The term morphotropic phase boundary (MPB) usually refers to a concertation region where different crystal structures with a low energy barrier can coexist [1]. The most common and well understood MPB exists between the polar rhombohedral (R3m) and tetragonal (P4mm) structures in lead zirconate titanate [2]. While such transitions are not allowed by symmetry rules, recently it has been explained that they are mediated by a formation by transitionary monoclinic phase. This phase coexistence region has attracted a lot of attention from scientists as it has been discovered that such compounds show large sensitivity to external stimuli. However, while currently dominant piezoelectric materials with MPB (PZT, PMN-BT) are extremely efficient they contain lead which is highly toxic. Due to this drawback and overall shift towards green and sustainable chemistry, which was partly induced by anti Pb legislations, a search for a greener alternative is ongoing.
Over the last couple of decades many different compounds were analyzed in search of a possible alternative. BiFeO3, a lead free multiferroic compound with perovskite type structure, was found to have a polar to non-polar morphotropic phase boundary upon doping with RE ions (La – Lu) [3]. The undoped compound is characterized by polar active rhombohedral structure described by R3c space group as well as magnetically active sublattice formed by iron ions which makes it to be room temperature multiferroic with Currie temperature of ~1100 K and Neel temperature of 643 K [1]. There are some drawbacks specific for the initial compound, viz. bismuth ferrite is characterized by a large leakage current as well as difficulties in preparation of single phase materials. Doping of the initial compound with rare earth ions has shown to at least partly solves these problems as well causes a polar (R3c) to non-polar (Pnma) structural transition, with an intermediate PbZrO3-like anti-polar (Pbam) phase [1]. While the concentration range of MPB varies depending on the type of RE ion, the stabilization of the anti-polar phase has only been possible with RE ions up to Sm within the MPB which is very narrow among similar oxide systems ~1% [4]. Moreover, correct determination of morphotropic phase boundary is quite difficult because it is strongly dependent on preparation technique and post synthesis treatment [3]. It should be noted, that structural characterization of MPB region performed based on X-Ray diffraction analysis and microscopic measurements such as Raman or IR spectroscopy can give different results as compared to local measurements such as TEM or PFM, which poses a great problem for further scientific research [5].
Hence the main aim of this work was to investigate the morphotropic phase boundary region in Sm-doped BiFeO3 compounds prepared by ethylene glycol assisted sol-gel technique, viz. to itemize the structural phase transition from the polar rhombohedral to the anti-polar orthorhombic followed by the non-polar orthorhombic phase driven by the dopant content using both local scale and microscopic measurement techniques. The obtained results highlight the differences specific for the MPB assuming the data obtained by microscopic measurements as compared to local scale structural results.