| Summary: | Lanthanide doping is widely employed to tune structural change temperature and electrical properties in ABO<sub>3</sub>-type perovskite ferroelectric materials. However, the reason that A-site lanthanide doping leads to the decrease of the Curie temperature is still not clear. Based on the reported Curie temperature of lanthanides (Ln) doped in two classic ferroelectrics PbTiO<sub>3</sub> and BaTiO<sub>3</sub> with A<sup>2+</sup>B<sup>4+</sup>O<sub>3</sub>-type perovskite structure, we discussed the relationship between the decrease rate of Curie temperature (Δ<i>T</i><sub>C</sub>) and the bond strength variance of A-site cation (<i>σ</i>). For Nd ion doped Pb(Mg<sub>1/3</sub>Nb<sub>2/3</sub>)O<sub>3</sub>-PbTiO<sub>3</sub> (Nd-PMNT) ferroelectric crystal as an example, the internal factors of the dramatic decline of the Curie temperature induced by A-site Nd doping were investigated under a systematic study. The strong covalent bonds of Ln-O play an important role in A-site Ln composition-induced structural change from ferroelectric to paraelectric phase, and it is responsible for the significant decrease in the Curie temperature. It is proposed that the cells become cubic around the Ln ions due to the strong covalent energy of Ln-O bonding in A-site Ln doped A<sup>2+</sup>B<sup>4+</sup>O<sub>3</sub> perovskite ferroelectrics.
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