Vibronic optical transitions in inorganic and organic rare earth materials

Journal

of the Less-Common

Metals,

112

(1985)

VIBRONIC OPTICAL TRANSITIONS RARE EARTH MATERIALS* P. CARO, CNRS

0. K. MOUNE,

ER 210,

(Received

March

I place

92195

Meudon

153

- 173

IN INORGANIC

E. ANTIC-FIDANCEV

A. Briand,

153

AND ORGANIC

and M. LEMAITRE-BLAISE (France)

4, 1985)

Summary The lanthanide elements are widely used as optical structural probes in solid state chemistry. Group theory and line positions are used to derive the symmetry and the number of crystallographic sites. For some compounds additional lines or illogical splittings make interpretation difficult. The Stark level widths depend on the phonon density of states in the material: if it is a continuous function the classical theory applies well, but if the phonon density of states is resolved into sharp peaks, zero-phonon line splittings may occur from resonance effects, with sharp phonons corresponding to the energy difference between Stark levels. Examples are shown for lanthanide organic compounds, and for some inorganic phases as well. Vibronic satellites occur prominently in some phases, especially if tetrahedral anions (phosphate etc...) are present. From the vibronics associated with simple electronic transitions it is possible to derive an effective density of phonon states. The puzzling case of Eu 3+ in GdNb04, where the fine structure of the emission spectrum depends on the optical excitation wavelength, is presented.

1. Introduction The sequence of the discrete energy levels of the lanthanide 4fN configurations has been recorded for a large number of solids over the years. The experimental data have been simulated by the theoreticians using a free atom plus crystal field Hamiltonian which involves a limited number of parameters, usually much less than the amount of available data. The phenomenological parameters obtained have been checked by the simulation of other properties depending on the wave vectors such as the paramagnetic susceptibility and its anisotropy, Zeemann splittings and, less frequently, intensities of the transitions. As a result phenomenological parameters are well established for *Paper presented at the International land, March 4 - 8,1985.

Rare Earth Conference,

0022-5088/85/$3.30

@ Elsevier Sequoia/Printed

ETH Zurich,

Switzer-

in The Netherlands

154

a fairly broad range of compounds, most of them simple structural types, exhibiting, but not always, a single site in the unit cell for the l~th~ide. Quite recently an important effort has been made to obtain crystal field parameter values directly from e& initio calculations based on structural data alone. Comparison with the well-known phenomenological values proves that this is reasonably successful provided that one uses a rather sophisticated approach employing much computer time and taking into account ah aspects of the contribution to the parameters from the electrostatic part of the bonding as well as from ,the covalent part, at least from first neighbors in the coordination polyhedron. Another trend in research has been the use of optical spectra as a tool in the analysis of complex problems in solid state chemistry. Typical questions concern the number and symmetry of the sites occupied by the l~th~ide in a given structure, or the number of phases in a mixture, observations of phase transitions, chemical reactions (for example, thermal decompositions, or complexations), detection of impurities, and so on. The optical techniques, absorption or fluorescence under several types of excitation conditions, are used in much the same way as in X-ray techniques, the energy levels sequence for a given site being, for example, as good an identification technique as an X-ray powder pattern. Glassy or crystallized solids, liquids, organic compounds, and biological materials, have been examined using the above techniques. Basically, the distinction of the number of sites is based on the existence of the nephelauxetic effect. Empirically, one knows that the energy level sequence is practically different for each ~~stallo~aphi~ environment. Moreover, one often uses several lanthanides to examine a given problem. The best optical probes are neodymium, europium, gadolinium, and terbium, which have part of their sequences well adapted to this type of work because of convenient quantum labels which facilitate interpretation. The advent of dye-laser technology, especially the popular rhodamine 6G system, has made europium a favorite optical probe because of the simple sequence of fluorescence lines originating from the 5Dd level. The displacement of the basic optical lines with local structure is small but easy to record with a high degree of precision on high resolution instruments. The displacement is due to a mixture of crystal field effects and to the proper nephelauxetic effect itself, which is a tiny change in values of the free atom parameters (Slater integrals and spin-orbit coupling constant). It is difficult to rationalize the displacement in term of structural data (distances and angle to the ligand coordination number) but practically its existence is well established, and with the exception, always possible, of accidental degeneracy, one can believe with confidence that a given line system is associated with a given discrete geometrical confi~ation around the lanthanide. Fluorescence line narrowing has been widely used to test, for instance, the sites in vitreous materials, in disordered solids or in liquid media, Group theory has been used with some success to derive the symmetry at the site associated with a given spectrum. Clever combinations of optical probes are

