (RE = Y, La; x = 0.001 and 0.01) powder samples were kindly supplied by Dr C. W. Struck. La203, Pr,O, and RE,02S are isostructural. They have P3m 1 (D:,) space group symmetry and the site symmetry is C,, for the RE3+ ions [15]. La,O, has a unit cell with a,,= 3.9373A and c,=6.1299A [15]. The powder samples of LiY, _xPr, F4 (x = 1 x 1O-4-O. 1) were prepared as described in [16]. LiYF, has the inverse scheelite structure, space group 14, /a(Ci*) [17]. The samples were checked by X-ray powder diffraction analysis and found to be single phase. Diffuse reflectance spectroscopy showed that the samples do not contain Pr4+ or other optical impurities. The low resolution luminescence spectra of all samples were measured by using a SPEX 1680 Spectrofluorometer with 0.22 m double monochromators and a 450 W Xe Lamp. The instrument is equipped with an Oxford helium flow cryostat. The spectra are corrected for the instrumental response. The excitation spectra of Pr’+ in LiYF, was also measured by using a Perkin-Elmer MPF3L 293
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Spectrofluorometer with a deuterium lamp as an excitation source. High resolution spectra and decay measurements were performed with an excimer dye-laser set-up, equipped with an Oxford Instruments liquid helium cryostat. The excitation of the samples is performed by a Lambda Physik LPD-3002 dye laser pumped by a LPX-100 excimer laser. The Coumarin 102 (LC 4800) dye is used. The dye laser output has a pulse width of 20 ns and a bandwidth of 0.18 cm-‘. The emission monochromator is a SPEX 1704 (focal length 1 m). Spectra are obtained by measuring the photocurrent of the cooled photomultiplier tube (RCA C31034) by a Philips DC-micrometer PM 2436. The excitation spectra are corrected for the variation of the dye laser output with wavelength. A digital oscilloscope Tektronix 2440 is used to obtain decay curves. 3.RESULTS Figure 1 shows the low resolution excitation spectrum of the 3Po+3F2 emission of Pti+ in La,O, for two different Pr3+ concentrations. The spectra consist of several sharp lines in the region 440-500 mn, corresponding to transitions within the 4f 2 configuration (‘Z& + 3Po,l,2and iZs). In addition to the sharp zero-phonon lines intense vibronic features are observed. Concentration enhancement is observed for several lines. Most of the increase of intensity can be attributed to vibronic transitions, but some
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+ ‘P2 and 3H$‘) + 3P,, ‘Z6 electronic lines in the 3Z-Z$i) region are also affected. The superscript (1) indicates the lowest crystal-field component. At 4.2 K the ‘H$‘) + ‘PO transition consists of only one zerophonon line because the ‘PO level is non-degenerate and the lowest crystal-field component of the 3Hq multiplet is the only one populated. Therefore, vibronic lines can be easily identified in this transition. This is a particularly suitable situation to investigate the effect of the Pr” concentration on the vibronic intensities. The integrated intensity ratio R of the vibronic features and the zero-phonon line can be determined and compared for samples with different concentrations of Pr’+. The vibronic part of the ‘Hi’) + ‘P,, high resolution excitation spectrum of Pr3+ in La,O, is shown in Fig. 2. The 3H!,‘) -+ ‘P,, ‘Z6 lines start at about 430 cn-’ higher energy than the ‘H&l)-+ ‘PO electronic line. This imposes 400 cm-’ as an upper limit to the frequency of the vibronics which can be included in the ratio R. This fact does not influence the comparison between different samples, because the positions of the zero-phonon and vibronic lines are not observably affected by the P?+ concentration and the ratio R includes the same lines for all cases. The position of the vibronic lines relative to the zero-phonon line is presented in Table 1. This table will be discussed later, since it also includes data from the high resolution emission spectrum. A logarithmic plot of the vibronic ratio R for the ‘Hi’) + ‘P,, transition of Pr3+ in (La, _IPr,)203 vs the
460
470
Wavelength
Fig. 1. Intraconfigurational excitation spectrum (‘If&‘)+ ‘P,,,,z and I&) of the ‘P,, + 2F2emission of Pr3+ in (La, _,TPrX)20, at 4.2 K, for x = 5 x low4 (dashed line) and x = 1 x IO-* (solid line). The vibronic side band of the 3H$‘)-+ ‘PO transition is indicated by v.
Vibronic transitions of the Pr’+ ion
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ZPL
Energy (cmd) Fig. 2. Vibronic part of the 3H$‘)-+ ‘POexcitation spectrum of the ‘PO+ 3F2emission of Pr3+ in La,O,
at 4.2 K. The ‘H$‘)+ ‘POzero-phonon line is at 20,281cn-‘.
Prj+ concentration x is shown in Fig. 3. A clear concentration enhancement of the vibronic coupling is observed. The data were obtained from high resolution spectra. The position of the zero-phonon line, 20,281 cm-‘, does not vary with concentration. The intensity distribution between the several vibronic lines is not affected by the increase of the concentration. The uncertainty in R is estimated as 20% for Pti+ concentrations equal to or lower than 0.1 mol%, and 10% for higher concentrations. The larger uncer-
tainty for the lower concentrations is due to the low emission intensity and to the relatively weak vibronic lines. These facts lead to a larger error in the integrated zero-phonon and vibronic intensities and in the background correction.
Table 1. Relative positions of the vibronic lines in the 3H$‘)+ ‘P,, excitation spectrum of the 3P,, emission (Fig. 2) and in the ‘P,, + ‘F, emission spectrum (Fig. 6) of Pr’+ in La,O, at 4.2 K. Vibrational data of Pr203 are included for comparison. All values are in cm-’ Vibrational frequency? [ 151 105 (R)
187 (R) 260 OR) 386 (IR) 406, 413 (R) 450 (IR)
Vibronic positionS Excitation Emission 70 (?) 1005 116§ 150 180 (?) 203 270 310 340 365 419 ?
tR = Raman; IR = infrared. $Relative to electronic origin. QSplit by 5 cm-‘.
