Elucidating the Role of Temperature and Pressure to the Thermodynamic Stability of Charged Defects in Complex Metal-Hydrides: A Case Study of NaAlH4

Complex metal hydrides are one of the most technologically relevant classes of hydrogen storage materials because of their huge applications as clean energy alternatives. In this class of materials, hydrogen-related point defects have been shown to play a crucial role in catalyzed dehydrogenation. We investigate the effects of environment (viz. finite temperature, hydrogen partial pressure, and doping) to understand the thermodynamic stability of point defects as a function of various charge states at a realistic condition in a bulk complex metal-hydride, using NaAlH4 as an example. Our approach employs density-functional theory (DFT) combined with ab initio atomistic thermodynamics, where the free energy of formation due to vibration of phonons is duly considered under harmonic approximation. We show that to understand the thermodynamic stability of various defects and its self-diffusion, the contribution of environmental effect to the free energy of formation is absolutely indispensable. We further validate that DFT with appropriate exchange and correlation functionals fails to predict the stable phases even at a moderately low temperature.