The vibrational spectra of lithium niobate, barium sodium niobate and barium sodium tantalate

Crystallographic studies of Ba 2 NaNb 5 O 15 have shown that the possible space groups are D 6 2 , C 11 2v and D 19 2h . The coincidences between the infrared and Raman spectra support the conclusion, based on the ferroelectric properties of the crystal, that the space group is C 11 2v . The anomalous tantalum compound has a spectrum compatible with a similar structure. The vibrations are classified as internal modes of the Mo 6 octahedron (M=Nb, Ta) and lattice translations involving motion of the cation. Force constants are calculated for internal modes using the Wilson FG matrix method and for lattice modes using the Born-von Karman theory.