Relationship between phase coexistence and properties in piezoelectric materials

Due to the toxicity of Pb, researchers are searching for high performance Pb free piezoelectrics to substitute for the Pb based ones, and the mechanism for developing high performance piezoelectrics are essentially needed. In this paper, we used a 6th order Landau-Devonshire model to calculate the free energy for different phase coexistence situations (single phase, two phases, three phases and four phases). Results showed that different phase coexistence statuses corresponded to different energy barriers for polarization rotation and extension: coexisted with more ferroelectric phases, the polarization rotation barrier will be lower, and coexisted with paraelectric phases, the polarization extension barrier will be extremely reduced, and different energy barriers result in different enhancement abilities for materials' piezoelectricity and permittivity. Experimental proofs were also provided to verify this finding. Our study clarifies the relationship between phase coexistence states with its properties performance, and re-interprets polarization rotation and extension mechanism (a free energy view) into a simple phase coexistence scenario (a material structure view), which may offer a fast way to find and engineer high performance piezoelectrics.