Stabilization of the Metastable Sn_XTi_1-XO₂ Alloy and Suppression of Its Spinodal Decomposition

Among the bimetallic oxide alloys, spinodal decomposition of the SnO₂-TiO₂ phase has garnered much attention due to its applications in gas sensors, photo anodes, and electron transport layers. The tin titanium oxide alloy usually undergoes spinodal decomposition at a critical temperature, exhibiting a characteristic lamellar microstructure upon decomposition. Unlike classical nucleation and growth, spinodal decomposition is a homogeneous phase transformation with no associated activation energy barrier. In this work, a non-decomposed, metastable Sn_XTi_1−XO₂ thin film alloy with the Sn:Ti atomic ratio in the range of 62:38 to 78:22 was deposited from a mixed oxide target by controlling the partial pressure of oxygen, sputter power, and deposition time in a radio frequency sputter chamber. The crystalline phase of the oxide film was obtained upon subsequent annealing at 500°C. While the thermodynamic equilibrium solubility limit of TiO₂ in SnO₂ at 500°C is of the order of 10%, this thin film alloy demonstrated a substantially higher cationic substitution of Sn by Ti, approximately 38%, in Sn_1-XTi_XO₂, highlighting its metastability. These Sn_1-XTi_XO₂ thin film samples were annealed from 500°C to 1200°C in steps of 100°C, but there was no observable decomposition. According to the (SnO₂-TiO₂) phase diagram, a metastable (Sn,Ti)O₂ alloy with Sn:Ti in the range of 62:38 to 78:22 should decompose at a critical temperature in the range 1100°C – 1200°C. Stacking faults and other planar defects observed in cross-sectional TEM analysis point toward a complex intergrowth of SnO₂ and TiO₂ which may suppress the decomposition and enhance the metastability window of the alloy to the temperature as high as1200°C.