Study on the relation between the crystal structure and thermal stability of FeUO4 and CrUO4

Abstract

FeUO4, CrUO4, and FexCr1-xUO4 are monouranates containing pentavalent U. Despite having similar crystal structures, the formation conditions and thermal stability of these compounds are significantly different. To determine the factors causing the thermal stability differences between FeUO4 and CrUO4, their crystal structures were evaluated in detail. A Raman band was observed at 700 cm−1 in all samples. This Raman band was derived from the stretching vibration of the O–U–O (‘uranyl’) axis band, indicating that FexCr1-xUO4 was composed of a uranyl-like structure in its lattice regardless of its “x” value. Mössbauer measurements indicated that the Fe in FeUO4 and FexCr1-xUO4 were trivalent. Furthermore, FexCr1-xUO4 lost its symmetry around FeIII with increasing electron densities around FeIII as the abundance of Cr increased. These results suggest no significant structural differences between FeUO4 and CrUO4. Thermogravimetric measurements for UO2, FeUO4, and CrUO4 showed that the temperature at which FeUO4 decomposed under an oxidizing condition (approximately 1073 K) was significantly lower than the temperature at which CrUO4 decomposition started (approximately 1523 K). Based on these results, we concluded that FeUO4 decomposition was triggered by an “in-crystal” redox reaction, i.e., FeIII + UV → FeII + UVI, which would not occur in the CrUO4 lattice because CrIII could never be reduced under the investigated condition. Finally, the existence of CrIII in FexCr1-xUO4 effectively suppressed FexCr1-xUO4 crystal decomposition, even at a very low Cr content.