Physicochemical properties of cesium chromate and ferrate: Experimental and first-principles insights for severe accident source-term modeling

Reliable thermodynamic data for cesium compounds are essential to predict fission products behavior during severe nuclear reactor accidents. This study establishes the physicochemical properties of cesium chromate (Cs₂CrO₄), cesium ferrate (Cs₂FeO₄), and its decomposition product CsFeO₂ through combined experimental and first-principles approaches. High-purity samples were synthesized by solid-state reaction and characterized using powder X-ray diffraction (XRD), thermogravimetric–differential scanning calorimetry (TG-DSC), and in-situ high-temperature XRD measurements up to 1050°C. First-principles DFT and quasi-harmonic phonon calculations provided complementary insights into vibrational and electronic properties. Cs₂CrO₄ crystallizes in an orthorhombic Pnma structure and undergoes one-step decomposition at temperatures of about 950°C. Cs₂FeO₄ exhibited a rapid dehydration at about 97°C, transformed to CsFeO₂ starting from 170°C, transiently reabsorbed oxygen at a temperature range of 400°C to 800°C, and finally decomposed to Fe₂O₃ above 1086°C. The computed heat capacities reproduced the experimental C_p up to 750 K (476.85°C), while the imaginary phonon modes reveal dynamic instabilities in ferrate phases. An electronic-structure analysis revealed band gaps of about 3.0 eV for Cs₂CrO₄ and about 1.2 eV for Cs₂FeO₄, while CsFeO₂ displays metallic behavior. By linking these results to severe accident scenarios, this work clarifies how decomposition thresholds, phase stability ranges, and redox sensitivity of cesium compounds govern their volatility, transport, and deposition in reactor containment. The validated dataset provides critical input for source-term databases (e.g., ECUME – effective chemistry database of fission products under multiphase reaction) and severe accident codes such as MELCOR and ASTEC, thereby improving the predictive accuracy of cesium release and radiological safety assessments.