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Correlation between ferroelectricity and torsional motion of acetyl groups in tris(4-acetylphenyl)amine observed by muon spin relaxation.

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Title: Correlation between ferroelectricity and torsional motion of acetyl groups in tris(4-acetylphenyl)amine observed by muon spin relaxation.
Authors: Nakamura, J. G., Hiraishi, M., Okabe, H., Koda, A., Kumai, R., Pratt, F. L., Kadono, R.
Source: Journal of Applied Physics; 1/28/2025, Vol. 137 Issue 4, p1-10, 10p
Subject Terms: MUON spin rotation, ACETYL group, PHENYL group, DENSITY functional theory, ATOMIC displacements
Abstract: It is demonstrated by muon spin relaxation and resonance experiments that the switchable spontaneous polarization of the organic ferroelectric compound tris(4-acetylphenyl)amine is governed by the local molecular dynamics of the acetyl group. The implanted muon forms paramagnetic states, which exhibit longitudinal spin relaxation due to the fluctuation of hyperfine fields exerted from unpaired electrons. The first-principle density functional theory calculations indicate that these states are muonated radicals localized at the phenyl group and on the carbon/oxygen of the acetyl group, thereby suggesting that the spin relaxation is dominated by the random torsional motion of an acetyl group around the C–C bond to the phenyl group. The stepwise change in the relative yield of radicals at T 0 ≈ 350 K and the gradual increase in the spin relaxation rate with temperature (T) indicate that the torsional motion is significantly enhanced by thermal excitation above T 0. This occurs concomitantly with the strong enhancement in the atomic displacement parameter of oxygen in the acetyl group (which is non-linear in T), indicating that it is the local molecular motion of the acetyl groups that drives the structural transition. [ABSTRACT FROM AUTHOR]
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Abstract:It is demonstrated by muon spin relaxation and resonance experiments that the switchable spontaneous polarization of the organic ferroelectric compound tris(4-acetylphenyl)amine is governed by the local molecular dynamics of the acetyl group. The implanted muon forms paramagnetic states, which exhibit longitudinal spin relaxation due to the fluctuation of hyperfine fields exerted from unpaired electrons. The first-principle density functional theory calculations indicate that these states are muonated radicals localized at the phenyl group and on the carbon/oxygen of the acetyl group, thereby suggesting that the spin relaxation is dominated by the random torsional motion of an acetyl group around the C–C bond to the phenyl group. The stepwise change in the relative yield of radicals at T 0 ≈ 350 K and the gradual increase in the spin relaxation rate with temperature (T) indicate that the torsional motion is significantly enhanced by thermal excitation above T 0. This occurs concomitantly with the strong enhancement in the atomic displacement parameter of oxygen in the acetyl group (which is non-linear in T), indicating that it is the local molecular motion of the acetyl groups that drives the structural transition. [ABSTRACT FROM AUTHOR]
ISSN:00218979
DOI:10.1063/5.0247652