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OFF-CENTER FLUORIDE DEFECTS IN ALKALI-BROMIDES AND -IODIDES

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HAL Id: jpa-00216924

https://hal.archives-ouvertes.fr/jpa-00216924

Submitted on 1 Jan 1976

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OFF-CENTER FLUORIDE DEFECTS IN ALKALI-BROMIDES AND -IODIDES

J. Wahl, A. Gongora, F. Lüty

To cite this version:

J. Wahl, A. Gongora, F. Lüty. OFF-CENTER FLUORIDE DEFECTS IN ALKALI- BROMIDES AND -IODIDES. Journal de Physique Colloques, 1976, 37 (C7), pp.C7-271-C7-271.

�10.1051/jphyscol:1976764�. �jpa-00216924�

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JOURNAL DE PHYSIQUE Colloque C7, suppliment au no 12, Tome 37, Dtcembre 1976, page C7-271

OFF-CENTER FLUORIDE DEFECTS IN ALKALI-BROMIDES AND -IODIDES (*)

J. WAHL (**), A. GONGORA (***) and F. LUTY

Physics Department, University of Utah, Salt Lake City, Utah, U. S . A.

Abstract. - A comprehensive study was performed in the sodium-, potassium-, and rubidium- bromides and -iodides for possible off-center effects of substitutional F- defects, using dielectric and Kerr-effect techniques. While normal centro-symmetric positions were found for F- in NaI, KBr and RbBr, a paraelectric behaviour

-

scaling with the F- content - was detected in KI, RbI and NaBr [I], indicating off-center positions for the fluoride ions. A

<

110

>

symmetry for the off- center defects was verified in all three cases by the anisotropy of the Kerr-effect and by dielectric measurements under stress of various symmetry. The temperature dependence of the optical and dielectric results yields a Curie-law (EIT) behaviour of the dipole alignment. The reorientation kinetics of the F- off-center ions was studied in detail by the rise- and decay-time of the Kerr-effect and by dielectric loss. The results show in all three cases predominant 90° reorientation at the lowesf temperatures, which changes for higher Tto a 60° reorientation process. A discussion in terms of the elastic dressing model will be given.

(*) Supported by NSF Grant # DMR 74-02516-A02.

(**) Present address : MPI Festkorperforschung, Stuttgart, Germany.

(***) Present address : Instituto Politecnico National, Mexico.

References

[I] < 110 > Off-center properties were previously reported for NaBr : F- by ROLLEFSON, R. J., Phys. Rev. B 5 (1972) 3235.

DISCUSSION

F. BBNIERE. - I guess that the reported contents in J. WAHL.

-

We d o not observe any clusters or fluoride ions (10-3-10-4) are over the solubility limit precipitates. The dipole contribution to the dielectric at the temperatures under consideration (1-10 K). constant and the Kerr effect scale with concentration.

Does this not give rise t o any additional effects in the A single Debye loss peak is observed which has less dielectric loss or dynamic Kerr effect measurements than 2 orders of magnitude halfwidth. The transient due to formation of clusters or precipitates ? Kerr-effect can be fitted with a single relaxation time.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976764

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