First-principles study of tetragonal ferroelectric structuer of K(Ta0.56Nb0.44)O3

Based on the results of total-energy calculations using full-potential linearized augmented plane wave method within the generalized gradient approximation (GGA),we determined the tetragonal fine structure of K(Ta0.56Nb0.44)O-3, i.e., the equilibrium configuration of B-site cations (Ta and Nb)in the ferroelectric phase. Ta is found to have an off-center displacement, relative to the oxygen octahedron, of about 0.005 nm along [00 (1) over bar]. The off-center displacement of Nh is as large as 0.016 nm,thus makes a major contribution to the occurrence of the ferroelectric instability.