Bonding properties and vibrational entropy of transition metal MeB 2(AlB 2) diborides

Abstract

The transition metal diborides MeB 2 were chosen as an example to illustrate an approach introduced by us, by which existing thermodynamic information can be systematized and new information generated. We consider trends in the melting temperature, the enthalpy of formation and lattice vibration properties as the metal Me varies along the 3d, 4d and 5d transition metal series. The trends show a consistent picture which can be explained from the electronic density of states by a rigid-band like argument. This fact gives us confidence to apply the regularities and obtain new information by interpolation and extrapolation procedures. In particular, we estimate the standard entropy S⊖ at 298.15 K for stable AlB 2 structure compounds where there are no previous measurements and for some metastable compounds ( Me ≡ Sc, V, Cr, Mn, Y, Mo, Tc, W and Re). The results are compared with estimates based on various empirical methods. In the Me-B systems, the MeB 2 phase may be stable at all temperatures, or be a high-temperature phase or a metastable phase. We explain the observed trend. Through the use of thermodynamic phase diagram calculations (the so-called CALPHAD method), we study the thermodynamic properties of the metastable AlB 2 structure phase FeB 2.