Origin of forbidden reflections in multiferroic Ba 2 CoGe 2 O 7 by neutron diffraction: symmetry lowering or Renninger effect?

• Published in: Journal of applied crystallography Vol. 49; no. 2; pp. 556 - 560
• Main Authors: Sazonov, Andrew, Meven, Martin, Roth, Georg, Georgii, Robert, Kézsmárki, István, Kocsis, Vilmos, Tokunaga, Yusuke, Taguchi, Yasujiro, Tokura, Yoshinori, Hutanu, Vladimir
• Format: Journal Article
• Language: English
• Published: 01.04.2016
• ISSN: 1600-5767; 1600-5767
• DOI: 10.1107/S1600576716002405

For a symmetry-consistent theoretical description of the multiferroic phase of Ba 2 CoGe 2 O 7 a precise knowledge of its crystal structure is a prerequisite. In a previous synchrotron X-ray diffraction experiment on multiferroic Ba 2 CoGe 2 O 7 at room temperature, forbidden reflections were found that favour the tetragonal-to-orthorhombic symmetry lowering of the compound [Hutanu, Sazonov, Murakawa, Tokura, Náfrádi & Chernyshov (2011), Phys. Rev. B , 84 , 212101]. Here, the results are reported of room-temperature single-crystal diffraction studies with both hot and cold neutrons to differentiate between genuine symmetry lowering and multiple diffraction (the Renninger effect). A comparison of the experimental multiple diffraction patterns with simulated ones rules out symmetry lowering. Thus, the structural model based on the tetragonal space group P\overline{4}2_{1}m was selected to describe the Ba 2 CoGe 2 O 7 symmetry at room temperature. The precise structural parameters from neutron diffraction at 300 K are presented and compared with the previous X-ray diffraction results.