Study on the mixed crystals (NH4)(2-x)KxSO4

A study on the mixed crystals of formula (NH4)(2-x)KxSO4 (1 > x > 0) has been carried out, using thermal analyses and x-ray diffraction on powder and single-crystal samples at different temperatures. It is shown that mixed crystals with x < 0.34 have a transition from the paraelectric to the ferroelectric phase; T-c decreases as the value of x increases. The orientation of the sulphate ion with respect to the crystallographic axes depends on the value of x. Mixed crystals with the paraelectric-ferroelectric transition turn around the b axis when the temperature decreases in the paraelectric phase, while in mixed crystals without a transition the orientation of the sulphate ion does not alter when the temperature varies. The transition is displacive, increasing the value of the parameter a without a soft mode. This is deduced from a study of the crystal structures, the translational and libration tensors and a density map of the sum of the bond valences.