1992 Volume 1992 Issue 11 Pages 1320-1325
Chromium borides were prepared by solid state reaction between chromium and amorphous boron powder at 600 to 1500 °C for 2 h in an argon atmosphere. Seven kinds of chromium boride phases such as Cr2B, Cr5B3, CrB, Cr3B4, Cr2B3, CrB2 and CrB4 were obtained from the starting materials with the atomic ratios B/Cr = O.1 0-2.94. The variation of the atomic ratio of the starting materials gave different product phases. With increased boron content, more boron-rich phases are formed, however the boride phase having the composition of the highest boride, CrB6, which does not exist was not detected. The formation of Cr2B began at 600 °C in boron-chromium mixtures with a large excess boron content (B/Cr=1.04and 2.15). Cr2B and CrB2 were obtained as a single phase at 1500 °C and 1400 °C, respectively, while, Cr5B3, CrB, Cr3B4, Cr2B3 and CrB4 were invariably obtained as a phase mixtures for various atomic ratios in the starting material at temperatures from 600 to 1500 °C. However, when starting materials with the atomic ratio of B/Cr=1.04 were used, the CrB phase shows the strongest relative X-ray intensities among CrB and Cr5B3 at above 1400 °C. The optimal conditons for the synthesis of Cr2B and CrB2 were found to include a reaction time of 2 h, reaction temperature and the atomic ratios of starting materials: 1500°C, B/Cr=0.50 for Cr2B, 1400°C, B/Cr=2.15 for CrB2. The lattice constants of chro mium borides are as follows: Cr2B (orthorhombic): a=7.4021( 9 ) Å, b=14.7044 ( 4 ) Å, c =4.2614 ( 7 ) Å, v=463.82 ( 5 ) Å3; Cr5B3 (tetragonal): a =5.4781( 4 )Å c=10.0920( 5 ) Å, V=302.85( 6 ) Å3; CrB (orthorhombic): a =2.9723( 1 )Å, b=7.8647( 5 ) Å, c=2.9303(3 ) Å, V=68.49( 9 ) Å3; Cr3B4(orthorhombic): a = 2.9923( 4 ) Å, b=13.0 154( 9 ) Å, c=2.9493( 1 ) Å, V=114.86( 3 )Å3; Cr2B3 (orthorhombic) a =3.0182( 5 )Å, b=18.1300( 6 )Å, c=2.9513( 9 ) Å, V=161.49( 5 ) Å.; CrB2(hexagonal): a=2.9728( 5 ) Å, c=3.0719( 4 ) Å, V=23.51(2 )Å3.
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