DIELECTRIC PROPERTIES OF ACu_3Ti_4O_(12)-TYPE PEROVSKITES

摘要

The synthesis, ceramic microstructure and electrical properties of ACu_3Ti_4O_(12)-type perovskites where A = Ca (CCTO), Na_(1/2)Bi_(1/2) (NBCTO) and Bi_(2/3) (BCTO) are presented. Phase-pure samples of CCTO and NBCTO were prepared from nominal stoichiometric compositions, however, a non-stoichiometric starting composition with 6 mol % TiO_2 deficiency was required to obtain phase-pure BCTO (BCTO'). All samples exhibit the so-called 'giant permittivity' effect at room temperature with ε' > 4000 at radio-frequencies. Impedance Spectroscopy (IS) reveals all ceramics to be electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries. The giant permittivity effect is attributed to an internal or grain boundary barrier layer capacitance mechanism.rnThe presence of the stereochemically active Bi~(3+) ion on the A-site has a dramatic effect on the magnitude of the intrinsic, or bulk relative permittivity (ε_r) for ACu_3Ti_4O_(12)-type perovskites. ε_r ～ 100 for CCTO but exceeds 200 for NBCTO and BCTO'. ε_r for all compounds is significantly higher than that calculated using the Clausius-Mossotti equation, implying the presence of a polarisation mechanism in addition to the polarisability of the constituent ions. This mechanism is attributed to incipient ferroelectricity.