A SUB-LATTICE MODEL FOR THE INCOMMENSURATE RAANSITION IN A_2BX_4-TYPE CRYSTALS

摘要

In order to describe a normal-incommensurate transition in A_2BX_4-type crystals, a two-sublattice model is presented, which is modified from the discrete four-sublattice model proposed recently by Ishibashi and Janssen (J. Phs. Soc. Jpn. 69 (2000) 3870). The new model describes the softening of an optical phonon branch connected to an acoustic branch in the extended zone scheme. The dispersion relation of the soft mode in K_2SeO_4 can be reproduced by terms up to the second-neighbor interactions, although the third-neighbor interactions are usually considered to be necessary.