The stoichiometric dependence of the modulation wave vector in the incommensurately modulated structures of and

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Abstract
Electron diffraction was used to study the stoichiometric dependence of the modulation wave vector in the incommensurately modulated structures of - and . For and the space group of the basic structure is Pmmn, and the superspace group characterizing the one-dimensionally modulated structure is found to be . The stoichiometry dependence of the modulation wave vector q can be described by a staircase function, with being constant at for . A second plateau has been observed in the stability field of for with . For the stoichiometry range varies continuously between and . In the present paper we will show that the system can be described by an extended Frenkel - Kontorova model. Despite its extreme simplicity in the one-dimensional case, the model exhibits most of the features necessary for understanding the variation of the dependence of the modulation wave vector on the lithium stoichiometry.