Deterministic Control of Sn ³⁺ Valence and Electronic Phase Evolution in AgSnSe ₂

Understanding how unusual oxidation states influence material properties is important for both fundamental science and energy applications. AgSnSe₂ is particularly intriguing because it stabilizes the rare and long-debated Sn³⁺ oxidation state, whose true existence and role have remained enigmatic for many years. In this work, we employ X-ray photoelectron spectroscopy, Mössbauer spectroscopy, and X-ray absorption spectroscopy to directly probe the oxidation state of Sn and its evolution under chemical substitution. All experimental evidence consistently confirms the presence of Sn in the +3 oxidation state in AgSnSe₂. Complementary density functional theory calculations further corroborate this assignment. By substituting Sn with Sb, we systematically control the electronic state and its impact on the material's physical properties. At low Sb concentrations, AgSnSe₂ retains superconductivity with a transition temperature of ∼5 K, while increasing Sb content deterministically drives a metallic-to-semiconducting transition through progressive suppression of superconductivity. Spectroscopic analyses show that Sb substitution provides deterministic control of the Sn oxidation state, evolving from a uniform +3 configuration in AgSnSe₂ to a mixed +2/+4 valence-skipping regime at higher Sb levels, thereby establishing a direct chemical handle over the material's electronic phase. This tunability demonstrates that the Sn oxidation state in AgSnSe₂ can be precisely engineered through Sb substitution, enabling controlled electronic phase transitions and establishing AgSnSe₂ as a promising platform for quantum and energy-related applications.