155

necessary, however, to derive information on coordination or distances. Some probes are more adapted to measure variations in free atom parameters than others, for instance, neodymium or gadolinium are better in this respect than europium which, in turn, is more convenient for the derivation of crystal field parameters. Terbium, from the variable intensity of its fluorescence, is quite useful in detecting changes in the chemical nature of the rare earth ligand. A decrease in free-atom parameters is associated with a lowering of the distance to the ligand (this can be proved by measuring, for instance, neodymium lines in a series of doped isomorphous compounds along the rare earth series from lanthanum to lutetium, which amounts to a pressure experiment on the pure neodymium compound). All this is routine practice in many laboratories. However, every experimentalist knows that in some compounds one observes optical signals which do not fit too well with the established simple picture of energy levels as supported by the theoretical background. This involves unexplained additional lines, unwanted splittings, contradictory evidence from different groups of lines from a single probe, or contradiction between probes. Most of those data have not been published, because of the difficulty of providing convenient, simple explanations or efficient theoretical support. In this paper we will present some typical cases, and we will try to offer an explanation of some of these phenomena.

2. The width of Stark component

spectral lines

It is well known that some high-lying Stark components in the ” + ‘LJ groups are broadened with respect to low-lying Stark levels. This can be seen in absorption or fluorescence and examples are given in Figs. 1 - 3. If absorption takes place from the ground state A to a couple of closely-spaced isolated levels, B and C, the higher level C is de-excited to B

I

4

Fig. 1. Absorption

at 4.2 K. 419,2 --f *P3,*, Nd(N03)3-6H,0.

Fig. 2. Absorption

at 4.2 K. 41g,2 + 4F9,2, NdP04.

156

7 912

4556

4592

41e/2-2KtsQ P

Fig. 3. Absorption at 4.2 K. 419,2 -+

4678

ZGv2,

4h~,

-

4736

4%,z,

2G,2

A

N~(NO~)~.~HZO.

by emission of a phonon, if a phonon with a frequency corresponding the energy difference between B and C exists. This is quantitatively pressed by the electric dipole probability [ 1,2] :

to ex-

W = (2n/h)f(X,OIP”‘IXc’)l*g(w)“c-B

(1)

where x is the product of the phonon state function and the electronic state function, J/, and g(u) is the density of phonon states at the frequency Y = EC-Eg. For most inorganic compounds the density of phonon states is more or less a continuous function. Some experimental techniques give access to part of this density: infrared absorption, Raman spectroscopy, inelastic neutron scattering and, as we will see, optical vibronic side-bands. If the bandwidth is broad, the phonon can propagate away from the rare earth site, and the upper level C is broadened (the width is inversely proportional to the lifetime) [3]. The width, I’, of the spectral line is then I’a x2&#-B X being a coupling

(2) constant.

157

In the absorption spectrum the transition from A to C appears to be broadened, and the transition from A to B is sharp (see Fig. 1 for an experimental example). This is also true for a manifold of several Stark components (Fig. 2) or for closely spaced J levels (Fig. 3). The energy difference between the high-lying level of a Stark group and the lower one usually falls in a region where the density of phonon states is highest. However, the broadening is not automatic. For some compounds there is no clear evidence of broadening. This is the case, for instance, in the absorption spectrum of Gd3+ ( 4f7) in gadolinium gallium garnet (GGG) (Fig. 4). For some compounds the density of phonon states becomes more or less sharply peaked. For organic compounds, the density is resolved in discrete peaks whose width depends on the correlation field between molecules. In this case there is generally no correspondence between the crystal field splittings and the peaks in the phonon density of states; the electronic lines are then very sharp. This can be observed in the absorption or fluorescence spectra of rare earth organic compounds. This somewhat contradicts the usual habit of the chemist of testing the “degree of crystallinity” of his material from the sharpness of the absorption or emission spectrum. Usually the “disorder” brings a contribution to the width of the lines (including the lower lying ones) by the fluctuation of the crystal field parameters from site to site. At the limit the spectrum may be the one for a glass. This is the case, for instance, for europium in stabilized zirconia [4] : although the cations are ordered, a systematic disorder in the position of the anions, which makes a sort of anion glass, brings the equivalent of an infinity of different crystallographic sites in the material. By contrast, in the organic compounds, the X-ray powder diffraction lines are broad and diffuse but the optical spectra are sharp. Usually the crystal field parameters appear to be very small and we can consider that we are dealing with a well-defined, localized, rare earth complex independent of the general organization of the crystal: the mole-

