Dipolar interaction in (NH 3.6 D 0.4 ) 2 PtCl 6

At 10% deuteration ammonium hexachloroplatinate stays in its cubic phase Fm3m at all temperatures. Rotational tunneling of NH4 and NH3D was measured by neutron spectroscopy. At temperatures T≥18 K, energies and intensities of the observed tunneling transitions agree with a statistical occurrence of ammonium isomers in a tetrahedral environment. Below T=18 K there is almost no change of the NH4 lines, while the NH3D line passes through an unresolved broad intensity distribution to a new three-line low-temperature spectrum. The intermediate spectrum is interpreted as the dynamical average of long-range dipolar-coupled orientationally disordered NH3D ions. The low-temperature spectra find an explanation as frozen minimum-energy configurations of NH3D monomers and dimers. A dipole moment of NH3D of 0.05 D can be estimated from the difference of the tunnel splittings.