Critical triple point as the origin of giant piezoelectricity in PbMg_1/3Nb_2/3O₃-PbTiO₃system

The morphotropic phase boundary (MPB) of PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) and its derivatives has been reported to exhibit a giant piezoelectric coefficient (d33). Hence, it is essential to understand the origin of excellent piezoelectric properties. In the present work, we observed that the cubic-tetragonal-rhombohedral triple point in the phase diagram of the PMN-xPT system is itself a critical point. The criticality of the triple point is evidenced by the nearly zero transition thermal hysteresis, the maximized dielectric permittivity, and the largest degree of elastic softening. The Landau modeling calculations reveal that such a critical triple point exhibits infinitely high piezoelectricity, which explains the origin of the giant piezoelectricity of the PMN-xPT system. This work illustrates that the near-criticality of the MPB composition is inherited from the criticality of the triple point. This study would stimulate work on searching materials with high piezoelectricity by designing a critical triple point.