80 115 150 205 250 310 350 420 508
Fig. 3. The integrated intensity ratio R of the vibronic part and the zero-phonon line for the ‘Hi’) + “P,, excitation transition of Pr3+ in (La, _,rPr,T),03 at 4.2 K as a function of the log of the concentration x. The solid line is a fit to eqn (1).
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\
i i '\
Wavekqth
\ ‘:.
\ :: \: \i \
(nm)
Fig. 4. The 4fz -* 4f 5d excitation spectra of the 3Po+ ‘F, emission of Pr3+ in (La, _XPrX)203at 4.2 K, for several Pr’+ concentrations x: 5 x 10m4(dot-dashed line), 1 x low2 (dashed line), 2 x IO-* (dotted line) and 5 x lo-* (solid line).
In addition to the sharp lines from transitions within the 4f2 configuration, two broad bands with maxima at 294 mn and 240 nm are observed in the excitation spectrum of Pr3+ f in La2 0,. These bands were assigned to the 4f2 + 4f Sd transition of Pr3+ and to the excitation via the host lattice absorption state, respectively. Figure 4 shows that the 4f Sd excitation bands shift to lower energies as the Pr3+ concentration increases. A constant value seems to be
reached at about 5 mol%. No shift is observed for the band at 240 nm. Figure 5 presents the emission spectrum of Pti’ in La,O, at 4.2 K under 4f2 + 4f 5d excitation. The Pr3+ concentration is 0.05 mol%. The emission lines correspond to the radiative transitions ‘PO + 3Hq,5,6,3F2,3,4and ‘D2 + 3Hq (Fig. 5). The ‘D, emission is hardly detectable under ‘P, excitation. This is expected since the energy gap between the ‘PO
Wmdength
Fig. 5. Emission spectrum of La,,,Pr,,,O,
Mm)
at 4.2 K upon 4f * + 4f Sd excitation.
Vibronic transitions of the Pr3+ ion
and ‘D, levels is 3900 cm-’ and the maximum energy of the lattice phonons in La,O, is 500 cm-‘. As a consequence the ‘PO+ ‘D, multiphonon relaxation is very inefficient since it requires at least eight phonons [ 181.The higher intensity of the ID, emission under 4f 2 + 4f 5d or host lattice excitation is due to the direct feeding of the ID, level [19]. The spectrum is essentially
the same
for all concentrations,
but
the ‘D, emission shows a pronounced concentration quenching. For concentrations of Pr3+ above 1 mol%, the ID2 emission is not observed. A similar phenomenon was observed before by e.g. Domauf and Heber [20] in La, _XPrXP,0,4, where it was attributed to cross relaxation between adjacent Pr3+ ions. Cross-relaxation is much more efficient for the ‘D, level than for the 3P0 level, because the energy mismatch for the cross-relaxation process for the latter level is much larger than for the former [20]. Vibronic transitions were also investigated in the emission spectrum. The 3P0 -+ 3H4 emission consists of several electronic lines due to the crystal-field splitting of the 3H4 level. This makes the analysis of the vibronic intensity rather complicated. The effect of the Pr3+ concentration on the vibronic intensity of the ‘PO + ‘H$‘) emission transition cannot be investigated due to increasing reabsorption of this emission line with increasing concentration of Pr3+ ions. The 3P0 + 3F2 emission is more suitable, because no reabsorption occurs and the crystal field splitting of the 3F2 level is small, allowing the observation of vibronic lines with frequencies higher than 100 cm-‘. Figure 6 shows the vibronic part of this transition at 4.2 K. In contradiction to the vibronic transitions in the excitation spectrum, these in the emission spectra do not depend on the Pr3+ concentration: the integrated intensity ratio R is 0.08 for all concentrations investigated.
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A comparison between the positions of the emission and excitation vibronics and the vibrational data of La,O, is presented in Table 1. The agreement between the emission and the excitation vibronics is good if one considers that the frequencies of the former are less accurate than those of the latter. In spite of the large difference between the R values, the vibronic transition probabilities for the 3P0 + 3F2 and ‘H$‘) + 3Po transitions are of the same order of magnitude, viz. 6 x lo3 and 2 x lo’, respectively. This is due to the fact that the zero-phonon transition probability for the former transition is much larger than that for the latter, 8.5 x 104s-’ and 2.3 x 10’ s-‘, respectively. These transition probabilities were determined as described in [4]. The agreement with the vibrational data is fairly good. Most of the vibronic lines can be assigned to coupling with i.r.- or Raman-active vibrational modes, but it is not possible to assign all of the observed vibronic features. Some of the vibronic lines seem to be split. A good example is the line centred at about 108 cm-’ which can be considered to be split into twin peaks at 100 and 116 cm-‘. Richardson et al. [21] reported the splitting of v3and v,-vibronic lines in the optical spectra of several rare-earth ions in elpasolites (Cs,NaRECl,). The v,-vibronic lines generally appear as doublets split by lo-15 cm-‘, and the v,-lines appear either as asymmetrically shaped singlets or as doublets split by 2-lOcm-‘. The splitting of these vibronics is attributed to lattice dynamics effects involving transverse (TO) and longitudinal (LO) optical phonon modes [21]. Only transverse lattice vibrational modes appear in the i.r. spectra, whereas both LO and TO modes contribute to vibronic spectra [21,22]. Similar effects may be responsible for the fact that we observe more vibronic lines than the vibrational
Energy (cm*)
Fig. 6. Vibronic part of the 3P0 + 3F2 emission spectrum of Pr ‘+ in La,O, at 4.2 K. The onset of the zero-phonon transition appears on the extreme right side of the spectrum. The zero-phonon line is at 14,998 cm-‘.
