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

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Abstract
Crystals of LuB4, LuAlB4, and Lu2AlB6 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 LuB4, LuAlB4, and Lu2AlB6 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 LuAlB4 and Lu2AlB6 single crystals are as follows: for LuAlB4 Hv=14.0(±0.6) GPa, ρ=86(±6) µ Ω· cm, and for Lu2AlB6 Hv=18.9(±0.7) GPa, ρ=31(±3) µ Ω· cm. The oxidation of LuB4, LuAlB4, and Lu2AlB6 crystals begins to occur at about 590° C, 960° C and 1030° C, respectively. The final oxidation products of LuB4 include LuBO3 and probably amorphous B2O3 or H3BO3. However, the oxidation products of LuAlB4 and Lu2AlB6 crystals were not detected by powder X-ray diffraction, probably due to insufficient amounts of these oxidation products.