Reduction Kinetics and Mechanism of Rare Earth Oxide (Tm_2O_3 and Yb_2O_3) with Metallic Lanthanum

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Reduction reaction and mechanism of samarium, europium, thulium and ytterbium oxides with metallic lanthanum have been investigated under the conditions of high temperature vacuum atmosphere. Reduction reaction of each rare earth oxide with metallic lanthanum parabolically proceeded with prolonged time. Reduction curve for each oxide could be arranged by using both logarithms of reactivity degree and time. The slope of straight line obtained was about 0.5 for the reduction of each rare earth oxide. Apparant activation energies of the reduction reaction calculated from Arrhenius plots were 96 kJ/mol for samarium oxide, 236 kJ/mol for europium oxide, 154 kJ/mol for thulium oxide and 243 kJ/mol for ytterbium oxide, respectively. An electrochemical reduction model based on Wagner's theory being used for high temperature oxidation of metal was applied for the reduction mechanism of each rare earth oxide with metallic lanthanum. From this model, a rate-determing step for the reduction reaction of each oxide could be considered to be the transfer process of lanthanum ion or oxygen ion in lanthanum oxide layer of reduction product.Dissolution behaviors of metallic lanthanum and neodymium have also been examined in LiF melts containing LaF_3 and NdF_3 at the temperature range of 1193 K to 1323 K. Saturated solubility of lanthanum decreased with an increase in the LaF_3 concentration in LiF-LaF_3 binary melt, and saturated solubility of neodymium was not dependent on the NdF_3 concentration in LiF-NdF_3 binary melts.Saturated solubility of lanthanum in LiF-20mol%LaF_3 binary melt was extremely smaller than that of neodymium in LiF-20mol%NdF_3 binary melts. Metallic lanthanum predominantly reacts with LiF in melt by the displacement reaction to form LaF_2 and LaF_3. On the other hand, metallic neodymium forms NdF_3 by the displacement reaction with LiF in melt at the initial stage of dissolution, and also reacts with NdF_3 in melt by the disproportional reaction to form NdF_2.