Ferroelectric phase transition in (NH4)2BeF4

The possibility of molecular distortion type (MD-type) microscopic mechanism (recently proposed for (NH4)2SO4) and the role of BeF42- ion in the phase transition of ammonium fluoberyllate are examined by analysing crystal structural data in both, the paraelectric and ferroelectric phases. The results show that BeF42- ion attains a dipole moment in the FE phase and contribute significantly to the spontaneous polarisation (Ps). The computed value of Ps due to NH4+ and BeF42- ions (0.21 μC/cm2) matches satisfactorily with the experimental results. The nature of the phase transition is found to be mainly of MD-type. The soft mode of the transition is proposed as a mixed mode having the internal mode of BeF42- ion and translatory modes of NH4+ ions. (author)