Micro-transitions or breathers in L-alanine?
Groupe de Dynamique des Phases Condensées, Université Montpellier 2, 34095 Montpellier Cedex 05, France
2 Hahn-Meitner Institut, Glienicker Str. 100, 14109 Berlin, Germany
3 Laboratoire de Physique des Solides, UMR 8502, bâtiment 510, Université Paris-XI, 91405 Orsay Cedex, France
4 CNRS, Laboratoire de Cristallographie, B.P. 166, 38042 Grenoble Cedex 09, France
5 ESRF, B.P. 220, 38043 Grenoble Cedex, France
Corresponding author: a mariette@GDPC.univ-montp2.fr
Revised: 19 December 2003
Published online: 2 April 2004
The lattice dynamics of crystalline L-alanine shows unusual features tentatively assigned to a localization of vibrational energy. However, other properties alternatively suggest the existence of a second order phase transition undetected in previous crystallographic work. In this paper, we present the results of a high resolution X-ray diffraction study down to 10 K, together with new structure determinations at intermediate temperatures. The data rule out the hypothesis of a conventional structural phase transition. No change in the space group symmetry is observed and an anomalous decrease of the lattice parameter c in discrete steps is discovered when heating the crystal from 10 K to room temperature. It could be ascribed to a progressive conformational change of the NH group of the zwitterionic molecule. An analysis of the physical properties of crystalline L-alanine suggests the existence of a strong dynamic Jahn-Teller-like effect owing to the NH charge-lattice coupling. This would explain both the splitting of some vibrational states and properties related to a microscopic lattice instability like the onset of depolarization in the transmitted light below ~250 K.
PACS: 61.10.Nz – X-ray diffraction / 05.45.Yv – Solitons / 64.70.Kb – Solid-solid transitions
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2004