The effect of heterovalent doping on the stability and properties of multiferroic Aurivillius phases

The effect of magnetoactive cations on the stability and properties of Aurivillius phases (1) Bi5Ti2(TiFe)1−x(NiNb)xO15, (2) Bi4Ti3−2.5x(Nb1/2Fe1/2)x(Nb2/3Ni1/3)1.5xO12, (3) Bi3Ti1−2.5xNb1+1.5x(Fe1/2Ni1/2)xO9, (4) Bim+1Fem−3Ti(3−x)(Ni1/3Nb2/3)xO3m+3 has been investigated. The range of existence of solid solutions was found to be limited to the values of x = 0–0.10 (1), 0–0.3 (2), and 0–0.2 (3). Although the solid solutions exist in the whole concentration range studied (x = 0–0.07) in the series (4), the stability is limited to the value of m = 6–7. The unit cell volumes increase with increasing x, which is expected due to the ionic radii of the dopant cations that are larger than those of the substituted cations. The coexistence of piezoelectric and magnetic properties was observed in the samples of solid solutions (1) and (4): the values of the piezoelectric coefficient d33 and piezoelectric voltage coefficient g33 reach 7 pC/N and 3.5 mV m/N, respectively; the saturation magnetization Ms and remanent magnetization Mr increase with increasing x, while the coercivity remains almost unchanged. Thermal studies indicate phase transitions in Bi5Ti2(TiFe)1−x(NiNb)xO15, and Bim+1Fem−3Ti(3−x)(Ni1/3Nb2/3)xO3m+3 at ~ 300–400, 750 and 840 °C. Additionally, these samples demonstrate a magnetodielectric effect of up to ~ 50% when applying a DC magnetic field of ~ 1 T in the temperature range of ~ 340–350 °C that is supposed to be a ferromagnetic Curie point.