Enhancement of piezoelectricity in a mixed ferroelectric

We use first-principles density-functional total-energy and polarization calculations to calculate the piezoelectric tenser at zero temperature for both cubic and simple-tetragonal ordered supercells of Pb3GeTe4. The largest piezoelectric coefficient for the tetragonal configuration is enhanced by a factor of about three with respect to that of the cubic configuration. This can be attributed to both the larger strain-induced motion of cations relative to anions and higher Born effective charges in the tetragonal case. A normal-mode decomposition shows that both cation ordering and local relaxation weaken the ferroelectric instability, enhancing piezoelectricity.