Valence of ce in T- and T′-La2-xCexCuO4

We have recently successfully doped the La site in T-La2CuO4 with Cc 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 T-c 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 similar to+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. (C) 2007 Elsevier B.V. All rights reserved.