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 <i>in situ</i> 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 SiH₆^2- units characteristic of hydridosilicates. Calculations predict superconductivity in LaSiH₂ and LaSiH₆ with <i>T</i>_c ≈ 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.