Spin Chains with Highly Quantum Character through Strong Covalency in Ca₃CrN₃

The insulating transition metal nitride Ca₃CrN₃ consists of sheets of triangular [CrN₃]^6- units with C₂ symmetry that are connected via quasi-1D zigzag chains. Due to strong covalency between Cr and N, Cr³⁺ ions are unusually low-spin, and S = 1/2. Magnetic susceptibility measurements reveal dominant quasi-1D spin correlations with very large nearest-neighbor antiferromagnetic exchange J = 340 K and yet no sign of magnetic order down to T = 0.1 K. Density functional theory calculations are used to model the local electronic structure and the magnetic interactions, supporting the low-spin assignment of Cr³⁺ that is driven by strong π donation from the nitride ligands. The surprising failure of interchain exchange to drive long-range magnetic order is accounted for by the complex connectivity of the spin chain pairs that further frustrates order. Our combined results establish Ca₃CrN₃ as a nearly ideal manifestation of a quantum spin chain whose dynamics remain unquenched down to extraordinarily low temperatures despite strong near-neighbor exchange coupling.