SOFT PHONON VALLEY NEAR THE TRANSITION TO AN INCOMMENSURATE PHASE IN BARIUM SODIUM NIOBATE J. Schneck, J.C. Toledano, J. Aubree, B. Joukoff and C. Joffrin J. Phys. Colloques, 42 C6 (1981) C6-734-C6-736 DOI: 10.1051/jphyscol:19816215