Proposal of a mechanism for the paraelectric-ferroelectric phase transition in (NH4)2BeF4 via the formation of solid solutions with Ti2BeF4

(NH4)2BeF4in its normal phase presents an incommensurate phase between -90 and -96°C and becomes ferroelectric at temperatures below -96°C. The Tl2BeF4has been synthesized and its crystal structure has been resolved. The existence of solid solutions of general formula (NH4)2-xTlxBeF4(2>x>0) is confirmed. From the study of these solid solutions by X-ray diffraction we can say that, the substitution of ammonium by thallium precludes the paraelectric-ferroelectric phase transition of (NH4)2BeF4. The displacement of the NH4 groups is hindered by thallium which disrupts the lattice periodicity. On the other hand the same thallium does not seem to have such an important influence on the twist of BeF4groups. The presence of thallium in (NH4)2BeF4precludes the phase transition, which indicates that the NH4+ ions play a most important role in triggering the phase transition.