Non-equilibrium defects and the statics and dynamics of incommensurate crystals

The statics and dynamics of the incommensurate crystals Ba2NaNb5O15, BaMnF4, and RbH3(SeO3)2 are discussed. In Ba2NaNb5O15 spectroscopic evidence for an unexpected high-speed diffusion process near T1 is presented; the diffusion constant is 1.3 ± 0.2 cm2/s and is 65 times the known thermal diffusivity. It is tentatively assigned to diffusion of discommensurations. In RbH3(SeO3)2 and BaMnF4 the asymmetry of sound velocities Vij with respect to interchange of subscripts is shown to arise not from spatial dispersion, as proposed by Dvorak and Esayan, but from antisymmetric contributions to the stress tensor due to local rotations; the explanation hypothesized is similar to that of Nelson and Lax for the photoelastic tensor, only for the special case of crystals having screw axes. Large thermal hysteresis in BaMnF4 within the IC phase follows the pattern observed initially in other incommensurates by Hamano et al. The high- and low-temperature endpoints of the IC phase in BaMnF4 satisfy one possible trajectory in phase space from the extended Ising model theory of Yamada and Hamaya: 2a*/5 ≥ q0 ≥ 5a*/13. Optical activity and monoclinic distortion angle in BaMnF4 fail to satisfy intrinsic theories, and both thermal expansion and the presence of “nearly commensurate” phases also seem compatible with defect models.