Phase transition and atomic structure of Cd₆Tb 1/1 approximant to quasicrystal

We present an in-situ temperature study of the atomic structure of the 1/1 Cd6Tb approximant to an icosahedral quasicrystal. It belongs to the `Tsai' type family of quasicrystal and approximants whose archetype is the well-studied Cd-Yb system[1]. Its high temperature structure can be described as a bcc packing of a large Tsai atomic cluster, whose inner shell is a disordered tetrahedron at room temperature. As for most of the Cd6RE (RE=rare earth) approximant, the Cd6Tb phase undergoes a phase transition at 190 K to a phase of lower symmetry, resulting from an ordering of the inner tetrahedron[2]. Moreover, it has been shown that this phase undergoes a magnetic phase transition below 20 K, with an antiferromagnetic ordering of the Tb moment bearing atoms. It is thus particularly important to have a detailed structural study of this phase. We have carried out a systematic in situ measurement on a single grain from room temperature down to 40K on the crystal beam line located at the Soleil synchrotron. The structural phase transition is observed at about 190K. Using different attenuation, we have collected integrated intensity in a large dynamical range, leading to more than 60000 unique reflections in the C2/c monoclinic low temperature phase. The final wR2 values for room temperature and 40K are equal to 0.0726 and 0.0905 respectively. The resulting atomic structure will be compared to the well-studied approximant Zn6Sc, which is isostructural to Cd6Tb[3]. The ordering of the innermost tetrahedron leads to the distortion of the successive shells. The evolution of the high temperature phase, in particular just above Tc where pretransitional diffuse scattering is observed will be presented.

Processing Twinned-Modulated and Composite Structures from CCD / CMOS Images

The Bruker APEX2 [1] software suite includes a full set of crystallographic programs for the solution and refinement of routine crystal structures. In addition to the standard tools required for routine problems, a number of advanced tools are also provided for analysis of non-merohedral twins and modulated structures. These tools include the CELL_NOW [2], SADABS [2] and TWINABS [2] programs and a Reciprocal Lattice Viewer with a q-vector analysis option. The Bruker SAINT [2] program is routinely used for the integration of image data collected with Bruker CCD (APEX II) or CMOS (PHOTON 100) two-dimensional detectors as part of the APEX2 software suite. SAINT has many advanced features for the processing of non-routine datasets. For example, a variety of new options have been implemented to facilitate the simultaneous integration of multiple domains for twinned and composite structures. Higher dimensional crystallography may be used to integrate modulated structures with up to three q-vectors. The output data for modulated structures may be output in either HKL5 or HKLF6 format suitable for input into the JANA2006[3] program. We will present results for one twinned-modulated structure and one composite structure to illustrate the use of the respective advanced options in SAINT to integrate images and JANA2006 to refine the structures.

Magnetic Option in Jana2006: A Unified Approach by Shubnikov (Super)space Groups

The concept of Shubnikov (magnetic) symmetry becomes frequently used for description, solution and refinement of magnetic structures. Its growing importance is connected with the ease of application to various classes of magnetic structures having the translation periodicity identical, commensurate or incommensurate with the nuclear one. Recently generalized superspace approach [1] for incommensurately modulated magnetic structures allows for combination of nuclear and magnetic modulations. This unified description helps fully understand e.g. multiferroic phases. The program Jana2006 (http://jana.fzu.cz) combines the concept of Shubnikov (super)space groups with the representational analysis based on the decomposition of the magnetic configuration space into basis modes, which transform according to different physically irreducible representations (irreps) of the space group of the paramagnetic phase [2]. Moreover, Jana2006 can launch the recently developed program ISODISTORT [3] to obtain similar but more general analysis. The generalized symmetry concept facilitates data processing where symmetry related reflections for single crystal data can be merged and the list of generated reflections for powder data can be reduced to independent ones. Another benefit concerns calculation of magnetic structure factors, stability of refinement and logical way to describe twin domains. Unlike in the Fullproff program [6], Jana2006 can combine the nuclear and magnetic scattering internally without necessity to introduce two phases. It can also calculate magnetic structures with modulated parent phase where the modulation appears before the magnetic phase transition. The lecture shows manifold possibilities how to refine modulated magnetic structures from various experiments. Several recently solved magnetic structures will be presented.