High-pressure stability relations, crystal structures, and physical properties of perovskite and post-perovskite of NaNiF3

NaNiF3 perovskite was found to transform to post-perovskite at 16–18 GPa and 1273–1473 K. The equilibrium transition boundary is expressed as P (GPa)=−2.0+0.014×T (K). Structure refinements indicated that NaNiF3 perovskite and post-perovskite have almost regular NiF6 octahedra consistent with absence of the first-order Jahn–Teller active ions. Both NaNiF3 perovskite and post-perovskite are insulators. The perovskite underwent a canted antiferromagnetic transition at 156 K, and the post-perovskite antiferromagnetic transition at 22 K. Magnetic exchange interaction of NaNiF3 post-perovskite is smaller than that of perovskite, reflecting larger distortion of Ni–F–Ni network and lower dimension of octahedral arrangement in post-perovskite than those in perovskite.