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Dielectric Relaxation and Structure Change at the Ferroelectric Phase Transition in Single Crystals BaTi03 and PbMg1/3NA2,3e3Dielectric dispersion at the ferroelectric phase transition shows up a s a decrease of the dielectric constant at maximum and a shift of that maximum towards higher temperatures when the measuring frequency increases. The above mentioned phenomenon is remarkable at diffuse phase transitions a s in PMN (KIRILLOV, ISUPOV) and PLZT (CARL, GEISEN). However, it is still detectable in such an abrupt phase change a s in BaTi03 (DEWANOV). In the last case the dielectric dispersion has been groundlessly interpreted a s a shift and an order change of the ferroelectric phase transition. The aim of our paper is to discuss the dispersion phenomenon and to reveal some general features for diffuse and abrupt phase transitions.The structure change and the dielectric dispersion have been investigated simultaneously by X-ray difkaction and dielectric measurements. The sufficiently high resolution in temperature allows to reveal common features in the temperature dependence of the lattice cell volume (in the phase coexistence regionthe mean volume) in both crystals: BaTiO3 ( Fig. 1) and PMN (Fig. 2). The curve's V(T) minimum for BaTiO3 corresponds to the emergence of the tetragonale phase, detectable by X-ray (Fig, 1). It is the beginning of the two-phase region. The end of the region or the moment of the disappearance of the cubic phase detectable by X-ray corresponds to the maximum volume value on the curve V(T) f o r BaTiO3. The shaded region of Figure 1 is the two-phase region for BaTi03. Small changes of the lattice parameters in PMN (Fig. 2) make it impossible not only to detect the two-phase region but also to fix the fact o f the phase coexistence at all. At the same time it is possible to detect the region of the two phase coexistence by using the analogical features in the volume change at the phase transition in both crystals. The temperature interval between the two extremes on the curve V(T) evidently corresponds to the two-phase region. The supposed coexistence region is shaded in Figure 2. So the analogy between the structure change and the dielectric dispersion in both crystals is clearly seen in Figure 1 and Figure 2. The differences are mainly quantitative. Consequently the dielectric constant at a sufficiently high frequency reaches its maximum value well before the temperature is lowered enough to shift the crystal in the two-phase region. In BaTiOB the dielectric constant reaches its maximum outside the two-phase region already at the frequency of 80 Hz. The limits for detecting a new phase are: concentrations more than one percent and the linear dimensions of nucleations bigger than 50 nm. Consequently the dielectric dispersion is not connected with the small regions or the nucleations of the ferroelectric phase exceeding these parameters. When lowering the frequency the maximum of the dielectric constant shifts to the lower temperatures, and there is a possibility for it to penetrate in...

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