(319c) Computationally Efficient Determination of Thermodynamic Stability of Five Metal Hydrides Using First Principles Including Hydrogen Isotope Effects

In Next Generation Nuclear Plants an application of metal hydrides as tritium sequestration materials requires materials that are thermodynamically stable to temperatures in excess of 1000 K. While the thermodynamics of metal hydrides at low to moderate temperatures have been successfully described by T = 0 K total energies and simple harmonic models, it is unclear as to what level of theory is sufficient to describe high temperature materials in which anharmonic effects may be significant. Since it is advantageous to apply computationally efficient methods in large scale material screening, we explored the predicted stabilities of five materials – ZrH₂, HfH₂, TiH₂, LiH_, and NaH – with four levels of theory using Density Functional Theory (DFT) to predict the dissociation temperatures, T_d, of very stable metal hydrides at one bar H₂ pressure. We also investigated the magnitude of the change in stability of hydrogen isotope-substituted metal hydrides with the aim of understanding if hydrogen isotope effects should be accounted for in material screening. Prediction of the Helmholtz free energy for the solid phases are based on 0 K total energy calculations, harmonic calculations at ground state volumes, quasiharmonic calculations, and quasiharmonic calculations with explicit anharmonic correction. It is determined that levels of theory which account for vibrational corrections to the crystal lattice (thermal expansion) are not necessary to get an accurate description of relative stabilities of metal hydrides. The simplest model with T_d described by DH/DS and DS=0.130 kJ (mol-K)^-1 predicts T_d to within 75 K of the much more computationally expensive quasiharmonic calculation for all materials.The shift in T_d upon substitution of tritium relative to protiated metal hydrides is within 50 K for all materials studied with larger shifts for lighter materials as expected. Accounting for vibrational effects due to isotope substitution in metal hydrides is unnecessary to accurately predict the relative stabilities of metal hydrides at high temperatures.