Naslov
Evidence of polaron excitations in Raman spectra of oxalic acid dihydrate at low temperatures

Autori
Mohaček Grošev, Vlasta ; Grdadolnik, Jože ; Hadži, Dušan

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Book of Abstracts, XXIth International Conference on Horizons in Hydrogen Bond Research “HBond2015” / Maria Wierzejewska, Zdzisław Latajka, Jan Lundell - Wrocław : Faculty of Chemistry, University of Wrocław, 2015, OC25-OC25

Skup
XXIth International Horizons in Hydrogen Bond Research “HBond2015”

Mjesto i datum
Wrocław, Poljska, 13.09.2015. - 18.09.2015

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
oxalic acid dihydrate; polaron; Raman spectroscopy; low temperature

Sažetak
Oxalic acid dihydrate is an example of molecular crystal with a very strong short hydrogen bond, O∙∙∙O distance being 2.49 Å at room temperature. Raman spectra revealed great richness of hydrogen bond transitions especially in the y(zz)x crystal orientation (1) . On cooling the system below 200 K one observes huge changes in the shape and position of the band observed around 1550 cm-1 at room temperature and moving to 1407 cm-1 at 10 K (Fig. 1). Also, a group of four bands occurs between 1152 and 1281 cm-1 below 200 K, gaining in strength on cooling, in spite of the fact that there is no phase transition reported for this crystal. Bearing in mind the results of measurements of the proton T1 relaxation time in the dependence of temperature, which become extremely prolonged around 200 K, we suggest that centre of inversion present in the P21/c crystal structure of (COOH) ∙2H2O disappears below 200 K due to proton disorder, making it possible for strong infrared band observed at 1253 cm-1 at room temperature to appear as a group of bands in Raman spectra below 200 K. Freezing of protons causes also self-trapping of an electron on one molecule inside the elementary cell, so called small polaron (2). The energy levels of this polaron have strong directional dispersion – e.g. they appear as very broad bands in the x(zz)y direction, a bit narrower in the y(zz)x direction and comparable in bandwidth with hydrogen bond transitions in the z(yy)x and other directions. Their overlap with ν(OH) and δ(COH) as well as photo conduction properties observed for similar crystal KCOOCOOH make them extremely difficult to calculate, as their intensity depends on polarization proportional to the wave function of the crystal (3). REFERENCES 1. V. Mohaček Grošev, J. Grdadolnik, J. Stare, D. Hadži, J. Raman Spectrosc.. 2009, 40, 1605. 2. A. Levstik, C. Filipič, V. Bobnar, I. Levstik, D. Hadži, Phys. Rev. B 2006, 74, 153104. 3. D. Emin: Polarons, Cambridge University Press 2013.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Kemija

POVEZANOST RADA