Investigation of the La–Al–H and La–Si–H Systems at High Pressures

Hydrogenation at gigapascal pressures can produce hydrides with potential superconducting, ionic, and hydrogen-storage properties. We studied the La-Al-H and La-Si-H systems up to 20 GPa using structure prediction and in situ synchrotron diffraction. In La-Al-H, only rhombohedral LaAlH6 is stable. The La-Si-H system forms an orthorhombic monohydride, LaSiH, at low pressure, while LaSiH2 and LaSiH7 are predicted to be stable at 20 GPa, and LaSiH6 is slightly unstable. LaSiH2 is structurally related to the monohydride, whereas LaSiH6 and LaSiH7 feature SiH62- units characteristic of hydridosilicates. Calculations predict superconductivity in LaSiH2 and LaSiH6 with Tc ≈ 10 and 6 K. Experimentally, LaSiH2 formation is indicated at 20 GPa, but higher hydrides were not observed due to decomposition into LaH3 and Si, suggesting that pressures above 20 GPa are required to stabilize these phases at synthesis temperatures.