Comment on "Origin of low-frequency dielectric dispersion in KH2PO4 and RbH2PO4 ferroelectric crystals" - art. no. 026101

Kim and Rim [Phys. Rev. B 59, 13 509 (1999)] have studied the frequency dep endence of the complex permittivity of KH2PO4 and RbH2PO4 in the ferroelect ric phase. In the low-frequency regime (f<10 kHz) they found a dispersion c onsisting of at least three distinctive components. Considering the tempera ture dependence of those relaxational modes they claim that each of them is associated with domain wall motion, and not with a heat diffusion central peak (HDCP). The aim of this Comment is twofold: First we want to demonstra te the importance and influence of the ac measuring field on the dielectric properties, in particular in the temperature range T< T-f = 100 K< T-c of domain freezing. Second, by investigating the dielectric nonlinearity in th e limit of small ac fields we exclude HDCP contributions in the low-frequen cy part of the spectrum.