Ammonium dihydrogen phosphate and potassium dihydrogen arsenate impurity effects on the dielectric behavior of potassium dihydrogen phosphate crystals

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Abstract
The dielectric constants of two types of mixed potassium dihydrogen phosphate crystals, one with ammonium dihydrogen phosphate (ADP) impurities and the other with potassium dihydrogen arsenate (KDA) impurities, have been measured at various impurity concentrations. The change of transition temperature and the dielectric behavior of the mixed crystals obtained from the dielectric-constant measurements are related to the role of impurities in the phase transition of the crystals. The transition temperatures of both mixed crystals decrease linearly as the impurity concentration increases, but the rate of decrease is strongly dependent on the type of impurity. The change of transition temperature in the mixed crystals is more sensitive to the concentration of ADP impurities than of KDA impurities. Domain-wall mobility in the region below the transition temperature remains high for crystals with ADP impurities, but becomes low for crystals with KDA impurities. A modified proton-lattice coupled model introduced here has proved to be a good theoretical model for explaining our experimental results and the roles of impurities in the crystals.