High Pressure Synthesis of Ca ₃ Ni ₂ O ₆ with Heterovalent Ni ²⁺ /Ni ⁴⁺ Chains

The A 3 BB′O 6 structure consists of alternating face-sharing trigonal prismatic B′O 6 and octahedral BO 6 units, forming one-dimensional (1D) chains arranged on a triangular lattice. Owing to the wide variety of possible A, B, and B′ cation combinations, this family exhibits diverse magnetic behaviors, including 1D magnetism and geometrical frustration. To date, A 3 Co 2 O 6 (A = Ca, Sr) represents the only reported case in which both the B and B′ sites are occupied by the same transition metal. Here, we report the high-pressure synthesis of Ca 3 Ni 2 O 6 under a highly oxidative environment, which reveals a heterovalent Ni 2+ /Ni 4+ distribution at the trigonal prismatic and octahedral sites, respectively, in contrast to A 3 Co 2 O 6, where both Co sites are trivalent. This distinct behavior between the Co and Ni systems is rationalized by crystal field stabilization at the octahedral site, which favors low-spin d 6 configurations for both Ni 4+ and Co 3+, as supported by DFT calculations. This study demonstrates that high-pressure synthesis combined with high-oxygen chemical potential provides a powerful strategy to access otherwise inaccessible valence states and to elucidate the interplay between coordination environment, electronic configuration, and magnetism in low-dimensional oxides.