title: Anharmonicity and isomorphic phase transition: a multi-temperature X-ray single-crystal and powder diffraction study of 1-(2'-aminophenyl)-2-methyl-4-nitroimidazole.
abstract: The harmonic model of atomic nuclear motions is usually enough for multipole modelling of high-resolution X-ray diffraction data; however, in some molecular crystals, such as 1-(2'-aminophenyl)-2-methyl-4-nitro-1H-imidazole [Paul, Kubicki, Jelsch et al. (2011 ▶). Acta Cryst. B67, 365-378], it may not be sufficient for a correct description of the charge-density distribution. Multipole refinement using harmonic atom vibrations does not lead to the best electron density model in this case and the so-called 'shashlik-like' pattern of positive and negative residual electron density peaks is observed in the vicinity of some atoms. This slight disorder, which cannot be modelled by split atoms, was solved using third-order anharmonic nuclear motion (ANM) parameters. Multipole refinement of the experimental high-resolution X-ray diffraction data of 1-(2'-aminophenyl)-2-methyl-4-nitro-1H-imidazole at three different temperatures (10, 35 and 70 K) and a series of powder diffraction experiments (20 ≤ T ≤ 300 K) were performed to relate this anharmonicity observed for several light atoms (N atoms of amino and nitro groups, and O atoms of nitro groups) to an isomorphic phase transition reflected by a change in the b cell parameter around 65 K. The observed disorder may result from the coexistence of domains of two phases over a large temperature range, as shown by low-temperature powder diffraction.
authors: CRM , Jean Barriol Institute, CNRS UMR 7036, University of Lorraine , BP 70239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France ; Faculty of Chemistry, Adam Mickiewicz University , Umultowska 89B, 61-614 Poznań, Poland.
authors: Agnieszka
authors: A
authors: Poulain
authors: CRM , Jean Barriol Institute, CNRS UMR 7036, University of Lorraine , BP 70239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France.
authors: Emmanuel
authors: E
authors: Wenger
authors: CRM , Jean Barriol Institute, CNRS UMR 7036, University of Lorraine , BP 70239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France.
authors: Pierrick
authors: P
authors: Durand
authors: Department of Chemistry, University of Warsaw , Pastuera 1, 02-093 Warszawa, Poland.
authors: Katarzyna N
authors: KN
authors: Jarzembska
authors: Department of Chemistry, University of Warsaw , Pastuera 1, 02-093 Warszawa, Poland.
authors: Radosław
authors: R
authors: Kamiński
authors: CRM , Jean Barriol Institute, CNRS UMR 7036, University of Lorraine , BP 70239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France.
authors: Pierre
authors: P
authors: Fertey
authors: Faculty of Chemistry, Adam Mickiewicz University , Umultowska 89B, 61-614 Poznań, Poland.
authors: Maciej
authors: M
authors: Kubicki
authors: CRM , Jean Barriol Institute, CNRS UMR 7036, University of Lorraine , BP 70239, Boulevard des Aiguillettes, 54506 Vandoeuvre-lès-Nancy, France.
authors: Claude
authors: C
authors: Lecomte
keywords: Hansen–Coppens model; X-ray closed-circuit helium cryostat; anharmonicity; experimental charge density; isomorphic phase transition; multiple-temperature powder diffraction