New applications of high-temperature solution calorimetry III. Enthalpies of formation of Mn 2B, MnB, and MnB 2

Abstract

The enthalpies of formation of three manganese borides have been measured by solution calorimetry in liquid copper at 1386 K. The following results are reported: αH f( 1 3 Mn 2B) = − (31.7±2.0)kJ ·mol −1, αH f( 1 2 MnB) = − (35.8±2.4)kJ ·mol −1, and αH f( 1 2 MnB 2) = − (21.1±3.4)kJ ·mol −1. The value for MnB 2 is only a preliminary one. We have also measured: (a) the enthalpies of mixing of { x BB(s) + (1 − x B)Cu(l)} (up to x B ≈ 0.015) at 1381 K, and (b) the enthalpies of solution of Mn(s) in Cu(l) at the same temperature. Comparisons between the enthalpies of solution of {(1 − x Mn)B + x MnMn} in Cu with the corresponding values for Mn in Cu and for B in Cu show evidence of strong chemical interaction between dissolved manganese and boron. This is interpreted to indicate extensive formation of a B a Mn b complex. The stoichiometry of this complex is not known. However, it tends to dissociate at higher a b = (1 − x Mn ) x Mn ratios.