DOUBLE-WELL POTENTIAL ENERGY SURFACES IN Cs2CdBr4 and Cs2ZnI4

We discuss the apparently anomalous behavior of the Pnam structure A(2)BX(4) compounds Cs2CdBr4 and Cs2ZnI4. At least for the former, it appears that while its first structural transformation is to an incomensurate phase, modulated along (a) over right arrow, it subsequently "locks in" to a (q) over right arrow =0 modulation leaving the periodicity along (a) over right arrow unchanged. We address this problem using lattice dynamics, and computer, molecular dynamics (CMD) employing parameter-free Gordon-Kim potentials. These appear to reproduce the transition temperatures in Cs2CdBr4, although the lowest temperature transformation remains ambiguous. We argue that the failure to form a tripled superlattice along (a) over right arrow reflects the fact that the incommensurate modulation is almost midway between (q) over right arrow = 0 and (q) over right arrow = (a) over right arrow*/3. Thus, given the substantially lower static energy of the (q) over right arrow = 0 modulation, the system favors the latter.