Hydrostatic pressure influence on dielectric permittivity of KH2PO4 and   KD2PO4 in the piezoelectric resonance region
  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. 
R. R. Levitskii, A. G. Slivka, A. P. Moina, P. M. Lukach, A. M. Guivan