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To date, all existing literature on the so-called 'high permittivity' perovskite oxide CaCu3Ti4O12 (CCTO) in the form of ceramics, single crystals and thin films show the grains (bulk) to exhibit semiconductivity with room temperature, RT, resistivity of similar to 10-100 Omega cm. Here we show that CCTO grains can be highly resistive with RT resistivity >1 G Omega cm when CCTO ceramics are processed at lower temperature (700 degrees C). With increasing processing temperature, the semiconducting CCTO phase commonly reported in the literature emerges from grain cores and grows at the expense of the insulating phase. For sintering temperatures of similar to 1000-1100 degrees C, the grains are dominated by the semiconducting phase and the insulating phase exists only as a thin layer grain shell/grain boundary region. This electrical microstructure results in the formation of the so-called Internal Barrier Layer Capacitance (IBLC) or Maxwell-Wagner mechanism that produces the commonly reported high effective permittivity at radio frequencies in dense ceramics. The relationship between Cu loss at elevated processing temperatures and the transformation of the grain resistivity from an insulating to semiconducting state with increasing processing temperature is also discussed

Modulus spectroscopy of CaCu3Ti4O12 ceramics: clues to the internal barrier layer capacitance mechanism