Crystal structure of Bi2.5Me0.5Nb2O9 (Me = Na, K):: A powder neutron diffraction study

The room temperature structures of the two-layer Aurivillius phases Bi2.5Me0.5Nb2O9 (Me = Na, Ii) have been refined with the Rietveld method from powder neutron diffraction data (lambda = 1.470 Angstrom). They consist of (Bi2O2)(2+) layers interleaved with perovskite (Bi0.5Me0.5Nb2O7)(2-) (Me = Na, K) slabs. The structures were refined in the orthorhombic space group A2(1)am, Z = 4, and the unit cell parameters of the two oxides are a 5.4937(3), b = 5.4571(4), c = 24.9169(14) Angstrom and a = 5.5005(8), b = 5.4958(8), c = 25.2524(16) Angstrom, respectively. The orthorhombic distortion increases with decreasing Met cation size in the perovskite layer (Bi/Me)(2+) site and the lone pair electrons from the Bi3+ cation are influencing the site distortion. This is in agreement with other two-layer Aurivillius phases and originates from bonding requirements depending on size and electronic environment. (C) 2001 Academic Press.