Anharmonicity in thermal motion and electrostatic forces on nuclei: Pulsed neutron diffraction fromMnF2

Abstract

Antiferromagnetic order in ${\mathrm{MnF}}_{2}$ induces a sharp polarization of the fluorine core electron shell. The dipolar core-deformation generates a sizable electric field at the nuclear site, thus giving rise to an apparent Coulomb force on the nucleus, which is not compensated by peripheral lattice contributions. A local source of an opposing electric field is proposed, consisting in a small skewness of the fluorine nuclear vibrational distribution. Pulsed single-crystal neutron diffraction $(T=15\mathrm{K})$ at the spallation source IPNS has been used to test the hypothesis. Data have been collected up to very high diffraction vectors, $(\mathrm{sin}\ensuremath{\Theta}/\ensuremath{\lambda}{)}_{\mathrm{max}}=2.75{\AA{}}^{\ensuremath{-}1},$ and statistically significant anharmonic parameters could be extracted. Though the predicted value for the electric field could not be confirmed quantitatively, the presence of anharmonicity at low temperature has been clearly established. The shape of the fluorine nuclear distribution function supports a subtle balance between the mean thermal electric fields caused by the electronic and the nuclear charge density distributions. The sense of the skewness around the equilibrium position is opposite to the one found for the paramagnetic state.