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

Abstract We have recently successfully doped the La site in T -La 2 CuO 4 with Ce by a molecular beam epitaxy. The resultant T -La 2− x Ce x CuO 4 was insulating up to x  = 0.06. This is in sharp contrast to T ′-La 2− x Ce x CuO 4 , which is superconducting with a T c of about 30 K. In order to clarify the difference between T -La 2− x Ce x CuO 4 and T ′-La 2− x Ce x CuO 4 , we estimate the valences of Ce in T -La 2− x Ce x CuO 4 and T ′-La 2− x Ce x CuO 4 by using Vegard’s law. The estimated valence of Ce is ⩾+3.84 in T -La 2− x Ce x CuO 4 and ∼+3.78 in T ′-La 2− x Ce x CuO 4 . The result indicates that electron doping is actually achieved in T -La 2− x Ce x CuO 4 , 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.