Erbia-Hafnia system

Abstract

The phase relations in the erbia-hafnia system were studied after fixed-temperature anneals and at high temperatures by X-ray diffraction with emphasis placed on the cubic fluorite solid solution region. Solubility of erbia in hafnia is less than 1 m/o. The slight solubility of erbia in hafnia lowered the upper limit of the monoclinic to tetragonal transformation temperature from 1825 degrees C to 1725 degrees C. The fluorite domain extends from 9 m/o to 50 m/o Er $sub 2$)$sub 3$ at 1450 degrees C and from 6 to 60 m/o erbia at 1900 degrees C. A two-phase domain composed of fluorite and erbia solid solutions extends from 50 m/o to 73 m/o erbia at 1450 degrees C. The evidence suggests that the two-phase region is domed and does not extend all the way to the liquidus curve. Above 73 m/o erbia, a single phase domain of erbia solid solution is present. the liquidus curve exhibits a maximum of 2845 degrees C at 20 m/o erbia. A eutectic exists at 85 m/o erbia and 2340 degrees C. The melting points of pure hafnia and erbia were found to be 2825 +- 20 degrees C and 2400 +- 20 degrees C respectively. X-raymore » structure determinations detected ordering of the pyrochlore type at 33.3 m/o erbia in the fluorite domain. A previously unrecognized anomaly in the activaton energy for total electrical conductivity was observed around 33.3 m/o erbia. The anomalous activation energy was related to the pyrochlore structure and, in particular, to the migration of the oxygen ions occupying the (a) positions in the pyrochlore structure. An anomaly was also observed in the conductivity vs composition relation. This anomaly was also related to ordering of the pyrochlore type. (author)« less