THE PARAELECTRIC-INCOMMENSURATE PHASE-TRANSITION AND CRYSTAL-STRUCTURE OF THE MIXED-CRYSTALS K-X(NH4)(2-X)SEO4

A study on the mixed crystals of general formula K-x(NH4)(2-x)SeO4 has been carried out, using thermal analyses and x-ray diffraction on pow der and single-crystal samples at different temperatures. The replacem ent of the potassium ion by the ammonium ion is only possible for x gr eater than or equal to 1.45. The transitions paraelectric-incommensura te-ferroelectric are only observed for x greater than or equal to 1.76 and the temperature of the paraelectric-incommensurate transition inc reases when the value of x decreases. The replacement of the K+ ion by the ammonium ion is only produced in the K(1) site. The different beh aviour of the mixed crystals and the K2SeO4 compound is explained from the structural results.