Spinel structure and liquidus temperature relationships in nuclear waste glass

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The liquidus temperature of spinel (AB2O4) crystals frequently limits the waste loading of vitrified nuclear waste. In other studies, glass structure-spinel liquidus temperature relationships were discovered for the non-spinel forming cations in glass, but no such relationship was identified for the spinel-forming metal-ions. In this paper, coefficients from an empirical first order mixture model describing spinel liquidus temperature were correlated with the octahedral site preference energy (OSPE) of the spinel-forming metal ions. The OSPE was calculated using approximations of glass and spinel structure made by extrapolating from data on similar glasses and spinels. A literature review indicated that Cr(III), Ni(II), and Fe(II) are most likely to be in the octahedral sites in spinel, Mn(II) in the tetrahedral site, and that Fe(III) will be common in both sites. These assignments were used as starting assumptions to calculate the OPSE for each metal ion. A strong correlation between the OSPE and the liquidus temperature coefficients of spinel-forming metal ions was observed. Further correlations determined that the Crystal Field Stabilization Energy was the most important portion of the OSPE for these spinels. These results indicate that the thermodynamic properties of spinel could be predicted based on assumptions of spinel structure, underpinned by fundamental properties, when scaled empirically.

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