Comparative study of the thermal stability of rare earth hexaborides

Abstract

At 1500-2500 Deg and 10-6 torr, MB6 (M = La, Ce, Pr, Nd, Sm) sublimed or melted without decompn. Both EuB6 and YbB6 dissocd. with preferential evapn. of the metal, and rhombohedral beta -B was formed at .apprx.2400 Deg in the case of EuB6, whereas YbB12 was formed at >=1650 Deg in the case of YbB6. LnB6 (Ln = Y, Gd, Tb, Dy) dissocd. at 1550 Deg to LnB4 and B. The thermal stability of the lanthanide hexaborides increases as the satd. vapor pressure of the lanthanide metal decreases; the hexaboride stability also increases as the lattice parameter of the hexaboride decreases, which corresponds to an increase in the strength of the B-B bonds of the covalent B lattice. [on SciFinder (R)]