THE STOICHIOMETRIC DEPENDENCE OF THE MODULATION WAVE VECTOR IN THE INCOMMENSURATELY MODULATED STRUCTURES OF EPSILON-LIXV2O5 AND EPSILON'-LIXV2O5

Electron diffraction was used to study the stoichiometric dependence o f the modulation wave vector in the incommensurately modulated structu res of epsilon- and epsilon'-LixV2O5. For epsilon-LixV2O5 (0.32 less t han or equal to x less than or equal to 0.52) and epsilon'-LixV2O5 (0. 52 less than or equal to x less than or equal to 0.80) the space group of the basic structure is Pmmn, and the superspace group characterizi ng the one-dimensionally modulated structure is found to be Pmmn (0 ga mma 1/2). The stoichiometry dependence of the modulation wave vector q can be described by a staircase function, with gamma being constant a t gamma = 0.435(5) for epsilon-LixV2O5 A second plateau has been obser ved in the stability field of epsilon'-LixV2O5 for 0.550 less than or equal to x less than or equal to 0.575 with gamma = 0.465(5). For the stoichiometry range 0.575 less than or equal to x less than or equal t o 0.707, gamma varies continuously between gamma = 0.465(5) and gamma = 0.565(5). In the present paper we will show that the system LixV2O5 can be described by an extended Frenkel-Kontorova model. Despite its e xtreme simplicity in the one-dimensional case, the model exhibits most of the features necessary for understanding the variation of the depe ndence of the modulation wave vector on the lithium stoichiometry.