LATTICE-DYNAMICAL ASPECTS OF ANTIFERROELECTRIC PHASE TRANSITION IN ND4D2PO4

Temperature-dependent quasi-elastic neutron scattering has been observed in the paraelectric tetragonal phase of ND4D2PO4 in the vicinity of the Z point of the Brillouin zone, i.e., at (h,0,l) where h+l is odd. The data are interpreted in terms of a condensing highly damped optic-phonon mode which initiates the transition to the antiferroelectric state. The intensity contours of the quasi-elastic scattering in the (a*,c*) plane are roughly elliptical in shape and are centered at the Z points of the extended Brillouin zone. The major axis of the intensity distribution is parallel to the c* direction and is about four times larger than the a*-oriented minor axis, thus suggesting a polarization fluctuation perpendicular to c*. As the temperature is lowered, the integrated intensities at the Z points increase as T(T-T0). The value for T0 of 195°K compares more closely with the actual transition temperature (Tn=234°K) than that obtained by dielectric constant measurements (T080°K). The inelasticity of the scattering is 0.70.1 meV at 244°K and increases linearly with temperature to 1.80.2 meV at 304°K. An interpretation of the scattering at the Z point in terms of an overdamped phonon mode results in a damping constant independent of temperature and a temperature dependence of the frequency of the form A2K(T-T0). In the antiferroelectric phase, the Z points become permissible Bragg peaks and quasi-elastic scattering is not observed. © 1969 The American Physical Society.