Neutron powder diffraction studies of stoichiometric NdOF between 1.5 and 300 K

Abstract The paramagnetic susceptibility of polycrystalline hexagonal neodymium oxyfluoride, NdOF, shows an anomalous evolution as a function of temperature. To explain the anomalies, the crystal structure of NdOF was studied between 1.5 and 300 K by neutron powder diffraction, Rietveld analyses, and bond valence calculations. The Rietveld refinements yielded good fits to the hexagonal space group R 3 m (No. 166). The unit cell volume and parameters a and c decrease with temperature, but the a parameter value reaches its minimum at 55 K, and thereafter starts to increase. The c parameter value decreases smoothly to 25 K but then drops sharply to compensate for the change in the a value. This behaviour typical of ferroelectric or -elastic compounds could be due to a small distortion from centrosymmetry—too small to be observed by powder diffraction. The bond valence calculations indicated that the stability of the hexagonal structure decreases with decreasing temperature but give no explanation for the anomalous cell parameter evolution.