RAMAN-SPECTROSCOPY OF INCOMMENSURATE BA2NANB5O15

We have performed Raman studies of the E-symmetry phonon modes in barium sodium niobate in the tetragonal ferroelectric phase above 582 K and measured carefully their splitting into pairs of B1 and B2 symmetry in the incommensurate (IC) phase between 543 and 582 K. For both heating and cooling cycles the data satisfy mean-field predictions with B1-B2 frequency splitting proportional to the order parameter. The data show that the incommensurate-incommensurate transition at 565 K (heating) does not alter the orthorhombic symmetry; that is, it is not a true 1q-2q transition, and tetragonal symmetry does not set in until the incommensurate-normal transition at 582 K. This is compatible with very recent electron microscopy data (Verweft, Van Tendeloo, and Van Landuyt), which show that the incommensurate-incommensurate transition near 565 K produces only short-range order in a direction perpendicular to the long-range 1q modulation, and with x-ray studies of this transition (Kiat, Toledano, and Schneck). An analogous situation is observed just below the incommensurate-commensurate transition at 105 K: Here the C11 and C22 elastic coefficients have minima at 105 K, signaling a phase transition, but they do not become equal (C11 = C22) until approximately 90 K, indicating that there is an incommensurate-incommensurate transition from orthorhombic to tetragonal near 90 K. The 90-K IC-IC transition seems quite analogous to the 565-K IC-IC transition, but electron microscopy shows that these phases are not reentrant.