The role of nearest neighbour anharmonic couplings in the phase diagram ofbetaine calcium chloride dihydrate (BCCD)

We present the results of mean-field calculations on a symmetry based semim icroscopic model for betaine calcium chloride dihydrate. The model takes in to account the three degrees of freedom associated with the lowest energy p honon branches as relevant variables and experimental inelastic neutron sca ttering data and observed transition temperatures are used to fit the coupl ing constants of the model potential. Although the method does not allow us to predict incommensurate phases, the results show the failure of previous models based on continuous variables to reproduce the observed phase diagr am. The insertion of a new term in the model potential corresponding to anh armonic interactions between nearest neighbours is shown to be crucial to s tabilize some experimentally observed commensurate phases.