Dielectric properties of donor-doped polycrystalline SrTiO₃

Abstract

Various donor dopants such as Y³⁺ and Nb⁵⁺ were incorporated in SrTiO₃ in amounts ⩽2.5 mol % during sintering in air at 1450° C for ⩽ 15 h. Dense ceramic materials with grains of optimum uniformity and largest size were obtained when the cation stoichiometry was adjusted to allow for charge compensation of the donor ions by strontium vacancies. For donor levels ≳ 0.3 mol%, the dielectric constant measured at 25° C and 1 kHz was up to two orders of magnitude higher than that of undoped strontium titanate. The increase in permittivity, however, was dependent on grain size, was influenced by the method of electrode application, and was suppressed by the presence of ≲ 0.1 mol% Mn in the ceramic. These observations, together with data obtained from electrical measurements at other temperatures and frequencies, were consistent with interpretation of the anomalously high dielectric constant as a boundary-layer effect resulting from semiconducting grains and weakly insulating grain-surfaces. It is suggested that donor doping influences the electrical properties of SrTiO₃ mainly by increasing the volatility of oxygen from the grains during sintering, and by decreasing the rate of re-oxidation during cooling.