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modes appearing in infrared and Raman spectra. A more detailed discussion of the intensity distribution between the vibronic lines lies beyond the scope of this paper. The effects of increasing concentration on the optical spectra of Pr3+ in RE,OrS (RE = Y, La) are similar to those observed for La,O,: Pr3+. The integrated intensity ratio R for the 3Hf) + ‘POexcitation transition increases by a factor of two when the Pr3+ concentration increases from 0.1 to 1.0 mol%. The intensity of the ‘D, emission decreases and the 4f * --t 4f Sd excitation transition shifts to lower energies with increasing Pr3+ concentration. The magnitude of these effects is similar for both host lattices. We turn now to the LiY, _XPr,F, samples. The intraconfigurational excitation spectrum and ‘Ze) of the ‘P,, -+ ‘H6 emission of (‘Z& + ‘PO 12 Pr3’ in LiYF, also shows vibronic side bands. The vibronic coupling is much weaker for Pr3+ in LiYF, than in La,O,. The variation of the vibronic intensity of Pr3+ with the host lattice has been discussed recently [4]. The vibronic part of the 3H$‘) -+ ‘PO excitation spectrum of Pr3+ in LiYF, has been discussed in [4]. The agreement between the vibronic spectra and the vibrational data of LiYF, is good [4]. In addition to the vibronic lines, several satellite lines were observed in the high resolution ‘Hi’) + ‘PO excitation spectrum of LiY 1_ XPrXF,. Similar lines have been observed by Barthem et al. [23] in LiY, _XPr,F, crystals. They are attributed to excitation transitions of pairs of Pr3+ ions. The main line is at 20,873 cm-’ in all investigated samples but the position of the satellites vary. Satellites are observed at 2.7 cm-’ higher energy (line A) and at 1.3 cm-’ (B), 2.6 cm-’ (C), 4.0 cm-’ (D) and 7.2 cm-’ (E) lower energies from the main line. Their relative intensities increase with x. Figure 7 shows the high resolution excitation spectrum of the LiY,,,, Pr,.,, F, sample at 4.2 K in the 3H$‘) + 3P,, region while monitoring fluorescence from the main site (Fig. 7a) and from one of the pair sites (Fig. 7b). The integrated intensity ratio R is 0.2 for the main site and 0.3 for the pair site. Within the experimental error the ratio R is the same for both sites. The ratio R for the ‘H$,‘) -+ 3Poexcitation transition is calculated from high-resolution excitation spectra monitoring emission from the main site. The ratio R is 0.2 for all the investigated concentrations. The zero-phonon line intensity includes the pair lines since the vibronic side band contains contributions from all sites. The decay time is 48 ps up to 10 mol% of Pr3+. The position of the 4f * --) 4f 5d excitation band does not depend on the Pr3+ concentration.
(b)
4..,‘.......
2mm
.-f
. ..._
2omo
,‘.......,,,
2aem
202m
Enargy (cm-3
Fig. 7. High resolution ‘Hi’) + ‘POexcitation spectrum of the ‘PO+ ‘H6 emission of LiYF,:O.S%Pr’+ at 4.2 K using site selective techniques: (a) monitoring main site (I), emission wavelength at 604.35nm and, (b) monitoring site D, emission wavelength at 607.83nm.
It has its maximum at about 50,0OOcm-’ for all samples. At 4.2 K the emission spectrum of Pr3+ in LiYF4 consists of transitions from the ‘P,, level (3P, -, 3H‘,, 5, 6, 3F2,3,4) only. The spectrum is the same under ‘PJ or 4f 5d excitation and it does not depend upon the concentration of Pr3+ ions. 4. DISCUSSION Before discussing interaction mechanisms leading to the concentration enhancement of the vibronic excitation transitions of Pr3+ in La,O,, we want to get an estimate of the distance over which the interaction between Pr3+ ion operates. We assume that any set of neighbouring Pr3+ ions within a distance shorter than r, has approximately the same vibronic intensity ratio. This distance r,, here called the critical distance, indicates the largest distance over which the interaction has observable consequences. The observed intensity ratio R can then be described by: R = &so x Go + &~,a x CMR,
(1)
where R,, is the vibronic intensity ratio for the isolated Pr3+ ions, C,, is the concentration of isolated Pr3+ ions R PA,Ris the vibronic intensity ratio for a pair of interacting Pr3+ ions and CPA,, is the concentration of these pairs in the host lattice. The concentration of pairs is given by: CP**a= 1 - cno = 1 - (1 - C)N
(2)
Vibronic transitions of the Pr’+ ion where C is the total concentration of Pr” ions and N is the number of sites within the critical distance r, for interaction between two Pr’+ ions, which is given by: N
=
Wd v ’
where V is the volume of the unit cell. The solid line in Fig. 3 is the best fit to eqn (l), taking the R value for the lowest x, 1.1, as RI,, and the R value for the highest x, 2.5, as RPAIR . This yields a critical distance r, of 9.5 A. This large value of r, shows that the mechanism responsible for the concentration enhancement of the vibronics involves an interaction between Pr’+ ions over some 10 A. Figure 3 shows also that there is not much sense in refining this analysis by taking into account a more refined distance dependence of the interaction, because the experimental data are rather inaccurate for the reasons mentioned above. The interaction between the Pr’+ ions obviously occurs over long distances and involves the intermediary between the interacting ions. In view of the long range of the interaction involved in the concentration enhancement of the vibronic coupling, direct 4f -4f interaction cannot be important. Danielmeyer [24] proposed a long-range interaction model based on the overlap between the 4f and 5d wavefunctions of neighbouring rare earth ions in order to account for the properties of stoichiometric rare earth laser materials. A non-vanishing overlap of the 4f and 5d wavefunctions of two Pr3+ ions, which may be increased by admixing host lattice wavefunctions, can lead to the observed interaction range, since the 4f wavefunction drops to 1% of the maximum amplitude at 2.5 A, and the 5d wavefunction extends to 8 A [24]. The shift of the 4f 5d configuration to lower energies can be seen as evidence that the 5d wavefunctions are involved in the interaction. Recently further support for 4f interaction via hybridization with 5d and 6s orbitals has been established by Gschneider [25]. His arguments originate from different types of compounds and different physical properties, and are based on the melting points of rare earth (RE) metals and alloys, on the hydrogen solid solubility in RE metals and on thermodynamic data of RE compounds (free energies of formation and stability constants of RE(EDTA) complexes). The 4f hybridization with the 5d and 6s orbitals of the lanthanide and non-rare earth elements in a compound or alloy is shown to be maximum for the lighter lanthanides (La-Nd). The 4f interaction is expected to be more effective in La,O, than in LiYF,, since the former compound
