Structural perspective on the anomalous weak-field piezoelectric response at the polymorphic phase boundaries of (Ba,Ca)(Ti,M)O₃ lead-free piezoelectrics ( M=Zr , Sn, Hf)

Abstract

Although, as part of a general phenomenon, the piezoelectric response of Ba (Ti_1−yMy )O₃ (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)O₃ . 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)O₃ , as per the nominal formula (1−x) BaTi_0.88Sn_0.12O₃ −(x)Ba_0.7Ca_0.3TiO₃ and (ii) without Ca modification, i.e., Ba(Ti_1−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).