An Sb3Nb3O13 [Sb(III)2Sb(v)Nb(v)3O13] phase having a defect pyrochlore structure has been prepared by heating a sol–gel derived powder in oxygen. X-ray powder diffraction results indicate that Sb3Nb3O13 has a face-centered cubic structure, S.G. Fd3m(227), with a refined unit cell parameter a =10.4965(1) Å, calculated density Dx=4.893, four molecules per unit cell (Z), and a calculated figure of merit SS/FOM F30=92.4(0.009,38).