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is more covalent than the latter. This implies that the interaction between Pr’+ ions via wavefunction overlap is much weaker in LiYF,:PIJ+ than in The absence of observable concenLa,O,:PI3+. tration effects in the excitation and emission spectra of Pr’+ in LiYF, can be seen as a consequence of the weak interaction between Pr’+ ions in this lattice. In this connection we note that superexchange interaction via the 2p orbitals of the F- ions has been proposed as an interaction mechanism between but only for associated Pr’+ ions in LiY,_,Pr,F,, distances shorter than 4.8 A [23]. We will now discuss how these interactions can affect the vibronic intensity. The vibronic transition probably is given by the summation of two contributions, usually indicated as the M and the A processes [l]. The former describes vibronically induced forced electric-dipole transition. The latter describes vibronics induced by a shift in the equilibrium position of the configurational coordinates of the excited state relative to the ground state [l]. If the wavefunction overlap is increased, the electron-phonon coupling strength (Huang-Rhys parameter S) is enhanced [26]. Therefore the A process contribution to the vibronic intensity becomes larger, since it is proportional to S [l]. Additionally, these interactions lead to an increase of the degree of covalency and to a shift of the 4f 5d configuration to lower energies. The latter fact causes an increase of the opposite-parity configuration admixing. It has been shown that the increase of covalency and opposite-parity configuration admixing increases the vibronic coupling strength due to both M and A processes [ 1,4]. However the ratio R does not depend on the position of the opposite-parity configuration [I, 271. Therefore the observed concentration enhancement of the ratio R has to be interpreted as a covalency effect. Consistently, the intensity of the vibronic transitions of Pr’+ in (RE, _,Pr,),O,S (RE = Y, La) also increase with x. The concentration enhancement is observed for the excitation vibronics only. The same phenomena have been observed for Ed+ in RE202S [8,9] and in NaSGd(WO,), [ll]. Hoshina et al. [8,9] suggested that superexchange interaction operates between two associated Eu’ + ions via the low energy chargetransfer state (‘CTS), which mixes with the ‘Fground state. Thus superexchange interaction operates between associated Eu’+ ions only when they are both in ‘F states, but not when one ion is in the ‘D excited state, because of the energy mismatch between the two Eu’+ ions [9]. Therefore concentration enhancement is observed only for the ‘F. + sD, excitation vibronics [8,9]. A similar explanation can hold for P?+. Here the superexchange interaction takes place
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via the 4f 5d state and is only effective when both ions are in the 3H4 ground state. In addition to the enhancement of the excitation vibronics, it appears that also the intensity of some pure electronic lines (3H$” -+ ‘Pz and ‘Hi’) + ‘Ze, see Fig. 1) increases with the concentration. This can be interpreted as an example of hypersensitivity, since A.Z = 2 [28]. Recently Garcia and Faucher [29] calculated the 4f -+ 4f oscillator strengths for dipolar electric transitions in PrCl,. Their results surprisingly show that the intensity of the absorption transitions 3H4 -+ 3P2,‘Ie and ‘D, can be explained using 4f 2 + 4f 5d configuration mixing, but that the 3H4 -+ 3P0,, transitions have no relation with the 4f2 + 4f 5d process. This fact implies that in our model the 3H4 -+ ‘P2 ,I& transitions should also show concentration enhancement, in good agreement with experiment. 5. CONCLUSIONS The intensities of the vibronic transitions and of the 3H$‘) + ‘P2 and 3H!,‘) -+ ‘Ze electronic transitions in the excitation spectrum of Pr’+ in La,O, and in RE202S (RE = La, Y) increase with increasing concentration of Pr3+ ions. This phenomenon can be ascribed to a superexchange interaction between Pr3+ ions. The interactions are assumed to take place via the overlap of the 4f and 5d wavefunctions of two Pr3+ ions over distances of about 10 A. The emission vibronics do not depend on the Pr3+ concentration. This fact suggests that the interaction between Pr3+ ions is effective only when the ions are both in the same state. The absence of concentration enhancement in LiYF,:Pr3+ shows that these effects require a low energy opposite-parity configuration and a high degree of covalency. Acknowledgements-C, M. Donega wishes to thank the University of Utrecht for a temporary appointment and the CNPq (Conselho National de Desenvolvimento Cientifico e Tecnolbgico-Brazil) for a study grant.
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0%?2-3697/93 $6.00 + 0.00 0 1993 Fwgamon F’ress Ltd
CONCENTRATION ENHANCEMENT OF THE VIBRONIC TRANSITIONS OF THE Pr3+ ION C. DE MELLO DONEG.& A. ELLENS, A. MEIJERINK and G. BLASSE Debye Research Institute, University of Utrecht, P.O. Box 8OCO0,3508TA Utrecht, The Netherlands (Received 21 September 1992; accepted 1 November 1992)
Abstract-The effects of the Pr3+ concentration on the excitation and emission spectra of Pr3+ in (La, _Jr.J203, (RE, _XPrX),02S (RE = Y, La) (x Q 0.2), and LiY, _,Pry4 (x 6 0.1) are investigated. Concentration enhancement of vibronic transitions and of certain zero-phonon transitions is observed for the excitation spectrum of Pr3+ m (La, _XPrX),03 and (RE, _XPrr)202S. These henomena can be ascribed to superexchange interaction between Pr 3+ ions over distances of about 10 1. Keywordr : Pr3+ spectroscopy, vibronics, concentration enhancement, superexchange interaction.
