Search for New High Performance Piezoelectrics

In an effort to design new high performance piezoelectric materials, we have performed first-principles calculations to study the energetics of several ABO$_{3}$ type materials, with the `A' site being occupied by two types of atoms. The motivation comes from recent findings of a morphotropic-phase boundary, which gives rise to large electromechanical coupling, at high pressure and low temperatures in pure PbTiO$_{3}$[1]. This prompted us to substitute the `A' site with a smaller atom compared to Pb to apply ``chemical pressure'' and tune the morphotropic-phase boundary to lower pressures. We have discovered (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$, (Pb$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ and (Sn$_{1/2}$ Ge$_{1/2})$TiO$_{3}$ to be promising new piezoelectric materials. (Pb$_{1/2}$ Sn$_{1/2})$TiO$_{3}$ shows lower energy monoclinic phases (space groups Cm with polarization along [xxz] and Pm with polarization along [x0z]) compared to the layered tetragonal phase (space group P4mm with polarization along [001]) while the remaining two compounds have a rhombohedral (space group R3m with polarization along [111]) ground-state compared to the tetragonal phase in the rock-salt pattern (space group I4mm with polarization along [001]).~Our results also suggest ease of polarization rotation and large electromechanical strain. [1] Zhigang Wu and Ronald E. Cohen, PRL, \textbf{95}, 037601, 2005