Processes of Europium Oxide Dissolution in Molten Alkali Metal Iodides

Abstract

The interactions of Eu2+ with oxide ions and the processes accompanying dissolution of Eu2O3 in molten CsI and NaI at 700 °C were studied by a potentiometric method and sequential addition method with electrochemical control of the saturation. An electrochemical cell with a membrane oxygen electrode Pt(O2)|ZrO2(Y2O3) was used for determination of the equilibrium molality of O2−. The solubility products of EuO are equal to PEuO = 7.94·10−14 mol2·kg−2 in molten CsI and 2.24·10−9 mol2·kg−2 in molten NaI. In the latter melt EuO is referred to as a weak base since its dissociation constant is 2.0·10−6. The interaction of Eu2O3 with both molten iodides leads to the formation of EuO and O2− in the solutions, which contain approximately 4·10−4 mol·kg−1 EuO in CsI and 4·10−3 mol·kg−1 EuO in NaI melts, and the dissociation of EuO in the melts is suppressed. The behavior of Eu oxo-species in the iodide melts is subjected to predictions of the Pearson “hard” and “soft” acids and bases concept. The admixtures formed by Eu2O3 dissolution in the molten iodides essentially affect the luminescence properties of single crystals grown from the saturated solutions as compared with the pure iodides. The obtained thermodynamic data permit consideration of Eu2+ as a good scavenger for the CsI melt, but it is not available for molten NaI.