Doping-induced transition from double exchange to charge order in La_1-xCa_xMnO₃ near x=0.50

We study a series of the perovskite manganite compounds La_1-xCa_xMnO₃ closely spaced in doping near x=0.50 where the ground state undergoes a doping-dependent metal-insulator transition (MIT) from a metallic to a charge-ordered state. A significant difference is found between the value of x and the Mn⁺⁴ fraction which defines the metal-insulator phase boundary. In the immediate doping regime of the MIT, the resistivity data can be quantitatively described by a model based on the coexistence of two carrier types: nearly localized carriers in the charge-ordered state exhibiting variable-range hopping and a parasitic population of free carriers which is tunable by either magnetic field or stoichiometry.