NMR study of molecular reorientations in NH4H2AsO4

Grosescu, R.

doi:10.1016/0009-2614(73)80019-3

Cited by 10 publications

(10 citation statements)

References 7 publications

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“…Clearly, the T 1 measurements primarily characterize the motion of the NH 4 + groups in ADA. Slowing down of the reorientational motion of the NH 4 + groups at T N evidently suggests that one of the four protons in the NH 4 + groups starts to form an N–H···O bond, thus leading to a localization of the O–H···O protons in one of the two minima of their O–H···O bond, supporting directly the theoretical model of antiferroelectric transition in ADP (and by implication, ADA) by Lasave et al It seems worth clarifying that the goal of the present work was quite distinct from that of Lasave et al and in fact all other earlier studies: − This study looked for evidence for displacive behavior of this model order–disorder lattice, whereas Lasave et al examined primarily the role of hydrogen bonding between the NH 4 + ion and the O’s of the PO 4 3– ion as the basis of the antiferroelectric character in ADP (and by extension for ADA) as compared to the ferroelectric nature of KH 2 PO 4 . Other differences are discussed in the Introduction.…”

Section: Results and Discussionsupporting

confidence: 85%

“…For both sets of protons, the activation energies at the AFE and PE phases are similar to those reported in the earlier static 1 H NMR studies. 13,14 Using the E a and τ 0 values, we calculated the correlation times (τ c ) of the acid and NH 4 + protons. Figure 9 shows that the τ c 's of both sets are nearly the same, and both change significantly below T N .…”

Section: Resultsmentioning

confidence: 99%

“…Earlier attempts to answer this question for several hydrogen-bonded compounds have utilized NMR spectroscopy, since the time scale of the anticipated proton-lattice motion falls in the NMR regime (millisecond to picosecond). 4−6 These studies include (i) 31 P NMR on KD 2 PO 4 , 7 (ii) 13 C on squaric acid, 8,9 (iii) 15 N for NH 4 H 2 AsO 4 (ammonium dihydrogen arsenate, ADA) 10,11 and ammonia borane 12 and 1 H for ADA 13,14 and NH 4 H 2 PO 4 (ammonium dihydrogen phosphate, ADP), 15,16 but the experimental methods used in the earlier studies were different from that of the present work. The 15 N study on ADA 10,11 cation as a probe of displacive character at the cation site and did not search for the order−disorder aspect.…”

Section: Introductionmentioning

confidence: 99%

“…The 15 N study on ADA , used the NH 4 + cation as a probe of displacive character at the cation site and did not search for the order–disorder aspect. Studies utilizing 1 H as a probe − used such a low resolution that the peaks from the NH 4 + and O–H···O protons were strongly overlapped; it was thus not possible to ascertain their specific roles. A closely relevant result is the earlier conclusion from 31 P chemical shift studies on KD 2 PO 4 that its ferroelectric transition is purely of the order–disorder type, based on the observation (within large error bars) that the 31 P isotropic chemical shift, δ iso , was found to be essentially temperature independent around T c .…”

Section: Introductionmentioning

confidence: 99%

“…Here, we have increased the resolution by orders of magnitude by working at the highest available uniform Zeeman field of 21.1 T (i.e., 900 MHz resonance frequency for 1 H) and utilizing the MAS technique employing single crystals. − We selected ADA because its T N of 216 K is well within the variable temperature range available with the 900 MHz spectrometer at the National High Magnetic Field Laboratory in Tallahassee, FL . The experiments involved the measurement of the temperature dependence of δ iso , as well as spin–lattice relaxation times ( T 1 ), of the O–H···O and NH 4 + protons individually, since these two parameters provide complementary information on the nature of the phase transition. − δ iso is known to be quite sensitive to electronic structure change in the whole molecular orbital framework around a given atom, but not to simple reorientational or translational changes. − An observed anomalous change in δ iso at a phase transition is therefore considered to be a direct evidence of a displacive character of the transition. For an order–disorder transition, δ iso should remain essentially unchanged, or it changes smoothly on cooling through a phase transition.…”

Section: Introductionmentioning

confidence: 99%

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Evidence from 900 MHz ¹H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH₄H₂AsO₄

Kweon

Kitchen

et al. 2015

J. Phys. Chem. C

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NH4H2AsO4 (ADA) is a model compound for understanding the mechanism of phase transitions in the KH2PO4 (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at T N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (1H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, T 1, of the O–H···O and NH4 + protons through the T N. As T → T N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its T N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at T N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH4 + protons. Both sets of protons exhibited order–disorder characteristics in their T 1 data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into order–disorder and displacive ones, should rather be “order–disorder cum displacive” type.

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