Coexistence of tetragonal and orthorhombic phases in K0.5Na0.5NbO3 – BiAlO3 single crystals modified by MnO2: Effects of non-180° domains and interlayers

Results of a comparative study on heterophase structures are discussed for lead-free ferroelectric K 0.5 Na 0.5 NbO 3 – BiAlO 3 single crystals modified by MnO 2 where the molar concentration x of MnO 2 is varied from 0 to 0.009. In these single crystals, the tetragonal P 4 mm and orthorhombic Amm 2 phases coexist at room temperature, and changes in heterophase structures are studied in the very narrow x range for the first time. A model of a heterophase sample contains vast regions of two coexisting phases, split into non-180° domains, and thin interlayers separating these regions. The interlayer is single-domain and belongs to either the tetragonal or orthorhombic phase, and this leads to a series of alternative heterophase structures. Planar interfaces between the coexisting phases and between the heterophase regions and interlayers obey conditions for a complete stress relief when changing volume fractions of the 90° domain types of the tetragonal phase in a wide range. These conditions hold for two main versions of heterophase structures (with either the 90° or 60° (120°) domain types in the orthorhombic phase) at specific domain orientations in the interlayer that influences volume fractions of the morphotropic phases to a certain degree. Evaluated volume fractions of the orthorhombic phase at x = 0–0.009 are consistent with experimental data.