Spontaneous formation of zigzag chains at the metal-insulator transition in the β-pyrochlore CsW₂O₆

A paramagnetic metal to nonmagnetic insulator transition at T _MIT=210 K is reported for the β-pyrochlore oxide CsW ₂O₆, accompanied by a first order structural transition that creates 110 oriented chains in the pyrochlore lattice. Comparison of CsW₂O₆, which has 1 electron per 2 W sites, to the fully d0 analog CsTaWO₆ shows that the transitions are electronically driven. Corefinement of high resolution synchrotron x-ray and neutron diffraction data shows that the structural distortion that creates the W chains cannot be attributed to simple charge or orbital ordering. Density functional theory calculations suggest that the phase transition is driven by a sharply peaked electronic density of states near the Fermi energy in the cubic β-pyrochlore phase. A further electronic instability is required to create the insulating ground state.