Valence of Ce in T- and T′-La2−xCexCuO4

We have recently successfully doped the La site in T-La2CuO4 with Ce by a molecular beam epitaxy. The resultant T-La2−xCexCuO4 was insulating up to x = 0.06. This is in sharp contrast to T′-La2−xCexCuO4, which is superconducting with a Tc of about 30 K. In order to clarify the difference between T-La2−xCexCuO4 and T′-La2−xCexCuO4, we estimate the valences of Ce in T-La2−xCexCuO4 and T′-La2−xCexCuO4 by using Vegard's law. The estimated valence of Ce is ⩾+3.84 in T-La2−xCexCuO4 and ∼+3.78 in T′-La2−xCexCuO4. The result indicates that electron doping is actually achieved in T-La2−xCexCuO4, but that this compound remains an insulator. This contradicts the general belief that the doping with either electrons or holes conducts or superconducts parent compounds.