New ternary compounds Sc3B0.75C3, Sc2B1.1C3.2, ScB15C1.60 and subsolidus phase relations in the Sc-B-C system at 1700 degrees C

The phase relations in the ternary system scandium-boron-carbon with specia l emphasis on the boron-rich section have been investigated in an isotherma l section at 1700 degrees C. Ternary compounds with the following stoichiom etries were identified: Sc2BC2, Sc3B0.75C3, Sc2B1.1C3.2, ScB2C2, ScB17C0.25 ,ScB15C0.80, and ScB15C1.60. Four of them, Sc3B0.75C3, Sc2B1.1C3.2, ScB15C1 .60, and ScB15C0.80, were synthesized for the first time. The first three c ould be indexed, i.e., Sc3BC3 shows tetragonal symmetry with 3.3308(3) Angs trom and c = 7.680(2) Angstrom; the basic hexagonal unit cell of Sc2B1.1C3. 2 is a = 3.3991(2) Angstrom, c = 6.7140(6) Angstrom, with a two-dimensional incommensurate structure with magnitude \q(i)\ =(2/7 + 0.005) q\(100)\Angs trom(-1) along [100] directions; and ScB15C1.60 shows a body centered ortho rhombic lattice a = 10.027(1) Angstrom, b = 8.0138(9) Angstrom and c = 5.66 68(6) Angstrom. A crystal of Sc2BC2 was grown by the floating zone method a nd the crystal structure was refined from single crystal data, which crysta llizes with tetragonal symmetry: I4/mmm, a = 3.3259(2) Angstrom, 10.6741(8) Angstrom, Z = 2, R = 0.011 for 263 unique reflections and 10 variables. (C ) 1999 Academic Press.