Abstract
The LaF3 structure has previously been described in the space group P63/mmc with two formula units per unit cell. Weak additional reflections on single-crystal photographs lead to a larger unit cell with a = 7.190 ± 0.002 Å, c = 7.367 ± 0.002 Å, space group P[unk]c1, and six LaF3 per unit cell. The structure has been refined by full-matrix least-squares computations. The deviation from the high symmetry is caused by displacements of the F ions. LaF3 is shown to be isotypic with HoD3. The calculated ionic-lattice energy (−1151.6kcal/mole) agrees with the energy calculated by a Born-Haber cycle (−1144.4 kcal/mole). The probable critical radius ratio for the change from the LaF3 to the orthorhombic YF3 structure is 0.94. For smaller ratios the LaF3 structure can be stabilized by anion vacancies.
