NMR OF PHASE TRANSITIONS IN SEMIDISORDERED CRYSTALS. R. Blinc, J. Stefan, Institute, University of Ljubljana, Ljubljana, Slovenia

The usefullness of NMR for the study of the local structure and phase transitions in partially disordered systems is illustrated on the example of: i) proton glasses and quadrupolar glasses, ii) triple q modulated incommensurate crystals and iii) ferroelectric and antiferroelectric smectic liquid crystals. Specifically we show how NMR is used to determine the local polarization distribution function and the Edwards-Anderson order parameter in the deuteron glass Rb_l-x(ND₄)_xD₂PO₄ and the quadrupolar glass K_1-xNa_xCN, the soliton density in incommensurate Ag₃AsS₃ and the polar and bipolar orientational order parameters in the ferroelectric liquid crystal CE-8 and the antiferroelectric liquid crystal MHPOBC. In all the above cases we deal with small deviations from the average structure which are hard to detect by scattering techniques but which can be easily seen by quadrupole perturbed NMR. The results of 87Rb and 2H NMR in Rb_1-x(ND₄))_xD₂PO₄ and of 14N NMR in K_1-xNa CN show that the T-dependence of the Edwards-Anderson order parameter can be described by the random bond- random field model but not by random fields only. 75As NMR and NQR results show that the soliton density is the order parameter of the I-C transition in proustite. In the ferroelectric liquid crystal CE-8 the bipolar order parameter is larger than the polar one over the whole ferroelectric SmC* phase except close to the SmA-SmC* transition.