Sn_2-2xSb_xFe_xO₄ Solid Solutions as Possible Inert Anode Materials in Aluminum Electrolysis

Single-phase samples of the Sn_2-2xSb_xFe_xO₄ solid solution were prepared by solid-state reaction in air at 1300 °C for 0.26 ≤ x ≤ 0.66. The crystal structure of the Sn_2-2xSb_xFe_xO₄ phases was studied by electron diffraction and X-ray powder diffraction. The compounds crystallize with the rutile type structure with a disordered arrangement of cations (P4₂/mnm space group, a = 4.7127(6) Å, c = 3.1595(4) Å, R_I = 0.020, R_P = 0.019 for Sn_1.48Sb_0.26Fe_0.26O₄ and a = 4.6682(8) Å, c = 3.1147(6) Å, R_I = 0.030, R_P = 0.022 for Sn_0.68Sb_0.66Fe_0.66O₄). The valence state of cations in the Sn_1.48Sb_0.26Fe_0.26O₄ and Sn_0.68Sb_0.66Fe_0.66O₄ samples was determined by means of ¹¹⁹Sn, ¹²¹Sb, and ⁵⁷Fe Mössbauer spectroscopy. The presence of Sn(II), Sb(III), or Fe(II) low valent state species was not detected in the samples. Resistivity vs temperature measurements revealed a semiconducting behavior from room temperature up to 900 °C with about four orders in magnitude decrease of the resistivity. Solubility tests in the cryolite-alumina melt showed that the steady-state concentration of dissolved tin for the samples with x = 0.26, 0.36 is significantly lower than that for SnO₂. The electrocatalytic activity and solid degradation products of Sn_2-2xSb_xFe_xO₄ are compared with those of another possible anode material (SnO₂ with small amount of the CuO and Sb₂O₃ additives). These tests allow evaluation of the prospects of the Sn_2-2xSb_xFe_xO₄ solid solutions as inert anode materials and to formulate an approach to further improvement of their degradation stability.