Structural perspective on the anomalous weak-field piezoelectric response at the polymorphic phase boundaries of (Ba, Ca)(Ti, M)O-3 lead-free piezoelectrics (M = Zr, Sn, Hf)

Although, as part of a general phenomenon, the piezoelectric response of Ba(Ti-1 M-y(y))O-3 (M = Zr, Sn, Hf) increases in the vicinity of the orthorhombic (Amm2)-tetragonal(P4mm) and orthorhombic (Amm2)-rhombohedral (R3m) polymorphic phase boundaries, experiments in the last few years have shown that the same phase boundaries show significantly enhanced weak-field piezoproperties in the Ca-modified variants of these ferroelectric alloys, i.e., (Ba, Ca)(Ti, M)O-3. So far there is a lack of clarity with regard to the unique feature(s) which Ca modification brings about that enables this significant enhancement. Here, we examine this issue from a structural standpoint with M = Sn as a case study. We carried out a comprehensive comparative structural, ferroelectric, and piezoelectric analysis of the Amm2 phase in the immediate vicinity of the P4mm-Amm2 phase boundaries of (i) Ca-modified Ba(Ti, Sn)O-3, as per the nominal formula (1-x)BaTi0.88Sn0.12O3-(x)Ba0.7Ca0.3TiO3 and (ii) without Ca modification, i.e., Ba(Ti1-ySny) O3. We found that the spontaneous lattice strain of the Amm2 phase is noticeably smaller in the Ca-modified counterpart. Interestingly, this happens along with an improved spontaneous polarization by enhancing the covalent character of the Ti-O bond. Our study suggests that the unique role of Ca modification lies in its ability to induce these seemingly contrasting features (reduction in spontaneous lattice strain but increase in polarization).