Enhanced reactivity of domain walls in WO3 with sodium

A possible new high temperature superconducting phase was recently reported in WO3:Na. The have examined the reaction between sodium vapour and WO3, a nd compared the phases formed by the reaction to previously known WO3 phase s. By using light microscopy and electron microprobe analysis, domain walls from the interior of the crystal are shown to ha ire a much higher Na cont ent than bulk material after reaction with Na vapour. This indicates prefer ential transport along the domain walls. The result is very similar to a re duction reaction of WO3 crystals in which twin walls lose oxygen preferenti ally. Oxygen deficient twin walls are superconducting with T-c approximate to 3 K.