‘G/2 1

2534

Fig. 4. Absorption

1

2543

at 300

I

2551

A

K. 8S7,2 -+ 6D,,,.

Gadolinium

gallium garnet

158

cules linked to the rare earth by the chemical bond behave independently of the other molecules in the solid. The vibrations are then localized and an emitted phonon may not move away. It may happen, however, that there is equality (resonance) of energy differences between Stark levels and sharp peaks in the density of phonon states. Then, eqn. (2) no longer applies: a zero-phonon line splitting occurs [ 21,.

3.Zero-phonon

line splittings

(Figs. 5 - 17)

The first evidence of abnormal splittings of Stark lines was obtained by one of us (O.K.M. [5]) in the course of a study of the absorption spectra of several carboxylic and aminopolycarboxylic neodymium salts. The 300 K 4I 9,2 --f 2P1,2 spectrum is reported, Fig. 5, and the 4 K spectrum is shown in Fig. 6. From these Figures the oxalate, the malonate and the hydrazino diacetate can be taken as exhibiting a single crystallographic site, the case of the EDTA salt or the glycolate being somewhat different. In the isolated 4F9/2 group, Fig. 8, however, there are more than five Stark components for the malonate, but there is perfect agreement with the number of theoretical lines for the same compound in the hypersensitive transition, Fig. 9. Discrepancies appear again in the 2G9,2 group, Fig. 11, whereas the other salts, including the glycolate, have a perfect number of lines. For the isolated 2Ps,2 group, Fig. 12, the glycolate is also correct but this time the oxalate has three times the number of theoretical lines! It is clear (putting aside the EDTA salt spe~t~m which is not very good) that the observed splittings do not make sense with regard to the definition of theoretical splittings for a single site. Further, the spectra are very similar, which indicates a similar structure at the crystallographic site for all compounds. It is a case of high coordination with rather small crystal field splittings, that is, low values for the crystal field parameters. The expl~ation may well be zero-phonon line splittings due to the resonance phenomenon mentioned above. The resonance is possible in a multiple way with numerous discrete peaks in the density of phonon states which extends up to 3600 cm-‘. These splittings are small. For instance, for the 2P3/2 case of the oxalate the six lines are at 26 200, 26 208, 26 220, 26 244,26 253 and 26 263 cm-‘. A similar phenomenon was encountered f6 3 in the absorption spectrum of the pure neodymium DPM chelate and its adducts with pyridine and phenylethylamine. In this particular case the crystal field parameters are large (it is an example of coordination seven), and the crystal field splittings are quite similar, qualitatively, to the ones for neodymium in the A-type oxide {also a case of coordination seven) but there are too many lines and many of them appear to be split: the 2P1,, line observed under high resolution at 4 K is a doublet at 23 1’71 and 23 177 cm-“, Fig. 14. Doublets also appear with roughly the same separation in the hypersensitive group at

Glycolare

:

/I Glycolate

K. 4I9,2 + 2P1,2. Neodymium

Fig. 5. Absorption

at 300

Fig. 6. Absorption

at 4.2 K, 419,2+

*P~,z,

salts of polycarboxylic

2Ds,2 neodymium

acids.

salts of polycarboxylic

acids.

r,~

,~

i

I

! Fig. 7. Absorption of polycarboxylic Fig. 8. Absorption acids.

at 4.2 acids. at 4.2

K, 4I9,2+ K,

(4F5,2,

4I9,2 --f 4F9,2,

‘H~,z), 2H11,2

f

(4S3,24F7,& . Neodymium

‘r’g~ ,8 8 “.

