X-RAY RIETVELD STRUCTURE DETERMINATION OF AMMONIUM ROCHELLE SALT AT 120-K (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 phas e transition at 109 K, was determined from X-ray powder data using the Rietveld refinement method. The largest relative displacements (great er than 0.30 angstrom) of atoms in the ferroelectric phase, compared w ith the paraelectric phase, were found for one of the water molecules, the ammonium cations and one of the hydroxyl groups. The structural m echanism of the ferroelectric phase transition seems to rely on a coop erative process of the reorientation of the ammonium cations and water dipoles and the reorganization of the complex hydrogen-bonding networ k. For the monoclinic ferroelectric phase, the value and sip of the sp ontaneous polarization vector P(s) (parallel to b) was found from a po int-charge model as + 0.22 (1) x 10(-6) C cm-2 at 100 K.