Cluster Model for K(Ta_0.56Nb_0.44)O₃ Solid Solution

Based on the results of total-energy calculations for embedded clusters using self-consistent-field X_α-scattered-wave method, we determined the tetragonal fine structure of K(Ta_0.56Nb_0.44)O₃, 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 of about 0.02Å along 001. The off-center displacement of Nb is much larger than that of Ta, thus it makes a major contribution to the occurrence of the ferroelectric instability.