An EPR and ENDOR study of the low frequency fluctuations and cluster formation in the hydrogen bonded ferroelectrics KH2PO4 and KD2PO4 and the antiferroelectrics NH4H2PO4 and ND4D2PO4

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EPR, ENDOR, and x-ray photoelectron (ESCA) spectroscopy are used here to characterize AsO4−4 and SeO3−4 radicals in KH2PO4, KD2PO4, NH4H2PO4, and ND4D2PO4 in their paraelectric and ferroelectric–antiferroelectric phases. Temperature dependence of the 75As or 77Se hyperfine structure and of the proton superhyperfine structure was used as a microscopic probe for studying low frequency (108–1011 Hz) fluctuations near the phase transitions. Above a certain temperature T*≳Tc, the EPR spectra show (the expected) axial symmetry. Below T*, the spectra exhibit the symmetry of either the ferroelectric or the antiferroelectric phase, and thus provide evidence for the formation of polarized clusters. From an analysis of the temperature dependence of the observed line shapes we deduce that the motion of the hydrogen bond protons is strongly coupled to that of the heavier ions; and the width of this mode is in the megahertz range around T*. This conclusion has been reached by using both probes in the phosphates. For arsenates, however, the SeO3−4 could not be stabilized.