PREPARATION OF CHROMIUM BORIDES BY SOLID-STATE REACTION BETWEEN CHROMIUM AND AMORPHOUS BORON

Chromium borides were prepared by solid state reaction between chromium and amorphous boron powder at 600 to 1500-degrees-C for 2h 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 = 0.10 approximately 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-degree-C in boron-chromium mixtures with a large excess boron content (B/Cr = 1.04 and 2.15). Cr2B and CrB2 were obtained as a single phase at 1500-degrees-C and 1400-degrees-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-degrees-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-degrees-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-degrees-C, B/Cr = 0.50 for Cr2B, 1400-degrees-C, B/Cr = 2.15 for CrB2. The lattice constants of chromium borides are as follows : Cr2B (orthorhombic) : a = 7.4021 (9) angstrom, b = 14.7044 (4) angstrom, c = 4. 2614 (7) angstrom, V = 463.82 (5) angstrom3 ; Cr5B3 (tetragonal) : a = 5.4781 (4) angstrom, c = 10.0920(5) angstrom, V = 302. 85(6) angstrom3 ; CrB (orthorhombic) : a = 2.9723(1) angstrom, b = 7.8647 (5) angstrom, c = 2.9303(3) angstrom, V = 68.49(9) angstrom3; Cr3B4 (orthorhombic) : a = 2.9923(4) angstrom, b = 13.0154 (9) angstrom, c = 2.9493(1) angstrom, V = 114.86(3) angstrom3; Cr2B3 (orthorhombic) : a = 3.0182(5) angstrom, b = 18.1300 (6)angstrom, c = 2.9513(9) angstrom, V = 161.49(5) angstrom3; CrB2 (hexagonal) : a = 2.9728(5) angstrom c = 3.0719(4) angstrom, V = 23.51(2) angstrom3.