Infrared absorption of OH- doped LiF

Solid State Communications, Vol. 32,pp. 551—552. Pergamon Press Ltd. 1979. Printed in Great Britain. INFRARED ABSORPTION OF OW DOPED LiF K. Guckelsberger* Service des Basses Temperatures, Centre d’Etudes Nucleaires de Grenoble, 85X, F38041 Grenoble Cedex, France and H.-R. Zelsmann~ Laboratoire de Spectrometrie Physique Universite Scientifique et Medicale de Grenoble F38000 St Martin d’Heres, France (Received 11 June 1979 by E.F. Bertaut) The low temperature IR spectra of OW doped UF shows no additional absorption in the vicinity of the main stretching band. This suggests that it is an “on-center” paraelectric system and that this feature suppresses most manifestations of this type of impurities. THE HYDROXYL ION in alkali halides is the subject of several investigations concerning its paraelectric [1, 2] and optical properties [2—4].However, little information is available concerning the properties of OW and 0D-ions in fluorides. These systems are interesting from a theoretical point of view since they represent a very “clean” system in the sense that the 0H-ions have a nearly identical mass and ionic radius compared to the fluorine ion they replace in the lattice and differ only by their permanent electric dipole moment. Unfortunately, experimental investigations are difficult for a number of reasons. In LiF, the electronic UV absorption of OW is near the fundamental absorption edge [5] and thus difficult to observe. The stretching mode is very accessible at 3731 cm’ for OW [6] and at 2741 cm’ for 0D [7]. No electro-caloric effect has been observed [8] and only thermal conductivity measurements [7, 9] provide information up to now. In this letter we wish to report briefly on IR-investigations obtained in connection with thermal conductivity work described elsewhere [9]. Specimens used were preparedby a diffusion treatment of “optical grade” Harshaw single crystals as described in [7]. Infrared absorption measurements were made in an improved CODERG spectrometer in high vacuum (10—s torr) with a typical resolution of ±3 cm~ as verified with water vapour lines and a reproducibility of better than ±1 cm~in the spectral range of interest. Further details are described elsewhere [10]. *

Present address: Max-Planck-Institut für Festkorperphysik Heisenbergstr, 1, D7000 Stuttgart 80, BRD. Present address: Departement de la Recherche Fondamentale, Centre d’Etudes Nucleaires de Grenoble, 85X, F3804l Grenoble Cedex, France. 551

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T (K) Fig. 1. The full width at half maximum W(OH) of the main stretching band of the OW ion in UF as a function of temperature. We have measured five specimens which had all a very similar OW-concentration, due to the particular doping process used. Wet chemical analysis indicated a concentration of 9 ±3 x 1018 cm3 for all of them. The temperature dependence of the full width at half maximum of the OW stretching mode in LiF, W(OH) is given in Fig. 1. Data points are a composite of all specimens measured. At 9K, recordings taken with a spectral slit-width of 5 cm1 and 2 cm~respectively gave a constant value of W(OH) = 8 cm~.We believe thus that this value is the upper limit of the linewidth at this temperature. One may conclude that its absolute value does not vary appreciably below about 100 K. Between 100 K and room temperature W(OH) increases proportional to T1~3as in other alkali h~.1ides[4]. A slight trend of the frequency of maximum absorption

552

INFRARED ABSORPTION OF OW DOPED UF

towards higher wavenumbers with decreasing temperature is observed. However, the rapid broadening of the peak at high temperatures makes a precise determination of this shift difficult. Within the limits of precision we could not detect a change of the absorption coefficient below 100 K for a given specimen. This means, together with a constant W(OH) a constant area under the absorption peak which remained remarkably symmetric down to about 8K, the lowest specimen temperature available We were unable to detect any new absorption feature between 2000 and 5000 cm~,neither in the vicinity of the main peak nor at frequencies some several 100 cm~ removed from it. Assuming a linewidth for such a new feature similar to that of the main stretching band, the photometric resolution of our spectrometer allows us to exclude absorption peaks about one hundred times weaker than that at 3731 cm1. The comparatively high value of W(OH) at low temperatures suggests very strong coupling to the lattice as observed in thermal conductivity [9]. The late onset of line.broadening at 100 K only is related to the absence of the so-called “non-Devonshire” lines as inferred from thermal conductivity data and their apparent lack in our low temperature spectra. This is expected since they are assumed to arise from an off-center motion of the center-of-mass of the hydroxyl ion around the lattice site in other alkali halides. In fluorides, and especially in LiF, the geometry should not allow such a displacement and no evidence is found for it. The apparent absence of the librational mode is more surprising. To explain the observed isotope shifts and oscillator strengths of these modes, Klein and coworkers [4, 11] derive the model of a torsional harmonic oscillator, again with a displaced center of mass to

Vol. 32, No.7

account for details but a librational mode should be observable even without it. It seems therefore that in LiF the librational band is either very broadened by strong lattice coupling or already “frozen out” at ternperatures where it becomes observable in other, less restricted surroundings. In summary, these results confirm further that the LiF(OH) is indeed a model for the pure reorientational motion of an electric dipole at a lattice site. The rather startling conclusion seems therefore to be that OW as a paraelectric impurity could only be studied in detail because it is in most cases also an “offcenter” impurity which thus becomes the dominant feature of the system.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

REFERENCES V. Narayanamurti & R.O. Pohi, Rev. Mod. Phys. 42, 201 (1970). S. Kapphan & F. Luty, J. Phys. Chem. Solids 35, 621Kapphan (1974). & F. Luty,J. Phys. Chem. Solids 34, S. 969 (1973). B. Wedding & M.V. Klein, Phys. Rev. 177, 3, 1274 (1969). P. Thacher, Thesis, quoted in [1]. T.G. Stoebe, J. Phys. Chem. Solids 28, 1375 (1967). K. Guckelsberger, K. Neumaier & H.-R. Zelsmann, Phys. Lett. 31A, 7, 397 (1970). W.E. Bron & R.W. Dreyfus,Phys. Rev. Lett. 16, 5, 165 (1966). K. Guckelsberger, Internal Report no SBT 74/354 (1974). H..R Zelsmann & Y. Marechal, Chem. Phys. 5, 367 (1974). M.V. Klein, B.M. Wedding & M.A. Levine, Phys. Rev. 180,902 (1969).