MODULATION AND INCOMMENSURABILITY IN A SUPERIONIC CONDUCTOR

The structure factor S(q) is calculated for the Frenkel-Kontorova model at high temperatures. Application to the 1-d ionic conductor K-Hollandite gives an explanation of diffuse X-ray scattering in terms of a modulation phenomenon. The positions of the diffuse sheets are predicted precisely from the stoichiometric composition. It is confirmed thereby that the mobile K+ ions behave like a chain with spacing b = 1/n where n is the concentration. The disorder is characterized as short-range crystalline order of wavelength b, competing with and modulated by the framework potential with a different period a. © 1979.