Growth of single crystals of alkaline earth metal hexaborides (CaB
6, SrB
6 and BaB
6) in molten aluminum in an argon atmosphere has been investigated.
Results obtained are as follows.
The optimum growth conditions of single crystals:
The mixing atomic ratios of raw materials: for CaB
6: (B/Ca)=3.20-5.70, (Al/Ca)=10.50-50.50, for SrB
6: (B/Sr)=2.00-3.50, (Al/Sr)=25.98-77.94, and for BaB
6: (B/Ba)=3.00-5.70, (Al/Ba)=34.81-76.28. The heating temperature and time: at above 1450°C for longer than 10h in any case.
Cubical single crystals composed of {100} faces, and needle-like and thick-plate-like crystals having well-developed (100) faces, respectively, were grown.
The lattice constants (
a0) and densities (
D) of single crystals:
CaB
6:
a0=4.15215±0.00008(Å),
D=2.44±0.02g/cm
3 SrB
6:
a0=4.19756±0.00009(Å),
D=3.37±0.03g/cm
3 BaB
6:
a0=4.26961±0.00008(Å),
D=4.26±0.03g/cm
3Values of Knoop-microhardness determined on (100) faces of single crystals:
CaB
6: 1840-1940kg/mm
2 SrB
6: 1680-1960kg/mm
2 BaB
6: 1770-2010kg/mm
2As-grown single crystals were used for the oxidation in air at temperatures between 700° and 1200°C. Oxidation began to proceed at 720°-730°C, and noncrystalline oxidation products were always obtained.
The oxidation proceeded in two stages. The initial stage of oxidation was found to be expressed by the general oxidation rate equation, (d
w)
n=
kt, and apparent activation energies of CaB
6, SrB
6 and BaB
6 single crystals calculated on the basis of the equation were 56kcal/mol, 55kcal/mol and 64kcal/mol, respectively.
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