Journal of Applied Crystallography

Volume 47, Part 2 (April 2014)

research papers

J. Appl. Cryst. (2014). 47, 532-538    [ doi:10.1107/S1600576713034560 ]

A symmetry-mode description of rigid-body rotations in crystalline solids: a case study of Mg(H2O)6RbBr3

M. Müller, R. E. Dinnebier, A.-C. Dippel, H. T. Stokes and B. J. Campbell

Abstract: The application of rotational symmetry modes to quantitative rigid-body analysis is demonstrated for octahedral rotations in Mg(H2O)6RbBr3. Rigid-body rotations are treated as axial-vector order parameters and projected using group-theoretical methods. The high-temperature crystal structure of the Mg(H2O)6RbBr3 double salt consists of a cubic perovskite-like corner-sharing network of RbBr6 octahedra with isolated MgO6 octahedra at the perovskite A sites. A phase transition occurs at 411 K upon cooling, whereupon the MgO6 octahedra experience a substantial rigid-body rotation, the RbBr6 octahedra are translated but not rotated, and both types of octahedra become slightly distorted. The MgO6 rotation has three orthogonal components associated with the X5-, [Gamma]4+ and X1- irreducible representations of the parent Pm{\overline 3}m space-group symmetry which, given the weakly first-order character of the transition, appear to be strongly coupled. Parametric and sequential refinements of the temperature-dependent structure were conducted using four model types: (1) traditional atomic xyz coordinates for each atom, (2) traditional rigid-body parameters, (3) purely displacive symmetry modes and (4) rigid-body rotational symmetry modes. We demonstrate that rigid-body rotational symmetry modes are an especially effective parameter set for the Rietveld characterization of phase transitions involving polyhedral rotations.

Keywords: rigid-body rotation; symmetry mode; irreducible representation; parametric refinement; phase transition.

txtdisplay filedownload file

Text file (18.6 kbytes)
[ doi:10.1107/S1600576713034560/to5065sup1.txt ]
Input text file for Rietveld refinement in TOPAS


To open or display or play some files, you may need to set your browser up to use the appropriate software. See the full list of file types for an explanation of the different file types and their related mime types and, where available links to sites from where the appropriate software may be obtained.

The download button will force most browsers to prompt for a file name to store the data on your hard disk.

Where possible, images are represented by thumbnails.

 bibliographic record in  format

  Find reference:   Volume   Page   
  Search:     From   to      Advanced search

Copyright © International Union of Crystallography
IUCr Webmaster