Potassium sodium (2R,3R)-tartrate tetrahydrate: the paraelectric phase of Rochelle salt at 105 K

Rochelle salt, K+·Na+·C4H4O6 2−·4H2O, is known for its remarkable ferroelectric state between 255 and 297 K. The current investigation, based on data collected at 105 K, provides very accurate structural information for the low-temperature paraelectric form. Unlike the ferroelectric form, there is only one tartrate molecule in the asymmetric unit, and the structure displays no disorder to large anisotropic atomic displacements.

Rochelle salt, K + ÁNa + ÁC 4 H 4 O 6 2À Á4H 2 O, is known for its remarkable ferroelectric state between 255 and 297 K. The current investigation, based on data collected at 105 K, provides very accurate structural information for the lowtemperature paraelectric form. Unlike the ferroelectric form, there is only one tartrate molecule in the asymmetric unit, and the structure displays no disorder to large anisotropic atomic displacements.

Comment
The radiation-induced free radical chemistry of dicarboxylic acids and their salts has received attention for several decades.
The Rochelle salt, is of particular interest as it as it exhibits a ferroelectric phase between 255 and 297 K, where the structure is monoclinic, space group P2 1 ; outside this temperature range the compound is paraelectric and presents orthorhombic phases in space group P2 1 2 1 2. The nature of the radicals formed in Rochelle salt is currently investigated in order to understand the mechanisms producing changes in the ferroelectric properties of this compound upon irradiation (Suzuki, 1974;Treeck, van & Windsch, 1977). For the analysis of the electron magnetic resonance data, precise knowledge of the low-temperature orthorhombic form is necessary. Structural data for the high-temperature orthorhombic form were first provided by Beevers & Hughes (1941). Iwata et al. (1989) carried out a neutron diffraction study for both orthorhombic forms; more accurate X-ray diffraction studies were later presented by Solans et al. (1997), who concluded that differences between the two P2 1 2 1 2 states are "small but significant". None of these structures are, however, available in the Cambridge Structural Database (Version 5.29 of November 2007;Allen, 2002). A high-precision redetermination of Rochelle salt at low temperature has therefore been executed.
Hydrogen bonds are listed in Table 1, the most unusual feature is the almost symmetric four-center interaction involving H31W.
When K + is replaced by NH 4 + [as, for instance, in II] the four shortest K2···O contacts are converted into hydrogen bonds, while only the two K1···O4 interactions are transformed into short hydrogen bonds, the K1···O1W and K1···O2W contacts being replaced by a three-center hydrogen bond.

Experimental
Rochelle salt was obtained from Sigma-Aldrich and tetrahydrate crystals were grown from saturated aqueous solutions. A large block-shaped speciemen was ground into a sphere in a mill and used for data collection.

Refinement
Full isotropic refinement was carried out for all H atoms. Fig. 1. : The molecular structure of (I). Displacement ellipsoids are shown at the 50% probability level. Metal ccordination has been indicated by dashed lines.   Refinement. Data were collected by measuring six sets of exposures with the detector set at 2θ = 29° and 65°, crystal-to-detector distance 5.00 cm. Refinement of F 2 against ALL reflections.