Enhanced reactivity of domain walls in

possible new high temperature superconducting phase was recently in WO3:Na. We have examined the reaction sodium vapour and WO3, and compared the phases by the reaction to previously known WO3 phases. using light microscopy and electron microprobe analysis, domain walls from the interior of the crystal are shown to have a much Na content than bulk material after reaction with Na vapour. indicates preferential transport along the domain walls. The is very similar to a reduction reaction of WO3 in which twin walls lose oxygen preferentially. Oxygen twin walls are superconducting with ≈3K.