  The effects of octahedral tilting of RbANb2O7 (A = Bi, Nd) compounds was
studied using density-functional theory. In this compound, the structural phase
transition was correlated with two octahedral tilting modes (a-a-c0 tilting and
a0a0c+ tilting), and magnitude of the octahedral tilting mode was analyzed in
the optimized structure. The theoretical results correlated well with the
recent experimental results on the ferroelectricity of RbBiNb2O7. The hybrid
improper ferroelectricity resulting from the coupling of two octahedral tilting
modes and off center displacement mode was analyzed by group theory and
symmetry mode analysis. The detailed relationship of the tilting modes to the
structural phase transition and the detailed physical properties of
ferroelectricity are also presented.
