Structural and Piezoelectric Properties of a Technologically Promising Ferroelectric Alloy From First Principles

with Laurent Bellaiche

Ferroelectric materials are of growing importance for a variety of actual and potential device applications. Examples include piezoelectric transducers and actuators, non-volatile ferroelectric memories, dielectrics for microelectronics and wireless communication, pyroelectric arrays, and non-linear optical applications.

An important class of ferroelectric materials are the perovskite oxides ABO₃ (e.g., PbTiO₃). At high temperature these all share the paraelectric simple-cubic perovskite structure: metal A at the cube corners, metal B at the cube center, and O atoms at the cube faces. As the temperature is reduced, many of the perovskite compounds undergo a structural phase transition and develop a switchable spontaneous electric polarization P. In other words, they become ferroelectric.

Interestingly, most of the perovskite compounds that are of greatest technological interests are not simple systems, but rather (A',A")BO₃ or A(B',B")O₃ alloys with two different kinds of A or B atoms, respectively. For example, the Pb(Zr,Ti)O₃ alloys are currently used in piezoelectric transducers and actuators. Other examples include new compounds - e.g., Pb(Mg,Nb)O₃, Pb(Zn,Nb)O₃ and Pb(Sc,Nb)O₃ - which exhibit so extraordinarily high values of the piezoelectric constants that they could usher in a new generation of piezoelectric devices that would improve everything from the resolution of ultrasound machines to the range of sonar listening devices.

Being able to predict properties of ferroelectric alloys can thus be of great technological importance - in addition to improve the currently poor knowledge of that class of materials.

The thrust of the proposed work is to predict the structural properties and the full piezoelectric tensor of Pb(Sc,Nb)O₃, and to identify the microscopic origins of the large piezoelectricity experimentally found in this compound. For that project of 10 weeks, two state-of-the-art computational methods will be used: (1) the First-principles method - for which W. Kohn received the1998 Nobel Prize of Chemistry - to determine the atomic positions inside the Pb(Sc,Nb)O₃ alloy, and (2) the modern theory of polarization to calculate the spontaneous polarization P and the piezoelectric coefficients.