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) A and c =13.13754(9) angstrom); with O/F anions being disordered at the apical sites, while the crystal structure of Si2NiO3Cl is described in the..tetragonal space group P4/mmm (a = b 3.85566(1) A. and c=14.43240(6) A) With O/Cl anions being fully Orflerecl.at the apical 'sites. Additionally, Sr2NiO3Cl undergoes :a longrange antiferiornagnetic order below T-N 7 33K, 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)-Opir-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 offcentering nickel ions and O/F site disordering.