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

The A3BB′O6 structure consists of alternating face-sharing trigonal prismatic B′O6 and octahedral BO6 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, A3Co2O6 (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 Ca3Ni2O6 under a highly oxidative environment, which reveals a heterovalent Ni2+/Ni4+ distribution at the trigonal prismatic and octahedral sites, respectively, in contrast to A3Co2O6, 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 d6 configurations for both Ni4+ and Co3+, 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.