Proton ordering and spontaneous polarization in mixed KDP-ADP crystals

Temperature dependences of the transverse permittivity of mixed K1−x(NH4)xH2PO4 crystals (x≈0, 0.04, 0.09, and 0.19) are experimentally investigated with the aim of determining the parameter PA that characterizes proton ordering in the region of the ferroelectric phase transition. A comparison of the temperature dependences of the PA parameter and spontaneous polarization shows that the spontaneous polarization is preceded by a partial ordering of protons in all the compositions studied. It is found that the crystals with a high ammonium content are characterized by a weaker effect of the lattice polarization on the proton ordering.