Phase diagram of the mixed crystals betaine phosphate and betaine phosphite: Experimental and Monte Carlo results

We propose a microscopic model that is capable of describing the features o f both deuterated and nondeuterated betaine phosphate (BP)-betaine phosphit e (BPI) solid solutions in the region of small concentration of BP, The siz e mismatch between the BP and BPI structural units, which leads to a drasti cal change of the nearest-neighbor interactions in the vicinity of single B P impurity, was taken into account in the model. We show that the model qui te accurately reproduces the experimentally observed abrupt drop of the fer roelectric phase transition temperature. The onset of the glass state given by the model also agrees well with the experimental results.