FAR INFRARED SPECTRA OF RbH2P04 CRYSTALS

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FAR INFRARED SPECTRA OF RbH2P04 CRYSTALS

M. Remoissenet, R. Gard

To cite this version:

M. Remoissenet, R. Gard. FAR INFRARED SPECTRA OF RbH2P04 CRYSTALS. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-106-C2-107. �10.1051/jphyscol:1972234�. �jpa-00214970�

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C2-106 M. REMOISSENET ET R. GARD

FIG. 3. - Partie imaginaire de la fonction dielectrique pour une polarisation parallele a c.

g a m e de temptrature. Mais le modele d'oscillateur harmonique amorti per met en premiere approximation de rendre compte des resultats. Deux parametres ajus- tables sont utilists : y la fonction d'amortissement et o, la frtquence de rtsonance de l'oscillateur non amorti ; on tient compte de la constante ditlectrique ~ ( 0 ) du cristal c contraint >> et des valeurs de A o . Les calculs

FIG. ^4.- Variation de w~ et A o en fonction de la tempkrature.

prtliminaires montrent que oo et y dependent de la temperature au voisinage de la transition, k 300 OK ils ont respectivement pour valeurs 60 cm-I et 100 cm-l.

La valeur ainsi calculCe de o, est plus faible que pour KDP 131 et elle varie en fonction de la tempirature d'une maniere difftrente de la loi en (T - T,)%.

D'autres calculs et mesures sont en cours et au voisinage de la transition, les risultats concernant les deux modes non amortis qui correspondent 2 o, et oh,

seront publiis ulterieurement.

FAR INFRARED SPECTRA OF RbH,P04 CRYSTALS

Abstract. - The far infrared reflectivity spectrum of RbHzP04 crystals is measured from 10 to 400 cm-1 in the temperature range from 300 OK to 140 OK. The complex dielectric constant is derived by a Kramers-Kronig dispersion relation. A preliminary analysis of the results shows that they are far from a Debye type relaxation, but the damped harmonic oscillator model fits them. A low frequency mode is found which is temperature dependent.

polarization fluctuations, piezoelectric and elastic constants [7].

Introduction. - The dielectric dispersion spectra of the angle of incidence is 120. The reflectivity measure- potassium dihydrogen phosphate, KDP, has been ments are made at several points from 3000K to investigated using far infrared [I], [2] and Raman [3]

I

0 100 203 303 4M) ^>

Experiments. - The polarized reflection measure-

ments are made with a far infrared R1lC FS720 Fourier FIG. ^1.- Far infrared reflectivity spectra of RbHzP04 crystal spectrophotometer. A wire grid polarizer is used and in c polarization.

spectroscopy. Theoretically, the occurrence of protons tunnelling, and their interaction with lattice distortions, 60.

predict that a collective excitation [4], [5] having a soft mode behaviour [6] is probable. The present

research is intended to make a detailed temperature _40, dependence measurement or the far infrared reflectivity

spectra of rubidium dihydrogen phosphate, RbDP, which is isomorphous with KDP. This work is a part of

a more complete study of RbDP which includes _20.

moreover the measurements of electrocaloric effect.

A R%

?/t//t

I ;! I - 300'~

: ^1. ^, ..-- ^154'K

:,, ^\%,

\. ,,, /,' ', I

, , :i

' ^{\ ~ %}

\;< , B ^I ^:I

5 .

!: u,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972234

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FAR INFRARED SPECTRA OF RbH2P04 CRYSTALS 107 140 OK for the wave numbers from 400 cm" to

10 cm-l. The samples for a and c polarization are cut from RbH,PO, crystals grown in our laboratory 181.

Results. - The far infrared reflection spectra are shown in figures 1 and 2. The complex dielectric

FIG. 2. - Far infrared reflectivity spectra of RbH2P04 crystal in a polarization.

constant is derived by a Kramers-Kronig dispersion relation, In the c polarization the imaginary part ~ " ( o ) of the complex dielectric constant (Fig. 3) exhibits two peaks centred at about oM

-

40 cm-I and

o h

-

180 cm-l, at 300 OK. The low frequency mode

FIG. 3. - Imaginary part of complex dielectric constant in c polarization.

w, is only slightly temperature dependent down to 165 OK. But it moves down quickly between this temperature and T, = 147 OK, the transition temperature which was determined from our elastic and dielectric measurements [7]. This decrease of frequency is accompanied by a decrease in the half width A o of the ~ " ( o ) spectrum (Fig. 4). The mode oh is also temperature dependent.

FIG. 4. - Temperature dependence of o~ and Am.

A preliminary analysis of the results shows that they are not consistent with a Debye relaxation model over the whole temperature range. But the damped harmonic oscillator model, fits the results. It requires the following two parameters : y the damping constant and wo the resonant frequency of the undamped oscillator. The values of the static dielectric constanfof the clamped crystal ~ ( 0 ) and the values A o are used.

Preliminary calculation shows that o, and y are temperature dependent near the transition point, their values are approximately 60 c m - b n d 100 cm-' at 300 OK.

This value of w0 is lower than that for KDP [3]

and its temperature dependence is different from the law

-

^(T^-^T,)%.

Further experiments and calculations are now in progress in the vicinity of the transition temperature, and the results concerning the two undamped modes which correspond to o, and o h will bepublished elsewhere.

References

[I] BARKER (A. S.) and TINKHAM (M.), Phys. Rev., 1963, [5] KOBAYASHI (K. K.), J. Phys. Soc. Japan, 1968,24,497.

38,2252. [6] COCHRAN (W.), Adv. Phys., 1960,9,387.

[2] SUGAWARA (F.) and NAKAMURA (T.), J. Phys. Soc.

Japan, 1970,28, 158. [7] PIERRE (C.), DUFOUR (J. P.) and REMOISSENET (M.), [3] KAMINOW (I. P.) and DAMEN (T. C.), Phys. Rev. Letters, to be published in Solid State Comm.

1968, 20, 1105. [S] REMOISSENET (M.), DESVIGNES (M.) and MARECEK (V.), [4] BLINC (R.), Phys. Rev. Letters, 1967,261, 8. Kvist und Tech., 1970, 5, 535.