We use first-principles density-functional total-energy and polarization calculations to calculate the piezoelectric tensor at zero temperature for both cubic and simple-tetragonal ordered supercells of ${\mathrm{Pb}}_3{\mathrm{GeTe}}_4.$ 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.

• Received 18 December 1997

DOI:https://doi.org/10.1103/PhysRevB.57.R13973