3 4F3,2. salts

D ”

;

Neodymium

f i

Cm-J

z z

salts

of polycarboxylic

161

Ethyl+nrdiamw

tttraocttatp

Hydrazano d;acCtate

Fig. 9. Absorption acids. Fig. 10. Absorption carboxylic acids.

at 4.2 at 4.2

K, K.

4I9,2+

4G5,2,

4I9,2 -+ 4G7,2,

*G 7 ,2. Neodymium (4G9,2,

*K13,2).

salts

of polycarboxylic

Neodymium

salts of poly-

162

tiydrazino

diocttats

Fig. 11. Absorption at 4.2 K. 4Ig,2 -+ ‘G~Q, 2(P, D)3,2, (4G,1,2, ‘K15~). Neodymium salts of polycarboxylic acids. Fig. 12. Absorption at 4.2 IL 4I9,2 + %2.

Neodymium salts of polycarboxylic

acids.

5800 A, together with additional lines (Fig. 15), this is also the case for the transition to 4G, ,* (Fig. 16). It seems that in this example, in agreement with the crystallography, there is basically a single site, but that some lines appear to be split in the Stark groups, whereas additional lines also appear. The situation here is somewhat more complicated because the molecule may be considered as a dimer [ 71 and there is possibly a pair effect in the spectrum. A third case is the splitting of the E representation of the ‘F, level of Eu 3+ in the Csv site of A-La203, and the S6 site of C-Y 203. For A-La203, the

163

1

Fig. 13. Absorption acids.

at 4.2 K. 4I9,2+ 4D3,2, ‘+D5,~. Neodymium salts of polycarboxylic

Fig. 14. Absorption spectra at 4.2 K. 4I9,2 + 2P1,2. (1) Nd(DPM)3; (2) Nd(DPM)J, (3) Nd(DPM)s, 2a-PEA. (Lines noted A, B, C in (1) are additional lines.)

2Py;

splitting of the levels is in agreement with the CJUsymmetry with the exception of the isolated ‘F, E level, which is split [8], Fig. 17. The Raman spectra of A-type sesquioxide single crystals show four rather sharp bands: E, = 106 cm-l (La203), 105 (Sm,O,); A,,:191(Laz0,) 188 (Sm203); A,, = 400 (La,Os), 444 (Smz03); E, = 408 (La,O,), 455 (Sm,O,) [9, lo], whereas the infrared assignments yield, for Nd,O,: AZu= 405 cm-‘, AZu: 228, E,: 412, E,: 228 ill]. Now at 77 K, from the optical data for Eu3+ in A-La,Os [S] we have the following sequence of energy levels O(‘F,, A,), 228 cm-l (‘F,, A1 in C&), 419 and 444 cm-’ (E split in C,,). It is clear that the values listed for the electronic levels and their energy differences are close to zone center vibrational frequencies and there is the possibility of resonance.

164

We will see in the next section that it is possible to measure the effective phonon density in the A-type oxide from the vibrational transitions, at 4 K, of the 3H4 + 3P0 transition in A-Pr203. A sharp line corresponds to the energy difference between the E and A levels of 7F,. In the isomorphous oxysulfide the splitting between the A and E representation of 7F, is much smaller (30 cm-‘) and there is, presumably, no appreciable phonon density to match this difference, hence the two lines are sharp and the degeneracy of the E level is not lifted. Recent experiments with synchrotron radiation [ 121 have shown that the E representation of 7F1 in the S6 site of Eu3+ doping C-type Y,O, was also split, the energy level sequence being 7F0, A, 0, 7F1, A, 126 cm-i, 7F,, E, 424 and 504 cm-‘. This is also possibly due to a resonance phenomenon with phonons of -340 cm-i. For both oxides there is no possibility of a structural distortion, and no evidence for it from the other optical data (for instance, there are no dipole electric-allowed-lines for the S6 site). Brawer [2] has developed a detailed theory for a two level system interacting with a continuum of lattice vibrations. Dynamical effects are to be

Fig. 15. Absorption spectrum at 4.2 K. 419,2 -+ 4G5,2, ‘GTj2, Nd(DPM)3 (numbers indicate the electronic lines).

