Studies of incommensurate barium sodium niobate

Abstract

New studies are reported on three different phases of barium sodium niobate: In the lowest-temperature “lock-in” phase from 0 – 40K, three different space group symmetries have been predicted theoretically (J. Schneck, Thesis, Univ. Paris, 1982) –P4₂bc, P4nc, and Bb. The present neutron study shows that P4nc is correct. In the incommensurate phase from 543K to 582K we find a dynamic central mode with width ca. 20 GHz that peaks in Raman intensity at 565K, in the middle of the IC phase within IK of the lq to 2q (orthorhombic to tetragonal) transformation inferred from the electron microscopy data (Feng Duan and Pan Xiaoqing, 1986). The q² dependence of central mode linewidth yields a value of zero for the gap energy (upper limit of 8 GHz), and the novel technique of measuring the ratio of intensities for forward and back-scattering yields a value of (12+3)% for the ratio of incommensurate modulation to the lattice constant (i.e., ca 0.2A distortion). This dynamic central mode may be related to the anti-phase boundary roughening transition predicted by Rice et al. Finally, the highest temperature transition, at Curie temperature 855K, is studied by boch dynamic central mode light scattering and a newly discovered thermal lens effect; these two techniques yield quite different diffusivities, indicating that more than one diffusion process is important near T_C.