Low-temperature chemical structure of the isostructural quantum multiferroics Cs2Cu2Mo3O12 and Rb2Cu2Mo3O12.

The dielectric response of the frustrated, quasi-1D quantum magnet Cs2Cu2Mo3O12 shows two components: a critical divergence at the quantum critical point, and an unusual relaxation at ultra-low temperatures. This feature is in complete agreement with a model of Debye relaxation of electric dipoles mediated by soft magnons, in a newly discovered multiferroic relaxation process. In contrast, the isostructural system Rb2Cu2Mo3O12 only shows the critical contribution. It is suspected that a reduction of lattice symmetry in Rb2Cu2Mo3O12 at low temperatures is responsible for the origin of the fundamentally distinct behavior. Therefore, determining the correct space groups at low temperatures of both materials is indispensable to understanding the origin of this exotic form of relaxation. Single crystal and powder diffraction will be combined to refine the chemical structures down to the lowest accessible temperature of 5 K.