HIGH-ORDER DIFFRACTION SATELLITES IN RB2ZNCL4 - EXPERIMENTAL-EVIDENCEAGAINST OVERALL PHASON THERMAL FACTORS

The temperature dependence of the diffraction pattern in the incommens urate phase of Rb2ZnCl4 can be explained by the temperature variation of 3 structural parameters: the amplitude of the primary mode of Sigma (2) symmetry, the soliton density defining its anharmonicity, and the amplitude of a third harmonic modulation. The addition of overall phas on thermal factors is shown to be inconsistent with the experimental d ata which include fifth- and seventh-order satellites. It is further d emonstrated that the use, in conventional structural analysis, of an i ncomplete 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 s purious character.