X-ray rietveld structure determination of ammonium Rochelle salt at 120 (paraelectric phase) and 100 K (ferroelectric phase)

Crystal structure of NaNH4(+)-C4H4O6.4H2O in both the paraelectric (at 120 K) and ferroelectric (at 100 K) phases, near the first-order phase transition at 109 K, was determined from X-ray powder data using the Rietveld refinement method. The largest relative displacements (greater than 0.30 A) of atoms in the ferroelectric phase, compared with the paraelectric phase, were found for one of the water molecules, the ammonium cations and one of the hydroxyl groups. The structural mechanism of the ferroelectric phase transition seems to rely on a cooperative process of the reorientation of the ammonium cations and water dipoles and the reorganization of the complex hydrogen-bonding network. For the monoclinic ferroelectric phase, the value and sign of the spontaneous polarization vector Ps (parallel to b) was found from a point-charge model as +0.22(1) x 10-6 C cm-2 at 100 K. (orig.)