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

Abstract

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.

Ab initiostudy of the one-dimensional H-bonded ferroelectricCsH2PO4

“…Recently, ab initio calculations based on the density functional theory (DFT) were conducted in KDP and its antiferroelectric (AFE) isomorph NH 4 H 2 PO 4 (ADP) [13,14,[27][28][29][30][31]. It was found that significant instabilities arise only when the heavy atoms are allowed to relax with the protons, a fact that is in accordance with the experiments [13,14,[32][33][34][35].…”

Hemimorphite as a natural sink for arsenic in zinc deposits and related mine tailings: Evidence from single-crystal EPR spectroscopy and hydrothermal synthesis

“…The [AsO 4 ] 4À radical in hemimorphite is similar in principal g and A values to its well-established counterparts in other crystalline hosts (e.g., Dalal et al, 1972Dalal et al, , 1977Murty et al, 1977). This radical has been shown to have the un- Table 3).…”

“…The average linewidth of this quartet is $0.3 mT. The presence of As from ICPMS analyses, together with the fitted spin Hamiltonian parameters (see below), allows us to identify this quartet to represent the [AsO 4 ] 4À radical (cf., Dalal et al, 1972Dalal et al, , 1977.…”

Ionic rearrangements and domain growth near antiferroelectric transitions: EPR study of NH4H2AsO4, ND4D2AsO4, NH4H2PO4, and ND4D2PO4

“…Hence the motional fluctuations observed via this probe will not be expected to be the same as those of As043-or ~0~~-units in the unperturbed lattice. Of course Se043-has the same charge as the ~0~~-or A S O~~-units, but electron nuclear double resonance (ENDOR) measurements have shown (6) that the disposition of the hydrogen-bonded protons around the ~e 0~~-center, and hence its observed site symmetry (D2d) is quite different from that (7) (C,) of a ~0 4~-unit below T,. On the other hand, c1-0~~-has the same charge and approximate size as a ~0~~-or A S O~~-unit, and recent ENDOR studies have shown (8) that its proton arrangement and hence site symmetry approximate those of a host antiferroelectric lattice.…”