Structure-Curie temperature relationships in BaTiO_3-based ferroelectric perovskites: Anomalous behavior of (Ba,Cd)TiO_3 from DFT, statistical inference, and experiments

摘要

One of the key impediments to the development of BaTiO_3-based materials as candidates to replace toxic-Pb-based solid solutions is their relatively low ferroelectric Curie temperature (T_C). Among many potential routes that are available to modify T_C, ionic substitutions at the Ba and Ti sites remain the most common approach. Here, we perform density functional theory (DFT) calculations on a series of ATiO_3 and BaBO_3 perovskites, where A = Ba, Ca, Sr, Pb, Cd, Sn, and Mg and B = Ti, Zr, Hf, and Sn. Our objective is to study the relative role of A and B cations in impacting the T_c of the tetragonal (P4mm) and rhombohedral {R3m) ferroelectric phases in BaTiO_3-based solid solutions, respectively. Using symmetry-mode analysis, we obtain a quantitative description of the relative contributions of various divalent (A) and tetravalent (B) cations to the ferroelectric distortions. Our results show that Ca, Pb, Cd, Sn, and Mg have large mode amplitudes for ferroelectric distortion in the tetragonal phase relative to Ba, whereas Sr suppresses the distortions. On the other hand, Zr, Hf, and Sn tetravalent cations severely suppress the ferroelectric distortion in the rhombohedral phase relative to Ti. In addition to symmetry modes, our calculated unit-cell volume also agrees with the experimental trends. We subsequently utilize the symmetry modes and unit-cell volumes as features within a machine learning approach to learn T_C via an inference model and uncover trends that provide insights into the design of new high-T_C BaTiO_3-based ferroelectrics. The inference model predicts CdTiO_3-BaTiO_3 solid solutions to have a higher T_c and, therefore, we experimentally synthesized these solid solutions and measured their T_C. Although the calculated mode strength for CdTiO_3 in the tetragonal phase is even larger than that for PbTiO_3, the T_c of CdTiO_3-BaTiO_3 solid solutions in the tetragonal phase does not show any appreciable enhancement. Thus, CdTiO_3-BaTiO_3 does not follow the inference model, which is based on established data and trends for ATiO_3. Rather, our experimental phase diagram for CdTiO_3-BaTiO_3 suggests that it behaves markedly differently from any other BaTiO_3-based systems studied so far.