First principles studies of the Born effective charges and electronic   dielectric tensors for the relaxor PMN (PbMg1/3Nb2/3O3)
  We report first principles density functional calculations of the Born effective charges and electronic dielectric tensors for the relaxor PMN (PbMg1/3Nb2/3O3). 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 [111] with a monoclinic C2 structure. 
Narayani Choudhury (Bhabha Atomic Research Centre, India), R. E. Cohen   (Carnegie Institution of Washington) and Eric J. Wa...