Continuous Evolution of Eu²⁺/Eu³⁺ Mixed Valency Driven by Pressure and Temperature

Continuous mixed valency involving Eu²⁺ and Eu³⁺ in Eu₄Bi₆Se₁₃ can be induced under applied pressure or at reduced temperatures. The monoclinic structure of Eu₄Bi₆Se₁₃, crystallizing in the P2₁/m space group (No. 11), features linear chains of Eu atoms aligned along the b-axis. Magnetic susceptibility measurements, conducted both parallel and perpendicular to the b-axis and analyzed using Curie-Weiss theory, alongside high-pressure partial fluorescence yield (PFY) data from X-ray absorption spectroscopy (XAS), indicate the material's propensity to adopt a mixed-valent state. Within this state, the trivalent Eu³⁺ configuration becomes increasingly favored as the pressure rises or the temperature decreases.