  We measured transverse and longitudinal dielectric permittivities of KDP and
DKDP in the piezoelectric resonance frequency region under the hydrostatic
pressure. The transverse permittivity decreases with pressure in the
paraelectric phase and increases in the ferroelectric phase. The pressure
dependence of the transverse permittivity of deuterated DKDP is well described
by the presented theory. The longitudinal permittivity of a pure KDP exhibits
several peaks in the vicinity of the transition point. Upon increasing
hydrostatic pressure, the peaks get closer and may merge.
