Critical temperature and carbon substitution in MgB2 prepared through the decomposition of Mg(BH4)2

We have prepared Mg(BH4)(2) and then obtained MgB2 through the decomposition of the Mg(BH4)(2). Precursor viscous liquid was first prepared by the metathesis reaction of MgCl2 and NaBH4 based on mechanochemical and solution processes. Subsequent drying of the liquid at 180 degrees C results in the formation of alpha-Mg(BH4)(2), whereas the treatment at high drying temperature of 220 degrees C forms beta-Mg(BH4)(2). Heat treatment in a flow of Ar gas decomposes the Mg(BH4)(2) into MgH2 and B at 300 degrees C, and subsequently the reaction of the MgH2 and B occurs at 450 degrees C, followed by the formation of MgB2 as main phase at 500 degrees C. Although drying of the liquid at 300 degrees C forms not Mg(BH4)(2) but nearly amorphous powder, the heat treatment of the amorphous powder brings about MgB2 as main phase as well with slightly increased impurity phases. The thus obtained MgB2 shows a critical temperature (T-c) of 35 K. This low T-c is attributed to the poor crystallinity and the substitution of carbon supplied from the solvent in the solution process.