The crystal structures of thallium(I) fluoride

The effects of temperature and pressure on the crystal structures of thalli um(I) fluoride have been investigated using powder neutron diffraction, wit h the aim of resolving the uncertainties present in the literature. Under a mbient conditions, TIF adopts an orthorhombic structure in space group Pbcm with Z = 4 and cell parameters a = 6.09556(8) Angstrom, b = 5.48860(7) Ang strom, and c = 5.18300(7) A. This structure can be derived from an idealize d rocksalt-type arrangement, though with extensive distortions of the anion sublattice due to the presence of the 6s(2) inert pair of the TI+ Above 35 5 K TIF becomes tetragonal with Z = 2, a = 3.78283(2) Angstrom, c = 6.12312 (5) Angstrom, and space group P4/nmm. The behavior of the compound is also studied under hydrostatic pressure but, contrary to previous reports, no st ructural transition was observed and TIF remains orthorhombic up to at leas t 3.5 GPa. The compressibility is greatest along the a and b axes. The rela tionship between the ambient- and high-temperature structures of TIF is des cribed and the influence of the inert pair discussed in relation to the mas sicot structured polymorph of PbO. (C) 2000 Academic Press.