Detection of slow modulation wave diffusion in incommensurate barium sodium niobate by 93Nb 2D exchange NMR

93Nb two-dimensional (2D) exchange NMR was used to study ultraslow motions in incommensurate barium sodium niobate (BSN) in the temperature range 500 - 560 K. These 2D exchange data provide strong evidence for diffusionlike motion of the modulation wave in BSN. Such motions in BSN had been previously detected by using a complementary NMR experiment, the Hahn spin-echo decay. However, since the Hahn spin-echo decay technique cannot always distinguish unambiguously between different motional mechanisms, this 2D experiment provides definitive confirmation of modulation wave diffusion and rules out other possible motional mechanisms for the Hahn spin-echo decay in this system.