Pressure-induced phase transitions of the LiAlD₄ system

Compression of the LiAlD₄ system has been studied using in situ time-of-flight neutron diffraction up to 7.15 GPa and 60 °C. At ambient temperature (T_a), the structure remains as monoclinic α-LiAlD₄ up to 7.05 GPa, displaying a 26.9% reduction in unit cell volume to 200.9 Å³, yielding a 28.7% increase in volumetric efficiency to 136.5 kg D₂∕m³. Decompression to a small 3 ton load at T_a and holding for 24 hours did not induce any phase transformation, and the starting 0 GPa α-LiAlD₄ structure was eventually recovered. Heating from T_a to 60 °C while under compression to 7.15 GPa produces new and intense reflections in the diffraction pattern, while also retaining α-LiAlD₄ reflections, indicating a two-phase α∕β-LiAlD₄ mixture. The β-LiAlD₄ structure indexed from this data is in the monoclinic space group I2∕b, with unit cell dimensions a=4.099(3) Å, b=4.321(4) Å, c=10.006(7) Å, γ=88.43(2)°, yielding a volumetric efficiency of 151.2 kg D₂∕m³. The atomic arrangement in I2∕b is identical to that in the theoretically predicted I4₁∕a super group, with isolated symmetric AlD₄ tetrahedra displaying AlD distances of 1.545(2) Å. Once again, decompression and cooling recovers the ambient α-LiAlD₄ structure.