Dalal, N. C. ; McDowell, C. A. ; Srinivasan, R. (1974) Magnetic resonance studies of the antiferroelectric phase transitions in the ammonium arsenates NH4H2AsO4 and ND4D2AsO4 Journal of Chemical Physics, 60 (10). pp. 3787-3794. ISSN 0021-9606
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Official URL: http://jcp.aip.org/resource/1/jcpsa6/v60/i10/p3787...
Related URL: http://dx.doi.org/10.1063/1.1680820
Abstract
The antiferroelectric phase transitions in NH4H2AsO4 and ND4D2AsO4 have been studied through EPR and NMR. The paramagnetic center AsO44-, created by x irradiation of these compounds, was used as a microscopic probe in the EPR studies. Both 75As hyperfine structure and the superhyperfine structure from the hydrogen-bond protons showed changes on passing through the Curie point. Some features of the EPR spectra have been identified with the configurations postulated by Nagamiya in his model of the phase transition in these antiferroelectrics. In conjunction with earlier Raman scattering, Mössbauer effect, NMR, and dielectric studies of ADP, the present EPR studies point to a model of the phase transition where the NH4+ ions play a dominant role. At high temperatures, well above the Curie point (Tc), the NH4+ ions are thought to be executing fast reorientations such that they behave effectively like the spherical ions K+, Rb+, or Cs+. At Tc there is sudden change in the motion of the NH4+ ions, and each NH4+ ion is thought to make a permanent hydrogen bond with an oxygen of an AsO4 group, leading to a distortion of the group and hence a phase transition. Measurements of the proton nuclear spin-lattice relaxation times (T1) support this model. Comparison with earlier results on NH4H2PO4(ADP) suggests that the same mechanism is operative in the phase transition of ADP also. Several anomalies mentioned in earlier studies on KH2PO4-type compounds could also be rationalized on the basis of the present model. Implications of the present work to the Nagamiya model and further experiments to check the conclusions of the present work are also pointed out.
Item Type: | Article |
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Source: | Copyright of this article belongs to American Institute of Physics. |
ID Code: | 36849 |
Deposited On: | 11 Apr 2011 13:32 |
Last Modified: | 11 Apr 2011 13:32 |
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