A nuclear magnetic resonance investigation of the crystal structure of LaF3

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Abstract
The perturbation upon the nuclear magnetic resonance (NMR) frequency of a nucleus due to the interaction between the gradient of the crystalline electric field and the electric quadrupole moment or the nucleus makes it possible to determine the symmetry of its surroundings. Measurements on La139 in single crystals of LaF3 lead to a lanthanum site symmetry of either 2–C 2, with the twofold axis normal to the crystal c axis, or m–Cs , or 2/m–C 2[unk] with the mirror plane containing the c axis. This low symmetry is inconsistent with earlier suggested models of the LaF3 structure. The NMR measurements show that the crystal has Laue symmetry 6/mmm and, with the help of data from x-ray measurements, the space group is determined to be P63 cm–C 6v 3 where the point symmetry of the La sites is m–Cs . A new model with six formula units in the unit cell is proposed. This model is also supported by NMR measurements of the chemical shifts of the F19 nuclei and is shown to be able to explain results obtained in electron spin resonance measurements.

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