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QQ1.05 - Finite Temperature Properties of Strongly Anharmonic and Mechanically Unstable Crystal Phases from First Principles 
April 22, 2014   9:45am - 10:00am

First principles harmonic stability analyses predict many important high-temperature crystal phases to be mechanically unstable, necessitating the development of new atomistic methods to rigorously account for anharmonic degrees of freedom at finite temperature. In this talk we present the recently-developed anharmonic potential cluster expansion framework and describe its use within Monte Carlo simulation to predict finite-temperature thermodynamic properties, mechanical properties, and structural phase transitions in strongly anharmonic and mechanically unstable phases. This framework allows a crystal's Born-Oppenheimer potential energy surface to be written as a polynomial expression that is invariant to finite rigid-body rotation and translation of the crystal, has been greatly simplified using the symmetries of a high-symmetry reference crystal, and can be parameterized from first principles electronic structure calculations, yielding an accurate, compact, and arbitrarily improvable model Hamiltonian. We have applied our method to ZrH, which exhibits a high-temperature cubic phase that, upon cooling, undergoes a symmetry-breaking second-order transition to one of three equivalent tetragonal phases. We find via Monte Carlo simulation that cubic ZrH, predicted by DFT to be dynamically unstable at 0K, can be anharmonically stabilized at high temperature, and our predicted cubic-to-tetragonal transition temperature is in good agreement with experiments. We also present calculated finite-temperature free energies and mechanical properties for both cubic and tetragonal ZrH, which can be used to parameterize continuum-scale constitutive models.

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