  In a recent paper (A.E. Glazounov and A.K. Tagantsev, Phys.Rev. Lett. 85,
2192 (2000)) it has been proposed a phenomenological model of the dynamic
non-linear response in relaxor ferroelectrics based on the averaging of the
Landau-Khalatnikov equation over the distribution of the relaxation times. The
purpose of the present comment is to clarify the physical assumptions involved
in the Glazounov-Tagantsev approach and show that models based on the
Landau-Khalatnikov equation can not describe properly the dynamic response in
relaxor ferroelectrics.