1. INTRODUCTION Recently one of us has reviewed the studies on vibronic transitions in the rare earth ions [l]. For several rare earth ions (Eu3+ [2], Gd3+ [3], Pti+ [4]) the intensity of these transitions has been shown to depend strongly on the host lattice. This host lattice dependence can be explained in terms of existing theories [5-71. The increase of the vibronic coupling strength is attributed to an increase of several related effects: covalency, the polarisability of the ligands, and the opposite-parity configuration admixing. Moreover, the vibronic intensity has been observed to depend on the concentration of the rare earth ions [l]. Hoshina et al. [8,9] observed concentration enhancement of the vibronic intensity of excitation transitions on the Eu3+ ion in the system (Y, _ xEu,), 0, S. Auzel et al. [lo] reported a doubling of the vibronic coupling strength going from x = 0.01 tox = l.Oin thesystem[(C,H,)&Y,_,Eu,(NCS),. Van Vliet and Blasse [l l] observed an increase of a factor four of the vibronic intensities in the excitation spectrum of Eu3+ in Na, Gd, _ xEu,(WO,), from x = 0.01 to x = 1.0. The concentration enhancement of the vibronic coupling was also observed for the 6P7,z + 8S7,2 emission transition of Gd3+ in (Y,_,Gd,)rOrS [12]. For Pr’+ such effects were reported by Strek and Galczynski [13] in LiLa,_,Pr,P,On and by Donega and Blasse [14] in (La,-,Pr,)@,. Our previous results on (La, _ xPrx)2 0, [ 141suggest that the concentration enhancement of the vibronic intensity is related to a high degree of covalency and the presence of opposite-parity configurations at low energies. In order to obtain insight in mechanisms responsible for the concentration dependence of
the vibronics, we have investigated the systems LiY, _xPr,F, and (La, _+Pr,),O, over a wide concentration range. These two systems were chosen because of the large differences in the degree of covalency, the position of the 4f 5d configuration and the vibronic coupling strength [4]. For comparison a few samples of (RE, _zPr,),02S (RE = Y, La) were also investigated. 2. EXPERIMENTAL
Powder samples of (La,_,Pr,),O, (x = 1 x 10m4-0.2) were prepared using procedures described in [3]. Firing was carried out in a N,:H2 (3: 1) atmosphere. The (RE,_,Pr,)202S (RE = Y, La; x = 0.001 and 0.01) powder samples were kindly supplied by Dr C. W. Struck. La203, Pr,O, and RE,02S are isostructural. They have P3m 1 (D:,) space group symmetry and the site symmetry is C,, for the RE3+ ions [15]. La,O, has a unit cell with a,,= 3.9373A and c,=6.1299A [15]. The powder samples of LiY, _xPr, F4 (x = 1 x 1O-4-O. 1) were prepared as described in [16]. LiYF, has the inverse scheelite structure, space group 14, /a(Ci*) [17]. The samples were checked by X-ray powder diffraction analysis and found to be single phase. Diffuse reflectance spectroscopy showed that the samples do not contain Pr4+ or other optical impurities. The low resolution luminescence spectra of all samples were measured by using a SPEX 1680 Spectrofluorometer with 0.22 m double monochromators and a 450 W Xe Lamp. The instrument is equipped with an Oxford helium flow cryostat. The spectra are corrected for the instrumental response. The excitation spectra of Pr’+ in LiYF, was also measured by using a Perkin-Elmer MPF3L 293
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Spectrofluorometer with a deuterium lamp as an excitation source. High resolution spectra and decay measurements were performed with an excimer dye-laser set-up, equipped with an Oxford Instruments liquid helium cryostat. The excitation of the samples is performed by a Lambda Physik LPD-3002 dye laser pumped by a LPX-100 excimer laser. The Coumarin 102 (LC 4800) dye is used. The dye laser output has a pulse width of 20 ns and a bandwidth of 0.18 cm-‘. The emission monochromator is a SPEX 1704 (focal length 1 m). Spectra are obtained by measuring the photocurrent of the cooled photomultiplier tube (RCA C31034) by a Philips DC-micrometer PM 2436. The excitation spectra are corrected for the variation of the dye laser output with wavelength. A digital oscilloscope Tektronix 2440 is used to obtain decay curves. 3.RESULTS Figure 1 shows the low resolution excitation spectrum of the 3Po+3F2 emission of Pti+ in La,O, for two different Pr3+ concentrations. The spectra consist of several sharp lines in the region 440-500 mn, corresponding to transitions within the 4f 2 configuration (‘Z& + 3Po,l,2and iZs). In addition to the sharp zero-phonon lines intense vibronic features are observed. Concentration enhancement is observed for several lines. Most of the increase of intensity can be attributed to vibronic transitions, but some
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+ ‘P2 and 3H$‘) + 3P,, ‘Z6 electronic lines in the 3Z-Z$i) region are also affected. The superscript (1) indicates the lowest crystal-field component. At 4.2 K the ‘H$‘) + ‘PO transition consists of only one zerophonon line because the ‘PO level is non-degenerate and the lowest crystal-field component of the 3Hq multiplet is the only one populated. Therefore, vibronic lines can be easily identified in this transition. This is a particularly suitable situation to investigate the effect of the Pr” concentration on the vibronic intensities. The integrated intensity ratio R of the vibronic features and the zero-phonon line can be determined and compared for samples with different concentrations of Pr’+. The vibronic part of the ‘Hi’) + ‘P,, high resolution excitation spectrum of Pr3+ in La,O, is shown in Fig. 2. The 3H!,‘) -+ ‘P,, ‘Z6 lines start at about 430 cn-’ higher energy than the ‘H&l)-+ ‘PO electronic line. This imposes 400 cm-’ as an upper limit to the frequency of the vibronics which can be included in the ratio R. This fact does not influence the comparison between different samples, because the positions of the zero-phonon and vibronic lines are not observably affected by the P?+ concentration and the ratio R includes the same lines for all cases. The position of the vibronic lines relative to the zero-phonon line is presented in Table 1. This table will be discussed later, since it also includes data from the high resolution emission spectrum. A logarithmic plot of the vibronic ratio R for the ‘Hi’) + ‘P,, transition of Pr3+ in (La, _IPr,)203 vs the
460
470
Wavelength
Fig. 1. Intraconfigurational excitation spectrum (‘If&‘)+ ‘P,,,,z and I&) of the ‘P,, + 2F2emission of Pr3+ in (La, _,TPrX)20, at 4.2 K, for x = 5 x low4 (dashed line) and x = 1 x IO-* (solid line). The vibronic side band of the 3H$‘)-+ ‘PO transition is indicated by v.
