Landau coefficients and the critical electric field in a KDP crystal

Analytical expressions for the Landau coefficients of deuterated potassium dihydrogen phosphate (DKDP) crystal are obtained in a four-particle cluster approximation. The critical electric field strengths calculated within the phenomenological and microscopic theories for DKDP are compared to experimental data. Calculations of the critical electric field in potassium dihydrogen phosphate (KDP) crystal in the cluster approximation with allowance for proton tunneling on hydrogen bonds allows us to determine the tunneling constant and the geometric factor associated with the change in the effective dipole moment upon deuteration. Good agreement with the experimental data is obtained only for positive values of the long-range interaction parameter.