165

(numbers

AE.

Fig. 17. Emission

181.

spectrum

under selective

indicate

the

%roA ,TK

excitation

of Eu3+ in A-La203

at 300 and 77 K

166

ignored in the usual case of the static crystal field, but when it is possible to isolate a complex around the rare earth in which the ligands interact weakly with the rest of the solid, a splitting of the upper electronic level into two sharp lines of equal intensity is possible through resonance. Intermediate cases will be characterized by a broad, central band and two sharp satellites of unequal intensities. Our organic compounds will be examples of the isolated complex case, and the rare earth sesquioxides ‘Fi, E level splitting may be an example of the intermediate situation. A close examination of the shape of the E level transition at 77 K in Fig. 17 shows that it may, indeed, correspond to three closely spaced lines. Resonant phonon interactions have been suggested to explain the shape of certain optical lines in semiconductors [13,143. To our knowledge the only example in the literature for a rare earth element occurs for Yb3* in YA103, where a small splitting of some Stark transitions in the 2F,,2 fs: 2F7,2, was observed experimentally and attributed by Perlin and. co-workers [15] to an electron-phonon resonance of the type described above, with a peak in the density of phonon states located at 190 - 200 cm-‘. The present work suggests that many more examples may be found in rare earth materials, especially organic salts in the solid state (see Figs. 6 16), because the sharp phonon energy extends up to 3600 cm-’ and the resonances may occur between Stark components of different 2s+1LJ levels.

4. Vibronic

satellites

Phonon sidebands are well known in rare earth spectra. They correspond to transitions for which the phonon wavefunction number has changed. The vibronic spectra associated with rare earth 4fN transitions, especially if it is with a single, well-isolated transition, provide useful information on the phonon frequency distribution g(w) of the host crystal. Absorption spectra have been widely used in this respect to derive effective density of phonon states. There are some advantages in respect of infrared or Raman absorption. Because of the localized character of the optical excitation, crystal vibration with wave vectors belonging to the entire Brillouin zone contributes instead of zone enter only for IR and Raman [ 201. One can have, consequently, a better picture of the strength of the ionlattice coupling. There are several examples in the literature of the use of rare earth vibronic spectra to derive information on the lattice dynamics of crystals, for instance, LnCl, [l, 161, YA103 [17 - 191, LaNb04 [20] and d transition ions such as Cr3+ and Ni2+ can also be used. Vibronic transitions are usually of a low intensity, roughly low3 of the parent transition [ 1 J. Ion pair spectra will be still less intense, of the order of 1O-5 [ 11. For most compounds one has to adjust the sensitivity of the spectrographs to bring out the vibronic satellites. There is, however, a class of compounds for which the vibronic satellite intensity is very large - in the same magnitude range as ordinary electronic lines.

167

We found that a large number of vibrational lines appear in absorption for compounds with tetrahedral complex anions: phosphate, vanadate, sulfate, niobate. The lines are very intense in the neighborhood of the hypersensitive neodymium transition at 5800 a; Judd has shown [21] that a pseudo-quadrupolar selection rule applies for the electronic part of the vibronic transition. The satellites are then prominent for the AJ = 2 transitions [ 221. It is, in fact, sometimes difficult to distinguish the zero-phonon electronic lines. Examples are shown in Figs. 18 - 22. The one-phonon vibrational spectrum extends some distance away from the electronic lines, up to 800 - 1000 cm-‘. The low frequency zone corresponds to external lattice vibrations, and the high frequency zone to internal vibrations: the v2 and v4 vibrations of the tetrahedral ion appear in the 300 400 cm-’ region, v3 in the 500 - 750 cm-’ region, and the V, vibration makes the highest frequency components [ 201.

?

5k

3k

5731 1

5615 1

5506 I Ai

Fig. 18. Absorption spectrum at 4.2 K of NdV04 2G7,2 (numbers indicate vibrational lines) [ 221.

