STOICHIOMETRIC YBA2CU3O7 IS OVERDOPED

There has been much disagreement about the origin of the 90 K transition temperature plateau in YBa2Cu3O7-delta. It is argued that the ''90 K plateau'' for YBa2Cu3O7-delta is simply the peak of the parabolic curve of T(c) as a function of hole concentration and that when delta almost-equal-to 0 then 1-2-3 is slightly overdoped by 0.03 to 0.04 holes per copper. This is demonstrated from studies on altervalent substitutions, oxygen deficiency, comparison with 1-2-4 and 2-4-7, thermoelectric power, normal state susceptibility, structural data and bond valence sums, muon spin relaxation, Raman studies on the delta-dependence of the superconducting energy gap, and the pressure dependence of T(c). It is also shown that cation-stoichiometric LaBa2Cu3O7 should have T(c max) almost-equal-to 100 K.