The Ferroelectric Phase Transition and Modulated Valence Electrons in the Incommensurate Phase of Ammonium Tetrafluoroberyllate

Abstract The incommensurate structure of (NH 4 ) 2 BeF 4 is determined from single-crystal x-ray diffraction at T = 175 K. Comparison of this structure with the low-temperature structure shows the ferroelectric phase transition to correspond to the simultaneous development of local and global polarisations. The modulations of the electrons in the chemical bonds are made visible by the Maximum Entropy Method with the modulated structure as superspace prior. Keywords: Maximum Entropy Methodaccurate charge densityammonium tetrafluoroberyllate Acknowledgment Financial support has been obtained from the Deutsche Forschungsgemeinschaft (DFG) and FCI.