[en] Full text: Bi_0.5Na_0.5TiO₃-Bi_0.5K_0.5TiO₃-BaTiO₃ (BNBK) lead-free piezoelectric ceramic is an ecofriendly candidate to substitution of lead zirconate titanate-based piezoelectric in technological applications. The morphotropic phase boundary (MPB) for BNBK ceramic system is under discussion and it has two controversial proposals [1,2]. With an aim studying the MPB region, in this study, x(Bi_0.5Na_0.5TiO₃) - 0.72(1 - x)Bi_0.5K_0.5TiO₃ - 0.28(1 - x)BaTiO₃ (abbreviated as BNBK100x, with x = 0.82, 0.86, 0.88, 0.91 and 0.93 mol %) piezoelectric ceramics were synthesized by solid state reaction followed by conventional sintering. The crystalline phases evolution of BNBK100x samples were investigated by a combination of the X-ray diffraction (XRD) and Raman spectroscopy techniques, at room temperature. The XRD patterns revealed a complex perovskite structure without spurious phases for all the studied compositions. Rietveld refinements of XRD patterns indicated the BNBK82 sample has a single tetragonal phase (P4mm) and the BNBK91 and BNBK93 samples, single rhombohedral phase (R3c). Raman spectra for all the samples shows wide Raman peaks, denoting a high degree of chemical disorder, as a consequence of four ions (Bi²⁺, Na⁺, K⁺ and Ba²⁺) occupying the same crystallographic A site of the perovskite structure. The deconvolution of the Raman spectra confirmed the results obtained by XRD for the BNBK82, BNBK91 and BNBK93 samples with a single phase of P4mm and R3c symmetries, respectively. For the BNBK86 and BNBK88 samples, the deconvolution of the Raman spectra revealed a mixture of rhombohedral and tetragonal phases, evidencing that these compositions are located in the MPB region of the BNBK system. References: [1] Nagata Hajime, et. al., Jpn. J. Appl. Phys., Vol. 42, N.12, pp. 7401–7403 (2003). [2] Trelcat, Jean-François et. al., Ceramics International, Vol. 38,N. 4,pp.2823 – 2827 (2012). (author)