A material of Formula (I) is provided

MyTxQvW1-vOz-tJt  (I)

where:

T represents one of tin, lead, antimony and germanium, T being present in the interstitial spaces or voids of the lattice,

M represents one or more species, each selected from the group consisting of (i) metals other than T, and (ii) polyatomic ionic species, said polyatomic species having an ionic radius of no more than 2 Å, M being present in the interstitial spaces or voids of the lattice,

W is tungsten,

O is oxygen,

Q represents one or more element having an oxidation state of at least +4, Q, if present, occupying a lattice point of W,

J represents one or more non-metallic element anion of chemical valence −1, J, if present, occupying a lattice point of O,

v is from 0 to 1.0, t is from 0 to 3.0, y is non-zero and up to and including 0.32, x is non-zero and up to and including 0.32, and z is from 2.5 to 4, provided that x+y≤0.33.