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On the Urbach rule in SbSI ferroelectric crystal 
1Studenyak I.P., 2Kranjčec M., 1Koperlyos B.M.

1Uzhhorod National University, 46 Pidhirna St., 88000 Uzhhorod, Ukraine
2Geotechnical Department Varaždin, University of Zagreb, 7 Hallerova Aleja, 42000 Varaždin, Croatia

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Optical absorption edge of SbSI ferroelectric crystal is studied in a broad temperature range. An Urbach behaviour of the absorption edge is observed in paraelectric (T > Tc) and partly in ferroelectric (130 K < T < Tc) phases, whereas deviations from the Urbach rule are revealed at T < 130 K. The influence of crystal lattice disordering of various types on the Urbach absorption edge parameters is studied. The non-Urbach behaviour of the absorption edge is explained by a presence of dynamic structural disordering due to temperature-dependent phonon-defect interactions.

Keywords: phase transition, ferroelectric, absorption edge, Urbach rule 

PACS: 77.84.-s; 78.40.-q
UDC: 535.341
Ukr. J. Phys. Opt. 10 61-70 
doi: 10.3116/16091833/10/2/61/2009
Received: 30.01.2009

Анотація. В роботі досліджений оптичний край поглинання сегнетоелектричних кристалів SbSI в широкому температурному інтервалі. В параелектричній фазі (T>Tc) і частково в сегнетоелектричній фазі (130 K<T<Tc) спостерігалась урбахівська поведінка краю поглинання, тоді як при T<130 K виявлене відхилення від правила Урбаха. Досліджений вплив розупорядкування кристалічної гратки різного типу на параметри урбахівського краю поглинання. Не урбахівська поведінка краю поглинання пояснена динамічним структурним розупорядкуванням викликаним температурно залежною фонон-дефектною взаємодією.
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