Vibronic transitions of the Pr’+ ion
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ZPL
Energy (cmd) Fig. 2. Vibronic part of the 3H$‘)-+ ‘POexcitation spectrum of the ‘PO+ 3F2emission of Pr3+ in La,O,
at 4.2 K. The ‘H$‘)+ ‘POzero-phonon line is at 20,281cn-‘.
Prj+ concentration x is shown in Fig. 3. A clear concentration enhancement of the vibronic coupling is observed. The data were obtained from high resolution spectra. The position of the zero-phonon line, 20,281 cm-‘, does not vary with concentration. The intensity distribution between the several vibronic lines is not affected by the increase of the concentration. The uncertainty in R is estimated as 20% for Pti+ concentrations equal to or lower than 0.1 mol%, and 10% for higher concentrations. The larger uncer-
tainty for the lower concentrations is due to the low emission intensity and to the relatively weak vibronic lines. These facts lead to a larger error in the integrated zero-phonon and vibronic intensities and in the background correction.
Table 1. Relative positions of the vibronic lines in the 3H$‘)+ ‘P,, excitation spectrum of the 3P,, emission (Fig. 2) and in the ‘P,, + ‘F, emission spectrum (Fig. 6) of Pr’+ in La,O, at 4.2 K. Vibrational data of Pr203 are included for comparison. All values are in cm-’ Vibrational frequency? [ 151 105 (R)
187 (R) 260 OR) 386 (IR) 406, 413 (R) 450 (IR)
Vibronic positionS Excitation Emission 70 (?) 1005 116§ 150 180 (?) 203 270 310 340 365 419 ?
tR = Raman; IR = infrared. $Relative to electronic origin. QSplit by 5 cm-‘.
80 115 150 205 250 310 350 420 508
Fig. 3. The integrated intensity ratio R of the vibronic part and the zero-phonon line for the ‘Hi’) + “P,, excitation transition of Pr3+ in (La, _,rPr,T),03 at 4.2 K as a function of the log of the concentration x. The solid line is a fit to eqn (1).
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\
i i '\
Wavekqth
\ ‘:.
\ :: \: \i \
(nm)
Fig. 4. The 4fz -* 4f 5d excitation spectra of the 3Po+ ‘F, emission of Pr3+ in (La, _XPrX)203at 4.2 K, for several Pr’+ concentrations x: 5 x 10m4(dot-dashed line), 1 x low2 (dashed line), 2 x IO-* (dotted line) and 5 x lo-* (solid line).
In addition to the sharp lines from transitions within the 4f2 configuration, two broad bands with maxima at 294 mn and 240 nm are observed in the excitation spectrum of Pr3+ f in La2 0,. These bands were assigned to the 4f2 + 4f Sd transition of Pr3+ and to the excitation via the host lattice absorption state, respectively. Figure 4 shows that the 4f Sd excitation bands shift to lower energies as the Pr3+ concentration increases. A constant value seems to be
reached at about 5 mol%. No shift is observed for the band at 240 nm. Figure 5 presents the emission spectrum of Pti’ in La,O, at 4.2 K under 4f2 + 4f 5d excitation. The Pr3+ concentration is 0.05 mol%. The emission lines correspond to the radiative transitions ‘PO + 3Hq,5,6,3F2,3,4and ‘D2 + 3Hq (Fig. 5). The ‘D, emission is hardly detectable under ‘P, excitation. This is expected since the energy gap between the ‘PO
Wmdength
Fig. 5. Emission spectrum of La,,,Pr,,,O,
Mm)
at 4.2 K upon 4f * + 4f Sd excitation.
Vibronic transitions of the Pr3+ ion
and ‘D, levels is 3900 cm-’ and the maximum energy of the lattice phonons in La,O, is 500 cm-‘. As a consequence the ‘PO+ ‘D, multiphonon relaxation is very inefficient since it requires at least eight phonons [ 181.The higher intensity of the ID, emission under 4f 2 + 4f 5d or host lattice excitation is due to the direct feeding of the ID, level [19]. The spectrum is essentially
the same
for all concentrations,
but
the ‘D, emission shows a pronounced concentration quenching. For concentrations of Pr3+ above 1 mol%, the ID2 emission is not observed. A similar phenomenon was observed before by e.g. Domauf and Heber [20] in La, _XPrXP,0,4, where it was attributed to cross relaxation between adjacent Pr3+ ions. Cross-relaxation is much more efficient for the ‘D, level than for the 3P0 level, because the energy mismatch for the cross-relaxation process for the latter level is much larger than for the former [20]. Vibronic transitions were also investigated in the emission spectrum. The 3P0 -+ 3H4 emission consists of several electronic lines due to the crystal-field splitting of the 3H4 level. This makes the analysis of the vibronic intensity rather complicated. The effect of the Pr3+ concentration on the vibronic intensity of the ‘PO + ‘H$‘) emission transition cannot be investigated due to increasing reabsorption of this emission line with increasing concentration of Pr3+ ions. The 3P0 + 3F2 emission is more suitable, because no reabsorption occurs and the crystal field splitting of the 3F2 level is small, allowing the observation of vibronic lines with frequencies higher than 100 cm-‘. Figure 6 shows the vibronic part of this transition at 4.2 K. In contradiction to the vibronic transitions in the excitation spectrum, these in the emission spectra do not depend on the Pr3+ concentration: the integrated intensity ratio R is 0.08 for all concentrations investigated.
