Deterministic Control of Sn³⁺ Valence and Electronic Phase Evolution in AgSnSe₂

Area of Science:

• Materials Science
• Solid-State Physics
• Quantum Materials

Background:

• Unusual oxidation states significantly impact material properties for fundamental science and energy applications.
• The Sn³⁺ oxidation state in AgSnSe₂ is rare, long-debated, and its role remains enigmatic.

Purpose of the Study:

• To definitively probe and confirm the Sn³⁺ oxidation state in AgSnSe₂.
• To investigate the evolution of the Sn oxidation state under chemical substitution (Sb).
• To establish a direct chemical handle for controlling electronic phase transitions in AgSnSe₂.

Main Methods:

• X-ray photoelectron spectroscopy (XPS)
• Mössbauer spectroscopy
• X-ray absorption spectroscopy (XAS)
• Density functional theory (DFT) calculations

Main Results:

• Experimental evidence consistently confirms the presence of Sn in the +3 oxidation state in AgSnSe₂.
• Sb substitution controls the electronic state, suppressing superconductivity (∼5 K) and inducing a metallic-to-semiconducting transition.
• Sb substitution tunes the Sn oxidation state from +3 to a mixed +2/+4 valence-skipping regime.

Conclusions:

• The Sn³⁺ oxidation state in AgSnSe₂ is experimentally validated.
• Sb substitution offers precise control over the Sn oxidation state and electronic phase.
• AgSnSe₂ is a tunable platform for quantum and energy applications.