THE STRUCTURAL Re-ARRANGEMENT IN CaF2 DUE TO TRIVALENT RARE EARTH IMPURITY IONS

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THE STRUCTURAL Re-ARRANGEMENT IN CaF2 DUE TO TRIVALENT RARE EARTH IMPURITY

IONS

D. Kiro, W. Low, E. Secemski

To cite this version:

D. Kiro, W. Low, E. Secemski. THE STRUCTURAL Re-ARRANGEMENT IN CaF2 DUE TO TRIVALENT RARE EARTH IMPURITY IONS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-950-C1-951. �10.1051/jphyscol:19711341�. �jpa-00214376�

JOURNAL DE PHYSIQUE Colloque C I , supplLment au no 2-3, Tome 32, Fhrier-Mars 1971, page C 1

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THE STRUCTURAL Re-ARRANGEMENT

IN CaF2 DUE TO TRIVALENT RARE EARTH IMPURITY IONS

Microwave Division, Department of Physics, The Hebrew University, Jerusalem, Israel

R6sum6. - Nous presentons le spectre d'ENDOR des premiere, deuxitme et troisitme couches de Ce3+, Nd3+, Yb3-+ et Us+ dans CaFz avec des F- ou H- cornme ions compensant la charge. Une etude detaillee du spectre nous permet de determiner les dtplacements des ions seconds voisins F- autour de leurs positions d'equilibre. Nous pouvons partiel- lement en deduire la distorsion de la premike couche des ions F-. La comparaison du recouvrement des fonctions d'ondes 4f et 5f sur les ions F- est rdalisee grlce aux spectres de Nd3+ et U3+.

Abstract. - We report the ENDOR spectra of the first, second and third shells of Ce3+, Nd3+, Yb3+ and U3+ in CaF2 with F- or H- as charge compensating ions. From a detailed analysis of the spectra, we can plot out the displa- cement of the next nearest F- ions from their usual equilibrium positions. We can also infer to some extent the distortion of the first shell of the F- ions. Comparison of the overlap of the 4f and Sf wavefunctions on the F- Ions is realized from the Nd3+ and U3+ spectra.

There are many types of imperfections in nearly ideal approximation for distant F- ions. The nearest F- crystals. One type of imperfection that often exists is shell shows considerable deviation from the dipole- that cation of different valence state substitutes for the dipole approximation [l, 21. The spin Hamiltonian is of cation in the original regular lattice. This must affect the form

the surroundings to some extent, or at least the

nearest and possible next nearest shells. Ordinary X = &H.S

+ C

t .

the lattice when the substitutional impurity is only a

Like any second rank tensor this can be written to minor constituent. The method employed here is to use consist of two components Ts and Tp where

ENDOR spectroscopy as a tool to study these small rearrangements.

Calcium fluoride is nearly an ideal lattice for this purpose. It is possible to substitute for the calcium ion various trivalent and isoelectric divalent rare earth ions. In some cases the trivalent and divalent ions are situated at the cubic site, leaving the point symmetry undisturbed. In other cases E. S. R. spectra of tri- valent rare earth ions have indicated lower point symmetries. A particular case is that when an F- ion is located at the adjacent F - cube, thus setting up a tetragonal field at the rare earth site. Another possi- bility is the location of an interstitial H- ion rather than F- ion in the adjacent cube. Lowcr symmetries arising from other charge compensating mechanisms are possible but have not been studied by ENDOR methods. The question arises what rearrangements of ions result from either the excess charge on the tri- valent ion or the interstitial charge compensating ion.

It is this problem which can be partially solved using ENDOR methods.

The following procedure is used. One compares in detail the ENDOR spectra and optical spectra of Yb3+

and TmZf in CaF,. Both have f t 3 configuration, one hole in the complete f14 shell. It is found that from the second shell onwards the ENDOR spectra can be accounted for by the dipole-dipole interaction to better than 3 % for the second shell and even better for F- shells lying at larger distances. The inference is, therefore, that dipole-dipole approximation is a good

(*) Supported in part by the U. S. National Bureau of Stan- dards through Grant NBS (G)-38.

+

.

The first shell for Yb3+ shows much larger values of Tp than accounted for by the dipolar contribution, thus indicating strongly a covalent contribution. A partial explanation is that the excess charge on Yb3+

causes a contraction of the lattice in the first shell giving rise to a larger dipole-dipole interaction as well as an additional covalent contribution, and larger crystal field. The fact that the dipole-dipole interaction accounts so well for the outer lying shells is a strong indication that the distortion of the lattice is only local and does not propagate.

The fact that the dipole-dipole approximation is such a good approximation is the starting point for the analysis of the distortions caused by the interstitial ion.

Taking the F- ion in the third shell one can compute by giving a least square fit the displacement of the rare earth ions towards the interstitial ions. Similarly the whole rearrangement of the second F- shell can be computed to fair accuracy. In general there is slight displacement of the F- second shell towards the F- interstitial ion, moving along the same direction as the rare earth ion.

The first shell spectra are more difficult to evaluate because of the uncertainty in the covalent contribution and its dependence on distance. We made a tentative assumption that the 111 F - ion has approximately the same displacement as the 113 F - ion. This gives them sufficient parameters to disentangle the dipolar

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711341

THE STRUCTURAL Re-ARRANGEMENT IN CaFz DUE TO TRIVALENT RARE EARTH IMPURITY IONS C 1

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and covalent contributions. From this then we can infer, by using the so-called t< calculated dipole-dipole interaction D, the displacement of the F- ions in the first shell. The resulting picture is that the 4 F - ions separating the rare earth ions and the interstitial ions are moving outwards, whereas the 4 F - ions next to the rare earth ions move inward towards the rare earth ions. The interionic distances, however, of the rare earth -F- ion is kept constant. Measurements were made also on the interstitial H- ion and the same picture emerges except that the 4 F - ions surrounding

the H- ions have moved out more than in the case of the interstitial F - ion [3, 4, 51.

Measurements were made of Ce3+, Nd3+ and U3+

in CaF,. Similar measurements are in progress on SrF,. The distortions caused by Ce3+ and Nd3+-F- pairs are very similar. In the case of Nd3+(f3) the covalent contributions are much larger, probably caused by n bonding. A comparison of Nd3+ (4f3) and U3+(5f3) clearly indicates that the 5f wave function extends much further into the lattice. The resulting distortions are somewhat larger [6, 71.

References

[I] KIRO @.), LOW (W.), Phys. Rev. Letters, 1968, 20,101 1. [5] WRO (D.), Low (W.), SCHIPPER (D. J.), Phys. Letters, [2] BAKER (J. M.), BLAKE (W. B. J.), COPLAND (G. M.), 1968, 2 9 A , 586.

Proc. Roy. Soc. A., 1968, 309, 119. [6] SECEMSKI (E.), KIRO (D.), LOW (W.), SCHIPPER (D. J.), [3] KIRO (D.), Low (W.), Phys. Letters, 1968, 29 A, 537, Phys. Letters, 1970, 31 A, 45.

[4] BAKER (J. M.), DAVIES (E. R.), HURRELL (J. P.), Proc. r]] KIRO @.), Ph. D . Thesis (Jerusalem 1970).

Roy. Soc. A., 1968, 308, 403.