Synthesis and Characterization of Sodium-Iron Antimonate Na₂FeSbO₅: One-Dimensional Antiferromagnetic Chain Compound with a Spin-Glass Ground State

A new oxide, sodium-iron antimonate, Na₂FeSbO₅, was synthesized and structurally characterized, and its static and dynamic magnetic properties were comprehensively studied both experimentally by dc and ac magnetic susceptibility, magnetization, specific heat, electron spin resonance (ESR) and Mössbauer measurements, and theoretically by density functional calculations. The resulting single-crystal structure (a = 15.6991(9) Å b = 5.3323 (4) Å c = 10.8875(6) Å S.G. Pbna) consists of edge-shared SbO₆ octahedral chains, which alternate with vertex-linked, magnetically active FeO₄ tetrahedral chains. The ⁵⁷Fe Mössbauer spectra confirmed the presence of high-spin Fe³⁺ (3d⁵) ions in a distorted tetrahedral oxygen coordination. The magnetic susceptibility and specific heat data show the absence of a long-range magnetic ordering in Na₂FeSbO₅ down to 2 K, but ac magnetic susceptibility unambigously demonstrates spin-glass-type behavior with a unique two-step freezing at T_f1 ≈ 80 K and T_f2 ≈ 35 K. Magnetic hyperfine splitting of ⁵⁷Fe Mössbauer spectra was observed below T∗ ≈ 104 K (T_f1 < T*). The spectra just below T∗ (T_f1 < T < T*) exhibit a relaxation behavior caused by critical spin fluctuations, indicating the existence of short-range correlations. The stochastic model of ionic spin relaxation was used to account for the shape of the Mössbauer spectra below the freezing temperature. A complex slow dynamics is further supported by ESR data revealing two different absorption modes presumably related to ordered and disordered segments of spin chains. The data imply a spin-cluster ground state for Na₂FeSbO₅.