Single crystals were prepared using the floating zone method. The crystal quality increased as the atomic number of the rare-earth metal increased. The difference in quality was explained in terms of high-temperature hardness and growth temperature. The Vickers hardness of the (100)[110] plane was determined as a function of temperature (figure 2).

Floating Zone Growth and High Temperature Hardness of Rare-Earth Hexaboride Crystals: LaB6, CeB6, PrB6, NdB6, and SmB6. Otani, S., Nakagawa, H., Nishi, Y., Kieda, N.: Journal of Solid State Chemistry, 2000, 154[1], 238-41