The thermodynamics of point defects such as vacancies, interstitials, and impurities, and their clusters in crystalline materials have been the subject of study for many years. Conventional approaches for characterizing the temperature-dependent thermodynamic properties of defects in crystals have been based on identifying energetic ground state structures followed by calculations of the vibrational degrees of freedom to add temperature dependence. In this talk it is shown that this approach can omit important entropic contributions that can dramatically alter the properties of crystal defects at high temperature.

In this work, the total free energy (excluding electronic contributions) of defects is computed using approaches borrowed from studies of supercooled liquids and glasses [1,2]. Molecular dynamics simulations identify a quasi-continuous distribution of stable configurations for each defect structure, each configuration corresponding to a distinct local minimum in the potential energy landscape [3]. The density of these local minima increases with defect complexity and is used to directly compute the total free energy, including the configurational entropy, which has often been neglected in previous studies of defect thermodynamics. The relative impacts of configurational and vibrational entropy on both vacancy and self-interstitial cluster thermodynamics are evaluated and shown to lead to rich behavior in the phase diagram of these defects. Finally, connections between the observed defect states and crystal melting are made on the basis of the computed density of states.

[1] S. Sastry, Nature, 409, 164 (2001). [2] F. Sciortino, W. Kob and P. Tartalia, Phys. Rev. Lett., 83, 3214 (1999). [3] F. H. Stillinger and T. A. Weber, Phys. Rev. A, 25, 978 (1982).