Anion ordering and defect structure in Ruddlesden-Popper strontium niobium oxynitrides.

The crystal structure of the n = 1 member of the Ruddlesden-Popper family (SrO)(SrNbO(2)N)(n) was refined by the Rietveld method using neutron powder diffraction data. This complex crystallizes in the I4/mmm space group with cell parameters a = 4.0506(2) and c = 12.5936(9) angstroms. The refined composition was Sr(2)NbO(3.28)N(0.72), which corresponds to a formal oxidation state for Nb of +4.72, meaning 72% Nb(V) and 28% Nb(IV). The nitrogen atoms order in the equatorial sites of the niobium octahedra according to Pauling's second crystal rule as the more charged anion occupies the site showing the larger bond strength sums. Pauling's second crystal rule is shown to be able to predict the distribution of anions in the available crystallographic sites in other mixed anion systems such as oxyhalides with K(2)NiF(4) structures and other oxynitrides. The defect structure of the n = 1 and n = 2 members of the same family was investigated by high-resolution electron microscopy. Recurrent intergrowth along the c axis with other Ruddlesden-Popper members (n = 3, 4, and perovskite) is observed, resulting in streaking along this direction in the corresponding electron diffraction patterns.