Low temperature elastic and dielectric properties of incommensurate barium sodium niobate

We have performed dielectric and Brillouin spectroscopic measurements on incommensurate (IC) Ba2NaNb5O15 from ca. 15 K to room temperature and have related our measurements to earlier studies of birefringence and x-ray and neutron crystallography, with special emphasis upon the structural phase transitions near 105 K and 40 K. The 105 K transition is from Bbm2 to a modulated structure, which appears to be a reentrant phase also found at atmospheric pressure from 565 K to 582 K. Such a reentrant IC/C/IC sequence is predicted in the phenomenological theory of Ishibashi. The lock-in phase below 40 K is tetragonal and probably of space group P4nc (structural refinements of preliminary neutron scattering data are in progress).