Anomalously large Born effective charges in cubic WO3

Within density-functional theory, we compute the Born effective charges of tungsten trioxyde in its reference cubic phase (defect-perovskite structure). For the tungsten atom, the effective charge tensor is isotropic, with Z(W)*=+12.51. For the oxygen atoms, the two independent components of the tensor, corresponding, respectively, to a displacement of the atom parallel or perpendicular to the W-O bond, have the values 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 nominal ionic charges (+6 on W and -2 on O), but compatible with the Born effective charges found in related ABO(3)-perovskite compounds.