Frustration of tilts and A-site driven ferroelectricity in   KNbO_3-LiNbO_3 alloys
  Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off-centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling. 
D. I. Bilc (1), D. J. Singh (1) ((1) Condensed Matter Sciences   Division, Oak Ridge National Laboratory, Oak Ridge, Tenness...