Open Access

Andrew Sazonov, Martin Meven, Georg Roth, Robert Georgii, István Kézsmárki, Vilmos Kocsis, Y. Tokunaga, Y. Taguchi, Y. Tokura, V. Hutanu

• For a symmetry-consistent theoretical description of the multiferroic phase of Ba2CoGe2O7 a precise knowledge of its crystal structure is a prerequisite. In a previous synchrotron X-ray diffraction experiment on multiferroic Ba2CoGe2O7 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 Ba2CoGe2O7 symmetry at room temperature. The precise structural parameters from neutron diffractionFor a symmetry-consistent theoretical description of the multiferroic phase of Ba2CoGe2O7 a precise knowledge of its crystal structure is a prerequisite. In a previous synchrotron X-ray diffraction experiment on multiferroic Ba2CoGe2O7 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 Ba2CoGe2O7 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.…