We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni7B3, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th7Fe3-type model (hexagonal crystal system, space group P63mc, no. 186, a = 696.836(4) pm, c = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni3B and Ni2B above 375 °C, and shows an exothermic effect at 424 °C.
Nickel Nickel I 7300 Room-Temperature Synthesis of Metal Borides. -Black amorphous precipitates of unknown composition are obtained from aqueous solutions of NaBH 4 and NiCl 2 . The samples are characterized by powder XRD, DTA/TGA, SEM, and EELS. The primary precipitate contains a metal boride formed at room temperature and exhibits catalytic activity, e.g. for arylations of arylhalogenides. After annealing at 500°C for 18 h, the primary product transforms into crystalline Ni 3 B. -(KAPFENBERGER, C.; HOFMANN, K.; ALBERT*, B.; Solid State Sci. 5 (2003) 6, 925-930; Inst. Anorg.
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