Preparation, thermal stability and crystal structure of Li3UF7: New insights into LiF-UF4 binary phase diagram

Abstract

In this manuscript the crystal structure of Li3UF7 from powder X-ray and neutron diffraction studies is reported. The existence of Li3UF7 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 UF4, and an optimum cooling rate to prepare Li3UF7. The Li3UF7 phase could be obtained by cooling the molten LiF-UF4 with large excess of LiF at a rate of 5 K/min while slower cooling tends to form LiUF5. The crystal structures of Li3UF7 and LiUF5 in the temperature range of 300–10 K have been obtained from the powder neutron diffraction studies. At 300 K, Li3UF7 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: Ccca), and is isostructural to orthorhombic Li3ThF7. On cooling, the orthorhombic distortion of Li3UF7 increases but does not reveal any structural transition down to 10 K. The crystal structure of Li3UF7 is built from the defective (Li-deficient) layers of edge shared octahedral LiF6 and layers corner connected UF9 polyhedra. Analogous studies on LiUF5 have also been carried out, which has a tetragonal (Space Group: I41/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 LiUF5 is built from the three-dimensional sub-lattice formed by U4F30 units and the empty tunnels of the sublattice are occupied by Li4F16 clusters formed by four edge shared LiF6 octahedra. Also, the details of crystal chemistry of the compounds in LiF-UF4 systems and their thermal stabilities are discussed. The complexity in the crystal chemistry in LiF-UF4 system is manifested in multiple close by thermal events in the DTA runs which has also been discussed in the manuscript.