ANHARMONICITY IN MnF₂ AT LOW TEMPERATURE: BEYOND THE FROZEN-CORE APPROXIMATION. By W. Jauch, Hahn-Meitner-Institut Berlin, Germany, A. J. Schultz, Argonne National Laboratory, IL, USA, R. F. Stewart, Carnegie-Mellon University, Pittsburgh, PA, USA

Antiferromagnetic order in MnF₂ induces a dipolar distortion of the fluorine inner electron shell. The core-deformation generates a substantial electric field at the nucleus, which is not compensated by peripheral lattice contributions, thus giving rise to an apparent Coulomb force on the nucleus. The force exerted on any nucleus should vanish in the stable equilibrium configuration. A local source of an opposing electric field could consist in a small skewness of the fluorine nuclear vibrational distribution which should persist even in the limit of zero temperature. This model rests upon the assumption that the electron deformation does not rigidly follow the nuclear motion.

Pulsed single-crystal neutron diffraction (T = 15 K) at the spallation source IPNS has been used to test this hypothesis. Data have been collected up to very high diffraction vectors, (sin[[theta]]/[[lambda]])_max = 2.75 Å^-1. The harmonic mean-square displacement parameters are in excellent agreement with previous results from both gamma-ray and neutron diffraction. Statistically significant third-order coefficients of a GramCharlier expansion could be extracted from the experiment. The shape of the antisymmetric part of the nuclear distribution function substantiates a subtle balance between the mean thermal electric fields due to the electronic and the nuclear charge density distribution. The sense of the skewness around the equilibrium position is opposite to the one found previously for the paramagnetic state. It is to be noted that data from nuclear scattering alone can provide valuable indications concerning a local redistribution of electron density.

Experiment thus confirms the conclusion that the core polarization in antiferromagnetic MnF₂ is dynamically stabilized. A theoretical description of the detailed physical mechanism is lacking at present.