Errors inherent in electron probe microanalysis of YBa₂Cu₃O₇ films and the atomic composition of films with a resolution of 2 microns have been found. Critical temperature values have been determined from the temperature dependence of the electron beam induced voltage (EBIV). Having plotted these results on a triple phase diagram of oxides, we found two tie lines with highest T_c (ridges) and two tie lines with lowest T_c (valleys). A mechanism of cation defect formation was proposed which accounts for the presence of this topology. The mechanism was verified by reconstructing cation defects observed in TEM images.