Miedema’s approach

One of the most useful models in evaluating both enthalpy of formation and volume effects in intermetalllc compounds is Miedema s approach (Miedema Niessen, 1982 de Boer et al., 1988). It uses an atomic cellular model of the alloy, based on the concept of Wigner-Seitz (W.S.) cell. The intermetalllc compound AB is considered as being formed by the atomic W.S. cells of the pure metals A and B (seeRgureS). 

Problem 4 Compare experimental and calculated volumes per atom for the six phases of the Ca - Pd system. To obtain calculated volumes apply Zen s law, Miedema s approach and Metlo s approach. 

Miedema s model and parameters and prediction of compound formation capability. A semi-empirical approach to the evaluation of the compound formation capability and of the heats of formation of alloys was proposed by Miedema and co-workers. This resulted in a model which became very popular and, especially because there was scarcity of experimental data, was frequently used in the evaluation, even if approximate and several times incorrect, of the formation enthalpies. The model suggested for energy effects in alloys is well known essentially it is based on the definition of two parameters. By assigning two coordinates... 

Miedema s theory and structural information. The Miedema model for energy effects in alloys, presented in 2.2.1.3 has been very useful in an evaluation, albeit approximate, of the formation enthalpies and in the prediction of compound formation capability. For an example of the application and limits of this model, see the comments on the thermochemistry of the Laves phases reported in 3.9.3. However notice that the general usefulness of the Miedema approaches has diminished with time, both for its inherent approximation and for... 

During this period, various aspects of Miedema s methods for predicting the heat of formation of binary compounds were assembled and eventually published in book form (de Boer et al. 1988). This included the application of the technique to predict the thermodynamic behaviour of some ternary compounds. Whilst only applicable to a restricted set of crystallographic structures, this was nevertheless a significant development, as a common objection to the CALPHAD approach was that the existence of ternary compounds could never be predicted solely from binary data. 

Most heats of formation of binary hydrides were determined experimentally, and they are used in Equation 15 with calculated AH(ABn) values. Miedema s (18) approach for calculating AH(ABn) is summarized by Stewart, Lakner, and Uribe (8) ... 

A good agreement is observed between experimental and calculated values by both Miedema s and Merio s approaches, confirming the validity of these methods. On the contrary, the values calculated by Zen s law are always higher, CaPd showing the maximum contraction. 

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