The angular dependence of the second-order quadrupole shifts of the Na 23 magnetic resonance has been measured in the two paraelectric phases of both deuterated and normal Rochelle salt, as well as in the intermediate ferroelectric phase, where an additional splitting of the Na 23 lines has been found. The electric quadrupole coupling constants, asymmetry parameters, and orientations of the principal axes of the electric-fieldgradient (EFG) tensors at the sodium sites have been determined. The results of a point-charge-model calculation show that the changes in the EFG tensors on going from the low-temperature paraelectric phase to the ferroelectric phase can be explained by a displacement of two out of the four 0(5)-H hydrogens and 0(8) water molecules in the unit cell, the principal component of the displacement vector pointing in the direction of the ferroelectric axis.T HE first crystal known to exhibit ferroelectric properties was Rochelle salt. 1 It belongs to the still rather small group of crystals with two ferroelectric Curie points. 2 The shifts of the upper Curie point from 24 to 35°C and of the lower from -18 to -22°C on deuteration of the OH groups and water molecules demonstrates the role of the hydrogen atoms in the ferroelectric behavior of this crystal. Although its structure has been the subject of many investigations 3-5 there is relatively little known about the detailed mechanism of the ferroelectric transition. The recent re-examinations of the structure of Rochelle salt by x-ray and neutron diffraction techniques 4 ' 5 revealed significant departures from the atomic coordinates given in the first x-ray analysis 3 of this crystal. Unfortunately, the single-crystal neutron diffraction study of Frazer and co-workers 5 is not yet completed. The crystal structure is orthorhombic (space group P2i2i2) in the two paraelectric phases and monoclinic (space group P2i) in the ferroelectric phase. There are four formula units NaKC4H 4 06-4H 2 0 in the unit cell. Some of the preliminary hydrogen positions, published so far, are at variance with the proton resonance line shape data by Losche. 6 Thus it appears that though the molecular mechanism proposed on the basis of the earlier structure 3 is definitely not acceptable, the detailed nature of the transition is not clear and further investigations are required. Some years ago the angular and temperature dependence of the proton magnetic-resonance absorption in Rochelle salt was studied in this laboratory. 7 Though the angular dependence of the second moments qualitatively agreed with the protonic arrangement as proposed by Losche, 6 we were not able to deduce which are the ferroelectric dipoles responsible for the changes in the second moments at the two transition temperatures. 1 In order to throw additional light upon the changes in the crystal structure and atomic arrangement associated with the ferroelectric transition, we decided to undertake a Na 23 magnetic-resonance study of both normal and deuterated Rochelle salt. Some preliminary results on ...
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