New Sequence of Structural Phase Transitions inNaxWO3

Using precise lattice-parameter measurements, "cubic" ${\mathrm{Na}}_x\mathrm{W}{\mathrm{O}}_3(0.62\mathrm{}<~x<~0.94)$ has been found to transform from the ideal perovskite structure to three other related forms. The transition sequence is described in terms of a single vectorlike order parameter involving static in-phase tilts of W${\mathrm{O}}_6$ octahedra and is the zone-boundary analog of the ferroelectric series of transitions in BaTi${\mathrm{O}}_3$, the [001] tilt component arising from the condensation of the $M_3$ mode. Unexplained features of structural and Raman studies can now be interpreted.