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A comparison between the positions of the emission and excitation vibronics and the vibrational data of La,O, is presented in Table 1. The agreement between the emission and the excitation vibronics is good if one considers that the frequencies of the former are less accurate than those of the latter. In spite of the large difference between the R values, the vibronic transition probabilities for the 3P0 + 3F2 and ‘H$‘) + 3Po transitions are of the same order of magnitude, viz. 6 x lo3 and 2 x lo’, respectively. This is due to the fact that the zero-phonon transition probability for the former transition is much larger than that for the latter, 8.5 x 104s-’ and 2.3 x 10’ s-‘, respectively. These transition probabilities were determined as described in [4]. The agreement with the vibrational data is fairly good. Most of the vibronic lines can be assigned to coupling with i.r.- or Raman-active vibrational modes, but it is not possible to assign all of the observed vibronic features. Some of the vibronic lines seem to be split. A good example is the line centred at about 108 cm-’ which can be considered to be split into twin peaks at 100 and 116 cm-‘. Richardson et al. [21] reported the splitting of v3and v,-vibronic lines in the optical spectra of several rare-earth ions in elpasolites (Cs,NaRECl,). The v,-vibronic lines generally appear as doublets split by lo-15 cm-‘, and the v,-lines appear either as asymmetrically shaped singlets or as doublets split by 2-lOcm-‘. The splitting of these vibronics is attributed to lattice dynamics effects involving transverse (TO) and longitudinal (LO) optical phonon modes [21]. Only transverse lattice vibrational modes appear in the i.r. spectra, whereas both LO and TO modes contribute to vibronic spectra [21,22]. Similar effects may be responsible for the fact that we observe more vibronic lines than the vibrational
Energy (cm*)
Fig. 6. Vibronic part of the 3P0 + 3F2 emission spectrum of Pr ‘+ in La,O, at 4.2 K. The onset of the zero-phonon transition appears on the extreme right side of the spectrum. The zero-phonon line is at 14,998 cm-‘.
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modes appearing in infrared and Raman spectra. A more detailed discussion of the intensity distribution between the vibronic lines lies beyond the scope of this paper. The effects of increasing concentration on the optical spectra of Pr3+ in RE,OrS (RE = Y, La) are similar to those observed for La,O,: Pr3+. The integrated intensity ratio R for the 3Hf) + ‘POexcitation transition increases by a factor of two when the Pr3+ concentration increases from 0.1 to 1.0 mol%. The intensity of the ‘D, emission decreases and the 4f * --t 4f Sd excitation transition shifts to lower energies with increasing Pr3+ concentration. The magnitude of these effects is similar for both host lattices. We turn now to the LiY, _XPr,F, samples. The intraconfigurational excitation spectrum and ‘Ze) of the ‘P,, -+ ‘H6 emission of (‘Z& + ‘PO 12 Pr3’ in LiYF, also shows vibronic side bands. The vibronic coupling is much weaker for Pr3+ in LiYF, than in La,O,. The variation of the vibronic intensity of Pr3+ with the host lattice has been discussed recently [4]. The vibronic part of the 3H$‘) -+ ‘PO excitation spectrum of Pr3+ in LiYF, has been discussed in [4]. The agreement between the vibronic spectra and the vibrational data of LiYF, is good [4]. In addition to the vibronic lines, several satellite lines were observed in the high resolution ‘Hi’) + ‘PO excitation spectrum of LiY 1_ XPrXF,. Similar lines have been observed by Barthem et al. [23] in LiY, _XPr,F, crystals. They are attributed to excitation transitions of pairs of Pr3+ ions. The main line is at 20,873 cm-’ in all investigated samples but the position of the satellites vary. Satellites are observed at 2.7 cm-’ higher energy (line A) and at 1.3 cm-’ (B), 2.6 cm-’ (C), 4.0 cm-’ (D) and 7.2 cm-’ (E) lower energies from the main line. Their relative intensities increase with x. Figure 7 shows the high resolution excitation spectrum of the LiY,,,, Pr,.,, F, sample at 4.2 K in the 3H$‘) + 3P,, region while monitoring fluorescence from the main site (Fig. 7a) and from one of the pair sites (Fig. 7b). The integrated intensity ratio R is 0.2 for the main site and 0.3 for the pair site. Within the experimental error the ratio R is the same for both sites. The ratio R for the ‘H$,‘) -+ 3Poexcitation transition is calculated from high-resolution excitation spectra monitoring emission from the main site. The ratio R is 0.2 for all the investigated concentrations. The zero-phonon line intensity includes the pair lines since the vibronic side band contains contributions from all sites. The decay time is 48 ps up to 10 mol% of Pr3+. The position of the 4f * --) 4f 5d excitation band does not depend on the Pr3+ concentration.
(b)
4..,‘.......
2mm
.-f
. ..._
2omo
,‘.......,,,
2aem
202m
Enargy (cm-3
Fig. 7. High resolution ‘Hi’) + ‘POexcitation spectrum of the ‘PO+ ‘H6 emission of LiYF,:O.S%Pr’+ at 4.2 K using site selective techniques: (a) monitoring main site (I), emission wavelength at 604.35nm and, (b) monitoring site D, emission wavelength at 607.83nm.
