The effects of neodymium content and site occupancy on spontaneous polarization of epitaxial (Bi _4-xNd _x)Ti ₃O ₁₂ films

Epitaxial (110) (Bi4-x Ndx) Ti3 O12 (x=0-0.73) films with a variety of neodymium content were grown by metal-organic chemical-vapor deposition, and the effects of neodymium content and site occupancy on spontaneous polarization along the a axis were systematically investigated. Raman spectra revealed that the neodymium is selectively substituted for the bismuth (A site) in the pseudoperovskite layer up to x=0.45; however, with further increasing neodymium content, neodymium is also incorporated into other bismuth sites in the (Bi2 O2) 2+ layer. The neodymium substitution led to a continuous decrease in the Curie temperature. The saturated coercive field showed slight dependency on the neodymium content, while the remanent polarization significantly improved to a maximum of 34 μC cm2 at x=0.35. The estimated value of the spontaneous polarization for the epitaxial films with x=0.26-0.45 exceeded 50 μC cm2, i.e., the reported value for pure Bi4 Ti3 O12 single crystal, but markedly decreased at x=0.73. The degraded spontaneous polarization appeared to be associated with the site occupancy of the neodymium at the two different kinds of bismuth sites.