Low temperature properties of incommensurate barium sodium niobate

We have performed dielectric and Brillouin spectroscopic measurements on incommensurate Ba 2 N a Nb 5 O 15 from ca. 15 K to room temperature and have related our measurements to earlier studies of birefringence, 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 C cm 2 1 to a modulated structure, which appears to be a reentrant phase also found at atmospheric pressure from 565 K to 582 K. Preliminary neutron diffraction results will be presented showing the 2q IC nature of BSN below ∼ 100 K. Finally, the role of pressure in stabilizing the different phases in BSN will be discussed.