Structure and thermodynamic properties of the NaMgH₃ perovskite: A comprehensive study

One of the bottlenecks in the implementation of a hydrogen economy is the development of storage materials that can uptake high content of H₂ and release it within a suitable temperature and pressure range. Among the proposed hydride systems, the perovskite NaMgH₃ is receiving increasing attention, not only as the Mg ternary based hydride with the highest hydrogen gravimetric (6 wt %) and volumetric density (88 g L^-1) but also as a stable hydride likely to be formed in the transformation reactions of mixed hydrides. However, there is a large scatter in the literature for both the structure of the NaMgH₃ compound and the thermodynamics of the hydrogenation/dehydrogenation processes. In this paper a critical review of the literature data, supported by a new set of experimental (in situ synchrotron X-ray diffraction, infrared spectroscopy, high-pressure differential scanning calorimetry, pressure composition isotherms) and theoretical data is presented. The influence of ball milling on the microstructure is studied in the NaMgH ₃ in comparison to NaH and MgH₂. The infrared spectrum of NaMgH₃ compound, assigned by calculated and experimental results, is characterized by vibrational regions around 1100 and 600 cm^-1. In situ synchrotron X-ray diffraction measurements show the desorption reaction of NaMgH₃ into NaH and Mg at about 673 K under 0.2 MPa H₂, and the successive reabsorption of NaH and Mg back to NaMgH₃ at 623 K under 0.5 MPa H₂. From high-pressure differential calorimetry, it was measured a formation enthalpy of 141 kJ/mol f.u for NaMgH₃ compound. It was confirmed the possible reaction of NaH with Mg with observation of NaMgH₃ formation in 1.0 MPa H₂. Finally, this work provides a thermodynamic description of the NaMgH₃ phase by a critical assessment of the available information using the CALPHAD approach and the equilibrium pressure-temperature phase diagram is presented.