Rochelle salt – a structural reinvestigation with improved tools. I. The high-temperature paraelectric phase at 308 K

The crystal structure of the high-temperature paraelectric phase of Rochelle salt (K⁺·Na⁺·C₄H₄O₆²⁻·4H₂O) at 308 K has been reinvestigated using synchrotron X-ray diffraction with refinement parametersR(int) = 0.0123, final (shift/e.s.d.)_max= 0.019,R₁(all) = 0.0371 andwR₂(all) = 0.0608. The application of a new gas-flow sample cell designed to control both temperature and relative humidity permitted collection of data of excellent quality and enabled unrestrained refinement of all parameters, including those of the isotropic hydrogen atoms. A precise description of the structure has ensued. One K atom is disordered between two symmetry-equivalent sites; three O atoms in three of the four water molecules exhibit very strong anisotropy. Refining one O atom as a split atom was successful, yielding small improvements in the bonding parameters of several H atoms. The H atoms of all water molecules behave as single pairs. Their finalUvalues are of moderate magnitude indicating that these atoms do not participate in the anisotropy of the parent O atoms. It is suggested that the three water O atoms are in part statically disordered, while the bonded H atoms are not. Except for the split K atom and the three water O atoms there is no evidence of general disorder in the structure.