Compression of the LiAlD4 system has been studied using in situ time-of-flight neutron diffraction up to 7.15 GPa and 60 î At ambient temperature (Ta), the structure remains as monoclinic a-LiAlD4 up to 7.05 GPa, displaying a 26.9% reduction in unit cell volume to 200.9 ų, yielding a 28.7% increase in volumetric efficiency to 136.5 kg D2/m3. Decompression to a small 3 ton load at Ta and holding for 24 hours did not induce any phase transformation, and the starting 0 GPa a-LiAlD4 structure was eventually recovered. Heating from Ta to 60 àwhile under compression to 7.15 GPa produces new and intense reflections in the diffraction pattern, while also retaining a-LiAlD4 reflections, indicating a two-phase a/߭LiAlD4 mixture. The ߭LiAlD4 structure indexed from this data is in the monoclinic space group I2/b, with unit cell dimensions a=4.099(3) Ŭ b=4.321(4) Ŭ c=10.006(7) Ŭ ?=88.43(2)ଠyielding 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) Ů Once again, decompression and cooling recovers the ambient a-LiAlD4 structure.