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Abstract
Identifying the forces that drive a phase transition is always challenging. The hcp-fcc phase transition that occurs in cobalt at ~700 K has not yet been fully understood, although early theoretical studies have suggested that magnetism plays a main role in the stabilization of the fcc phase at high temperatures. Here, we perform a first principles study of the free energies of these two phases, which we break into contributions arising from the vibration of the lattice, electronic and magnetic systems and volume expansion. Our analysis of the energy of the phases shows that magnetic effects alone cannot drive the fcc-hcp transition in Co and that the largest contribution to the stabilization of the fcc phase comes from the vibration of the ionic lattice. By including all the contributions to the free energy considered here we obtain a theoretical transition temperature of 825 K.
Highlights
Phase transitions are one of the most fundamental phenomena of matter
Since we believe that the PM state at higher temperatures is well described by disordered local moments approach (DLM), one must conclude that the withdrawal of the FM ordering does not destabilize hcp and other mechanisms must be invoked to explain the structural transition
This early explanation is at odds with the fact that fcc Co is still magnetic up to ~1400 K and that the magnitude of the local magnetic moments are reduced around 8% at the Curie temperature[13]
Summary
Phase transitions are one of the most fundamental phenomena of matter. Understanding the driving forces behind them enables development of new theories, discoveries and tailor-design of new materials. Söderlind et al extended Skriver’s theory to account for the magnetic 3d elements using the fractional filling of both, spin-up and spin-down sub-bands[10] Following these arguments one can conclude that if Co was not magnetic it would choose the fcc phase as a ground state. Uhl and Kübler concluded from their theory that spin-fluctuations and reduced magnetization at higher temperatures lower the free energy of the fcc phase with respect to the hcp, triggering the phase transition[8, 12] They reported a calculated transition temperature T = 590 K. from magnetic effects only, they did not rule out other mechanisms such as phonons, as responsible of the structural transformation
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