Structural, magnetic, and dielectric studies of the Aurivillius compounds SrBi5Ti4MnO18 and SrBi5Ti4Mn0.5Co0.5O18.

We have successfully synthesized the Aurivillius compounds SrBi₅Ti₄MnO₁₈ and SrBi₅Ti₄Mn_0.5 Co_0.5O₁₈ using a modified Pechini method. Both samples have an orthorhombic structure with the space group B2cb. The valence state of Mn is suggested to be +3 and the doped Co ions exist in the form of Co²⁺ and Co³⁺ based on the results of x-ray photoelectron spectroscopy. The sample SrBi₅Ti₄MnO₁₈ exhibits a dominant paramagnetic state with the existence of superparamagnetic state as evidenced by the electron paramagnetic resonance results, whereas SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ undergoes a ferrimagnetic transition at 161K originating from the antiferromagnetic coupling of Co-based and Mn-based sublattices, and a ferromagnetic transition at 45K arising from the Mn³⁺-O-Co³⁺ (low spin) interaction. The sample SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ exhibits two dielectric anomalies. One corresponds to a relaxor-like dielectric relaxation which follows the Vogel-Fulcher function and the other dielectric relaxation obeys the Arrhenius law arising from the collective motion of oxygen vacancies. In addition, the sample SrBi₅Ti₄Mn_0.5Co_0.5O₁₈ exhibits a magnetodielectric effect caused by the Maxwell-Wagner effect because of the conductivity of the sample. This is demonstrated by the fact that the activation energy in dielectric loss process is close to that for dc conductivity and the magnetodielectric effect is sensitive to the measured frequency. [ABSTRACT FROM AUTHOR]