Extended Ni(III) oxyhalide perovskite derivatives: Sr2NiO3X (X = F, Cl).

Extended layered oxyhalide compounds, Sr2NiO3X (X = F, Cl), with the square pyramidal coordination around the trivalent nickel ions in the low spin state (S = 1/2), are successfully synthesized by a high-pressure and high-temperature reaction. Both these compounds crystallize in the n = 1 Ruddlesden-Popper type structure, but the difference of halogen anions incorporated dictate the anion-site ordering patterns and the magnetic ground states. Sr2NiO3F adopts the tetragonal cell in the space group I4/mmm (a = b = 3.79125(2) Å and c = 13.13754(9) Å), with O/F anions being disordered at the apical sites, while the crystal structure of Sr2NiO3Cl is described in the tetragonal space group P4/nmm (a = b = 3.85566(1) Å and c = 14.43240(6) Å) with O/Cl anions being fully ordered at the apical sites. Additionally, Sr2NiO3Cl undergoes a long-range antiferromagnetic order below TN = 33 K, while the fluorine counterpart does not exhibit a long-range ordering but spin glass transition at T(SG) = 11 K. In light of the positive Weiss temperatures for both X = F and Cl, the unpaired electron likely occupies a d(xy) orbital. Namely, the superexchange interaction mediated by d(xy)-Opπ-d(xy) in the NiO2 basal plane is antiferromagnetic, while the direct exchange interaction between d(xy)-d(xy) along the diagonal directions is ferromagnetic. The origin of spin glass behavior observed in X = F is probably due to randomness of the direct d(xy)-d(xy) bonds caused by off-centering nickel ions and O/F site disordering.