Crystal Growth and Some Properties of LuB₄, LuAlB₄, and Lu₂AlB₆

Crystals of LuB 4 , LuAlB 4 , and Lu 2 AlB 6 were grown by the high-temperature aluminium solution method using lutetium oxide and crystalline boron powders as starting materials. The growth conditions for obtaining single crystals of relatively large size were investigated. The as-grown LuB 4 , LuAlB 4 , and Lu 2 AlB 6 crystals were subjected to chemical analysis and measurements of unit cell parameters. The Vickers microhardness and electrical resistivity of the crystals were measured, and oxidation at high temperature in air was studied. Average values of Vickers microhardness (Hv) and electrical resistivity (ρ) determined for LuAlB 4 and Lu 2 AlB 6 single crystals are as follows: for LuAlB 4 Hv=14.0(±0.6) GPa, ρ=86(±6) µ Ω· cm, and for Lu 2 AlB 6 Hv=18.9(±0.7) GPa, ρ=31(±3) µ Ω· cm. The oxidation of LuB 4 , LuAlB 4 , and Lu 2 AlB 6 crystals begins to occur at about 590° C, 960° C and 1030° C, respectively. The final oxidation products of LuB 4 include LuBO 3 and probably amorphous B 2 O 3 or H 3 BO 3 . However, the oxidation products of LuAlB 4 and Lu 2 AlB 6 crystals were not detected by powder X-ray diffraction, probably due to insufficient amounts of these oxidation products.