Cation disorder in ferroelectric Aurivillius phases of the type Bi2ANb2O9(A=Ba, Sr, Ca)

The structures of the ferroelectric two-layer Aurivillius phases Bi 2 ANb 2 O 9 (A=Ba, Sr, Ca) have been refined using a combination of X-ray and neutron powder diffraction data. Bi 2 BaNb 2 O 9 is not significantly distorted from idealised symmetry and has been refined in tetragonal space group I4/mmm, a=3.9362(1) and c=25.6582(7) Å. The Sr and Ca compounds have been refined in orthorhombic space group A2 1 am, with a=5.5193(3), b=5.5148(3), c=25.0857(6) Å and a=5.4833(1), b=5.4423(1), c=24.8984(6) Å, respectively. The orthorhombic distortion increases with decreasing A 2+ cation size and originates from bonding requirements at the perovskite A site, in agreement with previous work. However, in contrast to earlier work, we find a partial mixing of Bi and A cations on their respective sites, which increases in the order Ca