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 H-2 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 NaMgH3 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 H-2, and the successive reabsorption of NaH and Mg back to NaMgH3 at 623 K under 0.5 MPa H-2. 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 H-2. 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.