Theory of phase transition inHg3−δAsF6

It is shown that the critical behavior near the chain-ordering phase transition in ${\mathrm{Hg}}_{3-\delta }\mathrm{As}{\mathrm{F}}_6$ is described by an $n=4\mathrm{}$ vector model previously used to characterize the structural phase transition in Nb${\mathrm{O}}_2$ and the magnetic transitions in Ho, Dy, and Tb. The order parameter describes the condensation of an incommensurate mass density wave (MDW). It is predicted that the fluctuations of this order parameter drive the transition first order.