It has its maximum at about 50,0OOcm-’ for all samples. At 4.2 K the emission spectrum of Pr3+ in LiYF4 consists of transitions from the ‘P,, level (3P, -, 3H‘,, 5, 6, 3F2,3,4) only. The spectrum is the same under ‘PJ or 4f 5d excitation and it does not depend upon the concentration of Pr3+ ions. 4. DISCUSSION Before discussing interaction mechanisms leading to the concentration enhancement of the vibronic excitation transitions of Pr3+ in La,O,, we want to get an estimate of the distance over which the interaction between Pr3+ ion operates. We assume that any set of neighbouring Pr3+ ions within a distance shorter than r, has approximately the same vibronic intensity ratio. This distance r,, here called the critical distance, indicates the largest distance over which the interaction has observable consequences. The observed intensity ratio R can then be described by: R = &so x Go + &~,a x CMR,
(1)
where R,, is the vibronic intensity ratio for the isolated Pr3+ ions, C,, is the concentration of isolated Pr3+ ions R PA,Ris the vibronic intensity ratio for a pair of interacting Pr3+ ions and CPA,, is the concentration of these pairs in the host lattice. The concentration of pairs is given by: CP**a= 1 - cno = 1 - (1 - C)N
(2)
Vibronic transitions of the Pr’+ ion where C is the total concentration of Pr” ions and N is the number of sites within the critical distance r, for interaction between two Pr’+ ions, which is given by: N
=
Wd v ’
where V is the volume of the unit cell. The solid line in Fig. 3 is the best fit to eqn (l), taking the R value for the lowest x, 1.1, as RI,, and the R value for the highest x, 2.5, as RPAIR . This yields a critical distance r, of 9.5 A. This large value of r, shows that the mechanism responsible for the concentration enhancement of the vibronics involves an interaction between Pr’+ ions over some 10 A. Figure 3 shows also that there is not much sense in refining this analysis by taking into account a more refined distance dependence of the interaction, because the experimental data are rather inaccurate for the reasons mentioned above. The interaction between the Pr’+ ions obviously occurs over long distances and involves the intermediary between the interacting ions. In view of the long range of the interaction involved in the concentration enhancement of the vibronic coupling, direct 4f -4f interaction cannot be important. Danielmeyer [24] proposed a long-range interaction model based on the overlap between the 4f and 5d wavefunctions of neighbouring rare earth ions in order to account for the properties of stoichiometric rare earth laser materials. A non-vanishing overlap of the 4f and 5d wavefunctions of two Pr3+ ions, which may be increased by admixing host lattice wavefunctions, can lead to the observed interaction range, since the 4f wavefunction drops to 1% of the maximum amplitude at 2.5 A, and the 5d wavefunction extends to 8 A [24]. The shift of the 4f 5d configuration to lower energies can be seen as evidence that the 5d wavefunctions are involved in the interaction. Recently further support for 4f interaction via hybridization with 5d and 6s orbitals has been established by Gschneider [25]. His arguments originate from different types of compounds and different physical properties, and are based on the melting points of rare earth (RE) metals and alloys, on the hydrogen solid solubility in RE metals and on thermodynamic data of RE compounds (free energies of formation and stability constants of RE(EDTA) complexes). The 4f hybridization with the 5d and 6s orbitals of the lanthanide and non-rare earth elements in a compound or alloy is shown to be maximum for the lighter lanthanides (La-Nd). The 4f interaction is expected to be more effective in La,O, than in LiYF,, since the former compound
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is more covalent than the latter. This implies that the interaction between Pr’+ ions via wavefunction overlap is much weaker in LiYF,:PIJ+ than in The absence of observable concenLa,O,:PI3+. tration effects in the excitation and emission spectra of Pr’+ in LiYF, can be seen as a consequence of the weak interaction between Pr’+ ions in this lattice. In this connection we note that superexchange interaction via the 2p orbitals of the F- ions has been proposed as an interaction mechanism between but only for associated Pr’+ ions in LiY,_,Pr,F,, distances shorter than 4.8 A [23]. We will now discuss how these interactions can affect the vibronic intensity. The vibronic transition probably is given by the summation of two contributions, usually indicated as the M and the A processes [l]. The former describes vibronically induced forced electric-dipole transition. The latter describes vibronics induced by a shift in the equilibrium position of the configurational coordinates of the excited state relative to the ground state [l]. If the wavefunction overlap is increased, the electron-phonon coupling strength (Huang-Rhys parameter S) is enhanced [26]. Therefore the A process contribution to the vibronic intensity becomes larger, since it is proportional to S [l]. Additionally, these interactions lead to an increase of the degree of covalency and to a shift of the 4f 5d configuration to lower energies. The latter fact causes an increase of the opposite-parity configuration admixing. It has been shown that the increase of covalency and opposite-parity configuration admixing increases the vibronic coupling strength due to both M and A processes [ 1,4]. However the ratio R does not depend on the position of the opposite-parity configuration [I, 271. Therefore the observed concentration enhancement of the ratio R has to be interpreted as a covalency effect. Consistently, the intensity of the vibronic transitions of Pr’+ in (RE, _,Pr,),O,S (RE = Y, La) also increase with x. The concentration enhancement is observed for the excitation vibronics only. The same phenomena have been observed for Ed+ in RE202S [8,9] and in NaSGd(WO,), [ll]. Hoshina et al. [8,9] suggested that superexchange interaction operates between two associated Eu’ + ions via the low energy chargetransfer state (‘CTS), which mixes with the ‘Fground state. Thus superexchange interaction operates between associated Eu’+ ions only when they are both in ‘F states, but not when one ion is in the ‘D excited state, because of the energy mismatch between the two Eu’+ ions [9]. Therefore concentration enhancement is observed only for the ‘F. + sD, excitation vibronics [8,9]. A similar explanation can hold for P?+. Here the superexchange interaction takes place
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via the 4f 5d state and is only effective when both ions are in the 3H4 ground state. In addition to the enhancement of the excitation vibronics, it appears that also the intensity of some pure electronic lines (3H$” -+ ‘Pz and ‘Hi’) + ‘Ze, see Fig. 1) increases with the concentration. This can be interpreted as an example of hypersensitivity, since A.Z = 2 [28]. Recently Garcia and Faucher [29] calculated the 4f -+ 4f oscillator strengths for dipolar electric transitions in PrCl,. Their results surprisingly show that the intensity of the absorption transitions 3H4 -+ 3P2,‘Ie and ‘D, can be explained using 4f 2 + 4f 5d configuration mixing, but that the 3H4 -+ 3P0,, transitions have no relation with the 4f2 + 4f 5d process. This fact implies that in our model the 3H4 -+ ‘P2 ,I& transitions should also show concentration enhancement, in good agreement with experiment. 5. CONCLUSIONS The intensities of the vibronic transitions and of the 3H$‘) + ‘P2 and 3H!,‘) -+ ‘Ze electronic transitions in the excitation spectrum of Pr’+ in La,O, and in RE202S (RE = La, Y) increase with increasing concentration of Pr3+ ions. This phenomenon can be ascribed to a superexchange interaction between Pr3+ ions. The interactions are assumed to take place via the overlap of the 4f and 5d wavefunctions of two Pr3+ ions over distances of about 10 A. The emission vibronics do not depend on the Pr3+ concentration. This fact suggests that the interaction between Pr3+ ions is effective only when the ions are both in the same state. The absence of concentration enhancement in LiYF,:Pr3+ shows that these effects require a low energy opposite-parity configuration and a high degree of covalency. Acknowledgements-C, M. Donega wishes to thank the University of Utrecht for a temporary appointment and the CNPq (Conselho National de Desenvolvimento Cientifico e Tecnolbgico-Brazil) for a study grant.
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