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An electrostatic model for the rotational potential of an ammonium tetrahedron in a cubic field is proposed. The rotational potential can be written as a rapidly converging series of kubic rotator functions. The NX4 ion, where X is hydrogen or deuterium, performs small angular motions about the minimum of the potential in both the ordered and disordered states, giving rise to librational energy levels. The anharmonic portion of the energy levels, both from the kinetic and potential energies, accounts for good agreement with infrared absorption data in NH4Cl, ND4Cl, NH4Br, and ND4Br. We use a one parameter fit to these data and then compute the potential barriers for rotations about threefold and fourfold axes of the tetrahedron. The difference between the transition temperatures for ND4Cl and NH4Cl is also explained.

A variational method is developed to treat rotational excitations of molecules and polyatomic ions in crystals. The theory is applicable for all potential strengths and is exact in the limits of weak and strong potentials. Rotational motion with one, two, and three degrees of freedom are treated in a unified manner. Explicit calculations are performed for the one‐dimensional rotation of a CH3 group in a C6 potential and the three‐dimensional reorientational motion of a methane molecule in Td and D2d potentials. The results are compared with recent neutron scattering experiments.

Superlattice reflections below the I/II phase transition of solid heavy methane are connected with the orientational ordering that occurs. As a consequence of a temperature-dependent neutron measurement, they may be grouped into two classes, both with structure amplitudes following exponential laws but having different exponents $\ensuremath{\beta}=0.21$ and ${\ensuremath{\beta}}^{\ensuremath{'}}=0.31$. This experimental behavior is well understood on the basis of a molecular-field-type analysis, which also shows that the system is close to a hypercritical point.

We have observed a pronounced anisotropy of critical neutron scattering at the 27-K phase transition of solid heavy methane. The anisotropy is due to orientational fluctuations of mainly two-dimensional character. We develop a formalism to treat neutron scattering from orientational fluctuations. Agreement with experiment is reasonable.

Der Anionenaustausch mit Salpetersaure-Methanol-Gemischen kann erfolgreich zur Reindarstellung vieler Elemente genutzt werden. Es werden Moglichkeiten der praktischen Anwendung solcher Glemische fur die Trennung der leichten Lanthaniden, fur die Reinigung der Lanthaniden von Zr und Hf und die Ausnutzung fūr die Produktion von radioaktiven Isotopen genannt.

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Using a lattice dynamical model for the soft modes in BaTiO3 we calculate the displacement correlation function of the Ti-ions. Its large anisotropy enables us to explain the puzzling anomalies of recent X-ray scattering experiments.

The effect of many body Van der Waals forces on the elastic constants of solid rare gases is calculated. These results are combinedwith contributions from the two body potential and zero point energy. The violation of the Cauchy relation is studied.