5&S

lw*,

5615

5506

(xenotime

structure).

4I9,2-+

4G5,2,

(monazite

structure).

419,2+

4G5,2,

Ah

Fig. 19. Absorption spectrum at 4.2 K of NdP04 2G7,2 (numbers indicate vibrational lines) [ 221.

168

I

5975

5616

5731

1

5615

I

5506

AA

Fig. 20. Absorption spectrum at 4.2 K of Nd3+ in LaVO4 (monazite structure). 4G5,2, 2G-,,2 (numbers indicate vibrational lines) [ 221.

5429

50

5603

5679

57s3

5mlual

aw

54556

Fig. 21. Absorption spectrum at 4.2 K of (Nd0)2(S04). cate vibrational lines). Fig. 22. Absorption spectrum at 4.2 K of Nd(Nb04). vibrational lines).

557s

57im

4I9,2 +

WF.3

419/z + 4G5,2, *G7,2 (arrows indi-

419,2 + 4G5,2, *G7,2 (arrows indicate

If an electronic transition with a single Stark level is well isolated, it is possible to derive a picture of the effective density of phonon states for the material. Examples are shown here for the 3H4 + 3P0 transition of A-Pr20s, Fig. 23, and Pr(N03)s.6Hz0, Fig. 24, at liquid helium temperature. In the case of the oxide (Fig. 23) the vibronic lines are very intense, and between 3P, at the 3P, absorption at 20250 cm-’ and the first Stark component 20675 cm-‘, one sees sharp peaks at 94, 106, 157, 217, 241,250,318 and 339 cm-’ of the 3P0 line. The sharp peak at 217 cm-’ may be responsible for the splitting by resonance of the ‘Fi E level of Eu3+ in A-La203. Another example is shown in Fig. 25 for the 2P,,2 level of NdV04. Another level which can be used is the 5D, level of Eu3+ [20]. The vibronic lines appear to be very intense for the beginning of the lanthanide series, for praseodymium and neodymium. The intensity depends not only on the lanthanide but also on the structure. For instance, no vibra-

169

Fig. 23. Absorption

spectrum

at 4.2 K of A-Pr,Os.

3H~ + 3Po, 3P1.

t

P,(NOs)a 6HrO 3H4-3P,

4K

Fig. 24. Absorption

spectrum

at 4.2 K of Pr(NO&.GH20.

i

I

L315

%4+3P0

I.260

Fig. 25. Absorption 2P 1 I2 > ‘h/2.

L210

spectrum

3H4 + 3Po, 3P1.

)

L175 hIA)

at 4.2

K of NdV04

(xenotime

structure)

at 4.2 K. 4I9,2-+

tional series is seen for the 3H4 + 3P0 absorption spectrum of Pr3+ in YAG. The intensity depends on the coupling coefficient of the ion to the lattice

[=I.

170

5. Tbe case of Eu3+ in GdNb04 The case of the europium spectrum in the rare earth niobates is rather puzzling. The emission spectrum of Eu3+ in LaNb04 (which is a ferroelastic compound) has been investigated by Ignatev and Ovsyankin [20], They found a series of vibronic transitions on the ‘F, -j, 5D0 line which is in fair agreement with the Raman spectra and which is consequently an image of the effective phonon density of states in the material. The absorption spectrum of the ‘Fe+ 5Dz transition for EuNb04 (Fig. 26) clearly shows vibronic satellites following the five sharp lines of the ‘D, level. Of course, it is difficult to extract the density of phonon states from this spectrum because vibronics associated with the five Stark components overlap, but the highest one-phonon vibronics should be around 800 cm-‘.

LW

4525

4603

4619 ale,

Fig. 26. Absorption spectrum at 77 K of EuNb04. lines.

