BiFeO₃-BaTiO₃ (BF-BT) is one of potential systems for lead-free/high-temperature piezoceramics. 最近一次审稿时发表了如下一些评论：

Various perovskites as third endmember have been added into BF-BT system. Most of article discussed like “decreasing leakage”, “improving piezoresponse” and so on, what is the true problem? What is novelty or difference? For this concern, the references should be cited from various kinds of opinion but not just from similar story. For instance, please refer to: Effects of substitution element on physical properties of BiFeO₃-BaTiO₃ ferroelectric ceramics driven by data-mining oxide perovskites (http://engine.scichina.com/doi/10.1360/N972018-00689).

Please present frequency-dependent dielectric constant and loss spectra at room temperature! It is important to detect the state of vacancies and related concentration. Meanwhile, dielectric properties are also the most important properties of piezoceramics, which include both contributions from ferroelectric phase and charged defects. The comparison of piezo-response and ferroelectric properties should be performed at the same level of dielectric loss. On the BF-BT piezoceramics, there is a big variation in the reported d₃₃ value and most of articles did not present dielectric loss value. Therefore, it is nonsense simply to compare d₃₃ and P_r because different contribution from charged defects is unknown!

On the valence change between Fe³⁺ and Fe²⁺ ions in BiFeO₃-based system, 1) the method that authors used in this manuscript to analyze photoelectron spectroscopy is wrong, although most of people in this community do this way. Please refer to article: A. T. Kozakov, et al. X-ray photoelectron study of the valence state of iron in iron-containing single-crystal (BiFeO₃, PbFe_1/2Nb_1/2O₃) and ceramic (BaFe_1/2Nb_1/2O₃) multiferroics. Journal of Electron Spectroscopy and Related Phenomena 184 (2011) 16-23. 2) At the same time, even though most of articles mentioned the co-presence of Fe³⁺/Fe²⁺, what condition for this co-presence? To my knowledge, this mechanism was proposed in the work of thin films deposited under low oxygen atmosphere, the condition in ceramic processing is very different from it. 3) Alternatively, new mechanism for improved insulating ceramics was also proposed. Please refer to articles: L. L. Zhang, et al. Robust insulating La and Ti co-doped BiFeO₃ multiferroic ceramics, J. Mater. Sci.: Mater. Electron. 27(8), 8725-8733 (2016) and references therein. 

On the term of “doping”, adding some third perovskite oxide into BF-BT is doped while e.g. adding MnO₂ into BF-BT is also doped, what is difference in chemistry and effect on properties? If different, using same term is suitable or not?