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The authors acknowledge financial support from CICyT (Spain) through Grant Nos. PB96-0875, AEN99- 0990, FPA2000-0956, and FPA2000-1252. V.M.-M. was supported by E.C. Contract No. HPMF-CT-2000-00450.Considering the disorder caused in manganites by the substitution Mn→Fe or Ga, we accomplish a systematic study of doped manganites begun in previous papers. To this end, a disordered model is formulated and solved using the variational mean-field technique. The subtle interplay between double exchange, superexchange, and disorder causes similar effects on the dependence of T_(C) on the percentage of Mn substitution in the cases considered. Yet, in La_(2/3)Ca_(1/3)Mn_(1-y)Ga_(y)O_(3) our results suggest a quantum critical point (QCP) for y ≈ 0.1–0.2, associated to the localization of the electronic states of the conduction band. In the case of La_(x)Ca_(x)Mn_(1-y)Fe_(y)O_(3) (with x = 1/3,3/8) no such QCP is expected.engjournal articlehttp://doi.org/10.1103/PhysRevB.66.104430http://journals.aps.org/open access5351-73Colossal magnetoresistancePhase-diagramSystemsModelPerovskitesTransitionBehaviorPhysicsFe.Física (Física)Física-Modelos matemáticos22 Física