‘FQ --f 5D2, arrows indicate vibrational

If Eu3+ in GdNbOG is excited at 2500 A, one gets the classical spectrum of Fig. 27, which shows a low symmetry site with small crystal field splittings and the following energy levels: 7F0: 0; 7F1 : 297,373,407 cm-‘; 7Fz: 876, 894, 945, 1175, 1255 cm-’ . . . . . . 5D 0 : 17 210’ cm-‘. Dye-laser excitation in the ‘Da line yields the same spectrum. However, if one uses the argon laser line at 4658 A (21468 cm-‘) to excit the spectrum, one obtains something different, much more complex, with a large number of lines (Fig. 28), and a complex emission in the 5D0 + 7F, region with at least four components! In this case the argon laser line falls exactly on the Stark components of 5Dz as recorded for EuNb04 (21379, 21399, 21450, 21461, 21474 cm-‘). Through excitation with UV light at 3650 A (27 397 cm-‘) one obtains another complex spectrum, but this time the ‘Do is split in only two lines at 17 208 and 17 216 cm-‘! (Fig. 29). Again dye-laser excitation in the ‘D, produces a spectrum, and a simple one, only for the excitation wavelength at 17 210 cm-‘. What is the explanation of this strange behavior? One may consider that the structure is perturbed by the high energy excitation, which may induce some sort of structural disorder, but we may be witnessing just another much more complex type of zero-phonon line splitting through resonance. The difference between the lowest ‘Di level at 18 947 cm-’ and

171

QO, is 1761 cm- ‘. The upper 5D, level is at 18 971 cm-‘, its very small theoretical splitting, of 6 cm-‘, was seen. It is quite reasonable to assume that the 7F, -+ 7Fz transition should also have vibronic satellites. The positions of some levels can be seen in the simple spectrum on the red side of the 5D, + 7F, transition. The highest vibronics, a sharp peak associated with the vi vibration of Nb04 tetrahedra, should extend from 876 + 800 = 1676 cm-’ to 1255 + 800 = 2055 cm- ‘, that is, in the energy range of the difference between ‘D, and ‘Do. The de-excitation of ‘D, may occur through resonance with the transition from 7F, to the vibronic levels of 7F2, whose transition probabilities are high in this matrix. In the case of the 2500 i$ excitation, ‘D, is direc tl y excited through the CTS, and the 5D, level is bypassed in the de-excitation cascade. Enhancement of the intensity of vibronics in resonance with the Stark energies of the ground 41,,, state in NdCl, has been observed in ref. 1 for vibronic satellites of the *Pi,, level at 1.4 K.

Fig. 27. Emission spectrum of Eu3+ (3%) in GdNb04, 2500 A, 77 K.

SDo+ 7F0, sDo+

7F,, excitation

Fig. 28. Emission spectrum of Eu3+ (3%) in GdNbOJ, ‘D,, -+ 7F,,, ‘Do--f 7F1, laser excitation at 4658 A,77 K.

172

?z--zox cm-’

mtz

I

5ooox

UC>

1604, em-1

Fig. 29. Emission 3650 A, 77 K.

spectrum

,724,

I

50008 10722 SW*

of Eu3+ (3%) in GdNb04,

5Do -+ ‘Fo, ‘Do--+ ‘F1, excitation

at

Of course there may be other explanations, such as especially efficient excitation of imp~ty sites. Sites associated with the high temperature phases of GdNbO,, were ruled out by creating traces of these phases using a CO2 laser beam fusion process followed by a fast quenching: the spectra corresponding to the high temperature phases are different. Work is in progress on the matter.

6. Conclusion It is clear that the utility of rare earth spectroscopy as a tool of the solid state chemist depends on the phonon density of states in the material. If it is a more or less continuous curve, there is no special problem and the classical procedures may be used with confidence. However, if the density of phonon states is sharply peaked as in A-type oxides, organic compounds, or tetrahedral anion compounds, one should exercise some care in interpretation, especially if, because of large values of the coupling coefficients,

173

the vibronic satellite transition probabilities become large. The occurrence of resonance can then lead, as predicted by Brawer [ 21, to complex splittings of the electronic optical lines in transitions to the levels involved in the resonance process. The advantages of all those “spurious” splittings and supplementary lines due to interactions with phonons by resonance, is obvious: they provide new analytical information, they can be used for characterization of the nature of the ligands bonded to the rare earths. This is especially impo~~t in the context of the great development of the use of rare earths as analytical probes in biological systems.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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