Abstract—

A powder mixture of eutectic composition in the LaB₆–NbB₂ system has been prepared by carboborothermic reduction of a La(OH)₃ + NbO_0.5–2(OH)_4–1 hydroxide mixture in vacuum at an isothermal holding temperature of 1650°C. The hydroxide mixture was prepared by reverse coprecipitation in a suspension of amorphous boron and carbon from aqueous solutions of lanthanum nitrate, niobium fluoride, and niobium oxyfluoride. We have examined the effect of the boron-to-carbon ratio on the elemental and phase compositions of the boride mixture. The average particle diameter of the synthesized borides has been determined to be 200–250 nm. The powders have been sintered at 1700°C to a relative density of 87% and arc-remelted. The resultant rodlike eutectic structures had a NbB₂ rod diameter near 600 nm. The structure of the solidified materials has been studied using scanning electron microscopy, X-ray microanalysis, and X-ray diffraction, and the eutectic composition has been accurately determined (58 mol % LaB₆).