Abstract
We report first principles density functional calculations of the Born effective charges and electronic dielectric tensors for the relaxor PMN (PbMg 1/3Nb 2/3O 3). Visualization of the Born charge tensors as “charge ellipsoids” have provided microscopic insights on the factors governing piezoelectric enhancements with polarization rotation. Several 15 and 30-atom ferroelectric and antiferroelectric supercells of PMN involving 1:2 and 1:1 chemical ordering have been studied. A cascading set of ferroelectric phonon instabilities lead to several low symmetry monoclinic structures. We find a ground state with a 15-atom unit cell with 1:2 chemical ordering along [1 1 1] with a monoclinic C2 structure.
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