Abstract
The heavier alkaline-earth hydrides AeH2 (Ae = Ca, Sr, and Ba) are considered as promising materials for hydrogen energy storage. Pressure-induced structural changes in AeH2 materials could improve hydrogen transport properties and result in a better understanding of the structure-property relationship. In this work, pressure evolution of carrier transport properties of SrH2 was investigated using impedance spectroscopy measurements at room temperature and first-principles calculations. The pressure-induced structure phase transition from a Pnma phase to a P63/mmc phase was accompanied by a transition from pure electronic conduction to mixed ionic-electronic conduction, which was related to the ionic migration barrier energy. In the P63/mmc phase, the H− ionic and electronic resistances of bulk and grain boundaries were distinguished, respectively. The total resistance of SrH2 decreased by about four orders of magnitude after the phase transition. This work provides critical insight into the structure-conduction relationship and the role of grain boundaries in the transport process of alkaline-earth hydrides under high pressure.
Highlights
The pressure-induced structure phase transition from a Pnma phase to a P63/mmc phase was accompanied by a transition from pure electronic conduction to mixed ionic-electronic conduction, which was related to the ionic migration barrier energy
The total resistance of SrH2 decreased by about four orders of magnitude after the phase transition
Hydride ion conductors may be applied in energy storage and conversion devices with high energy densities
Summary
This work provides critical insight into the structure-conduction relationship and the role of grain boundaries in the transport process of alkaline-earth hydrides under high pressure. Discontinuous changes were observed at 8.4 GPa, which could be attributed to the pressure-induced structural phase transition from orthorhombic (Pnma) to hexagonal (P63/mmc).26 We calculated the enthalpy of the P63/mmc phase with respect to the Pnma phase with pressure, as shown in Fig. S2 (see the supplementary material), which indicates that the phase transition occurred at around 8.2 GPa. For the electronic transport process, the bulk electronic resistance (Rbe) decreased with the increasing pressure in the Pnma phase, while it increased in the P63/mmc phase [Fig. 3(a)].
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