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In this manuscript the crystal structure of Li 3 UF 7 from powder X-ray and neutron diffraction studies is reported. The existence of Li 3 UF 7 has been debated for a long time, as it is often missed in a number of phase diagram studies. This study adopted a judicious composition of LiF and UF 4 , and an optimum cooling rate to prepare Li 3 UF 7. The Li 3 UF 7 phase could be obtained by cooling the molten LiF-UF 4 with large excess of LiF at a rate of 5 K/min while slower cooling tends to form LiUF 5. The crystal structures of Li 3 UF 7 and LiUF 5 in the temperature range of 300–10 K have been obtained from the powder neutron diffraction studies. At 300 K, Li 3 UF 7 has an orthorhombic lattice with unit cell parameters: a = 8.698(2) Å, b = 8.692(2) Å, c = 12.803(1) Å, V = 968.0(4) Å3; Z = 8 (Space group: C cca), and is isostructural to orthorhombic Li 3 ThF 7. On cooling, the orthorhombic distortion of Li 3 UF 7 increases but does not reveal any structural transition down to 10 K. The crystal structure of Li 3 UF 7 is built from the defective (Li-deficient) layers of edge shared octahedral LiF 6 and layers corner connected UF 9 polyhedra. Analogous studies on LiUF 5 have also been carried out, which has a tetragonal (Space Group: I 4 1 /a) lattice with unit cell parameters: a = 14.8882(8) Å, c = 6.5438(7) Å, V = 1450.5(2) Å3; Z = 16. The structure of LiUF 5 is built from the three-dimensional sub -lattice formed by U 4 F 30 units and the empty tunnels of the sublattice are occupied by Li 4 F 16 clusters formed by four edge shared LiF 6 octahedra. Also, the details of crystal chemistry of the compounds in LiF-UF 4 systems and their thermal stabilities are discussed. The complexity in the crystal chemistry in LiF-UF 4 system is manifested in multiple close by thermal events in the DTA runs which has also been discussed in the manuscript. ga1 • Synthesis and crystal structure of metastable Li 3 UF 7 in LiF-UF 4 are reported. • Conditions for formation of different phases in LiF-UF 4 system are delineated. • Crystal structures of Li 3 UF 7 and LiUF 5 down to 5 K are studied by LT-PND. • Anisotropic thermal expansion in Li 3 UF 7 is observed in between 5 and 300 K. • LiF-UF 4 phase diagram and comparative crystal chemistry phases in it are discussed. [ABSTRACT FROM AUTHOR]