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Autori: M.N. Grecu, S. Constantinescu, V.V. Grecu
Editorial: A.G.Feio and J.G. Moura, ISBN: 972-98802-0-4, Proceeding of 30th AMPER 2000 on Magnetic Resonance and Related Phenomena, VI, Lisbon, Portugal, p.449-453, 2001.
The present EPR and Mössbauer results on Hg+, VO2+ and 57Fe2+ probe-ions into substituted ammonium positions of AS provide new evidence of lattice dynamics and critical anomalies near Tc together with coexistence of the paraelectric and ferroelectric phases. Numerical calculations of the electric field properties and spectral parameters of the probe-ions at two ammonium sites were performed employing specialized computing programs . The temperature dependencies of hyperfine constant (Hg+), EPR line splitting (VO2+), and Debye-Waller factor (57Fe2+) in the critical region 0<|T-Tc|>10K were analyzed and discussed in terms of the existence of a vibration mode whose softening occurs due to local anharmonicity. The Debye temperatures TETAD and critical exponents (beta=0.38, gama=1.24) have been estimated as well and the important role of ammonium groups in the phase transition was confirmed
Cuvinte cheie: EPR, 57Fe-Mössbauer spectroscopy, ammonium sulfate, soft-mode, phase transition