High-order diffraction satellites in Rb2ZnCl4: Experimental evidence against overall phason thermal factors

The temperature dependence of the diffraction pattern in the in com men su ra te phase of Rb2ZnCl4 can be explained by the temperature variation of 3 structural parameters: the amplitude of the primary mode of Σ2 symmetry, the soliton density defining its anharmonicity, and the amplitude of a third harmonic modulation. The addition of overall phason thermal factors is shown to be inconsistent with the experimental data which include fifth- and seventh-order satellites. It is further demonstrated that the use, in conventional structural analysis, of an incomplete structural model with null or small anharmonicity, together with a reduced dataset lacking higher-order weak satellites, may lead to the successful refinement of a phason thermal factor, which has a spurious character.