First-Principles Theoretical Explanation of Incommensurate Behavior inRb2ZnCl4

We have carried out an ab initio theoretical study of the potential energy and molecular dynamics of ${\mathrm{Rb}}_2$Zn${\mathrm{Cl}}_4$. These calculations demonstrate that the incommensurate transition in this compound is caused by the relaxation of imperfect-hexagonal spirals of highly unstable Zn${\mathrm{Cl}}_4^{2-}$ ions. This leads to angstrom-size displacements with essentially zero energy change. We argue that such "latent" (or imperfect) symmetry could be the general cause of incommensurate transitions in insulators.