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 NaMgH 3 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 À1 ) 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 NaMgH 3 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 MgH 2 . The infrared spectrum of NaMgH 3 compound, assigned by calculated and experimental results, is characterized by vibrational regions around 1100 and 600 cm À1 . In situ synchrotron X-ray diffraction measurements show the desorption reaction of NaMgH 3 into NaH and Mg at about 673 K under 0.2 MPa H 2 , and the successive reabsorption of NaH and Mg back to NaMgH 3 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 NaMgH 3 compound. It was confirmed the possible reaction of NaH with Mg with observation of NaMgH 3 formation in 1.0 MPa H 2 . Finally, this work provides a thermodynamic description of the NaMgH 3 phase by a critical assessment of the available information using the CALPHAD approach and the equilibrium pressureÀtemperature phase diagram is presented.

After a screening of possible systems prone to give an enthalpy of decomposition close to 30 kJ·mol -1 H2 , i.e. suitable for a dehydrogenation process close to room temperature and pressure, the Zn dissolution into Mg(BH 4 ) 2 has been investigated. The total energy of pure compounds and solid solutions has been computed by DFT calculations using the CRYSTAL09 code. The phase mixture obtained after the synthesis strongly depends on the milling conditions. For prolonged times, the formation of Zn and MgCl 2 has been observed, suggesting the delivering of B-containing species during the milling. After heating, a hydrogen release, coupled with diborane delivering, has been observed for temperatures close to 100 °C, suggesting a significant decrease of the decomposition temperature with respect to pure Mg(BH 4 ) 2 . Theoretical and experimental results have been discussed on the basis of the possibile reaction paths, as estimated from available thermodynamic databases.

a b s t r a c t Thermodynamic data of the LiBH 4 compound are reviewed and critically assessed. On the basis of literature data of heat capacity, heat of formation, temperature and enthalpy of phase transitions, a CALPHAD optimized Gibbs energy function is derived for the condensed phases i.e. orthorhombic and hexagonal solid phases and the liquid phase. Considering hydrogen as an ideal gas phase, the thermodynamics of decomposition reactions of LiBH 4 is calculated, showing good agreement with existing experimental data.

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