Quantum disorder in (NH4)2SnCl6: a quantum mechanical single-particle model interpretation of neutron diffraction data

Abstract

The authors report a direct measurement of the quantum mechanical ground-state density distribution of a 3D rotator (NH4) in a crystalline rotational potential V( omega ). Neutron Bragg scattering data of (NH4)2SnCl6 taken at T=6K are analysed in terms of a density distribution, which is expanded into a series of symmetry-adapted functions. The coefficients in this series are determined in two ways: (i) directly from the experimental data and (ii) from a complete diagonalisation of a single-particle Hamiltonian. In the latter case three potentials V( omega ) known from the literature are used. They find that the potential derived recently by two of the authors (Vogt and Prandl, 1983) from room-temperature neutron data with a classical Boltzmann-type approach provides the best fit to the 6K measurement. None of the potentials is able to reproduce completely the tunnel splitting observed by other authors.