Fabrication and Characterization of Rare Earth Hexaboride Single-Crystal Materials

摘要

Oriented single crystals of LaB6, CeB6 and PrB6 have been prepared by the arc float-zone refining technique, which is described. A critique of the various possible preparation techniques is given. Wet chemical methods were used to determine bulk stoichiometries of samples prepared from starting materials of various R/B compositions (where R = rare earth). It was found that LaB6.09, a desired composition, could be grown in single-crystal form from LaB6.2 starting material, the excess boron compensating for preferential boron vaporization. Starting materials with B/La < 6.2 produce a B/La gradient in two-pass zone-refined specimens. The level of metallic impurities in arc zone-refined LaB6 is lower than that reported elsewhere for laser zone-refined LaB6. Residual oxygen present in the zone-refined specimens was found to produce second-phase inclusions, which were studied in detail. The effect of contamination of LaB6 surfaces by evaporated refractory metals has also been investigated. Keywords: Arc; Float-zone, Refining, Technique rare earth, Hexaborides, Borides, Thermionic, Cathodes, Boron compounds.