ANOMALOUSLY LARGE BORN EFFECTIVE CHARGES IN CUBIC WO3

Within density-functional theory, we compute the Born effective charge s of tungsten trioxyde in its reference cubic phase (defect-perovskite structure). For the tungsten atom, the effective charge tensor is iso tropic, with Z(W)=+12.51. For the oxygen atoms, the two independent c omponents of the tensor, corresponding, respectively, to a displacemen t of the atom parallel or perpendicular to the W-O bond, have the valu es Z(O parallel to) = -9.13 and Z(O perpendicular to)* = -1.68. Z(W)* and Z(O parallel to) are anomalously large with respect to the nomin al ionic charges (+6 on W and -2 on O), but compatible with the Born e ffective charges found in related ABO(3)-perovskite compounds.