Structural ordering and Raman scattering in M5Nb3OF18 (M = NH4, K, Rb) ferroelectric crystals

M5Nb3OF18 (M = NH4, K, Rb) ferroelectric crystals were studied by vibration al spectroscopy and x-ray diffraction at different temperatures. Thermally stimulated structural transformations in these crystals are often accompani ed by chemical changes. Rb5Nb3OF18 undergoes a reversible ferroelectric pha se transition at T-c= 600 K and an irreversible transformation accompanied by breaking of Nb-O-Nb bridge bonds, leading to the rupture of oxyfluoronio bate chains and yielding new chemical compounds. The mechanism of the latte r transformation is elucidated by calculating vibrational spectra for vario us configurations of oxyfluoroniobate chains.