Evolution of crystalline phases and morphotropic phase boundary of the (Bi,Na)TiO3-(Bi,K)TiO3-BaTiO3 lead-free ceramic system

In this work, high density Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-BaTiO3 (BNBK) lead-free ceramics with nominal compositions crossing the morphotropic phase boundary region were prepared and their crystalline phases were investigated by a combination of the X-ray diffraction and Raman spectroscopy techniques, at room temperature. Results show that the BNBK82 sample has a single tetragonal phase (P4mm) and the BNBK91 and BNBK93 samples, single rhombohedral phase (R3c), while the BNBK88 and BNBK86 compositions showed coexistence of the above two phases with a not previously reported additional tetragonal P4bm phase. The Raman spectra deconvolution, for the BNBK86 and BNBK88 samples, also demonstrates the coexistence of rhombohedral and tetragonal phases, with phase fractions in very good agreement with the Rietveld refinement results, evidencing these compositions are located in the MPB region of the BNBK system. Results in this work suggest the MPB region proposed by Takenaka, for the BNBK system, is more appropriate than the one proposed by Trelcat.