The complex dielectric constant in KDP is determined a t w = 4.62 cm-l (d = 2.16 mm) for E 11 c and E 1 c between 100 and 295 K by measurement of the complex reflection coefficient using a Michelson interferometer. At room temperature the complex dielectric function g(w) is determined for E 11 c and E 1 c in the far infrared using an asymmetric

Abstract. The far-infrared reflection spectra of betaine phosphite single crystals in the three crystallographic directions are investigated in the range from 5 to 600 cm-' at temperatures between 203 and 323 K using a dispersive Fourier spectrometer. The dielectric function E(2) is evaluated from the complex amplitude reflection coefficient P ( J ) measured. In the spectrum polarized along the ferroelectric direction a soft mode of relaxation type is observed which accounts for a large part of the relevant static dielectric constant at T, = 210 K. Among the oscillator modes, which generally exhibit normal temperature dependence, there is a heavily overdamped mode at 58 cm-', polarized along the crystallographic uI axis, with anomalous damping behaviour. The damping constant decreases nearly linearly with temperature up to 323 K which is close to T = 355 K where a second phase transition is known to exist.

The infra-red spectra of LiF, NaC1 and CsBr are interpreted according to the dispersion theory of 1BO~N and HVANG 1. This theory takes into account anharmonic terms in the lattice potential and leads to a frequency-dependent damping which describes the absorption. This damping function is first obtained by evaluatingthe empirical data of the optical constants and is then compared with an approximate computation based on the frequency spectrum of lattice vibrations. The result of this comparison shows that theory and experiment correspond qualitatively very closely, especially with respect to the secondary structure in dispersion.Jetzt 12hysikalisches Institut der Universit~it Freiburg i. ~Br.

Abstract, The complex dielectric function C(w) along the a axis of PbHPO, single crystals is determined from reflection measurements at frequencies between 70 and 225 GHz in the paraelectric and the ferroelectric phase near the transition temperature T,. = 310 K. The data are complemented by a remeasurement of the properties of the lowest far-infrared mode. The mm-wave data can be described by a simple Debye formula. The relaxation time exhibits critical slowing down, and the corresponding relaxation frequency varies between about 2 and 60 GHz near T,. Proton tunneling appears to play no essential role in this dielectric mechanism related to proton ordering.

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