Polymorphism in Metals II. Electronic and Magnetic Free Energy

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Abstract
The paper considers the role of the electronic and magnetic free energy in stabilizing the frequently observed high temperature bcc phase in metals. For transition metals, the conduction band density of states depends on the lattice structure but yet the corresponding difference in the temperature dependence of the free energy is in general not large enough to be the main reason for the polymorphism. This is discussed in some detail for titanium and zirconium. For rare earths, neither the conduction electrons nor the localized 4fstates are effective in stabilizing a high temperature bcc structure. The form of the free energy well above the Curie temperature is compared for the localized and the itinerant models of magnetism. Although the band model gives a good description for most weakly magnetic systems, iron is an exception in this respect. Finally, it is concluded that a somewhat lower Debye temperature for the bcc lattice remains a major reason for the close-packed to bcc structural transformations of transition metals.

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