title: The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7.
abstract: In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectric coupling. For a symmetry-consistent theoretical description of this multiferroic phase, precise knowledge of its crystal structure is a prerequisite. Here we report the results of single-crystal neutron diffraction on Ca2CoSi2O7 at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space group P2(1)2(1)2 with a 3 × 3 × 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) Å] compared with the high-temperature normal state [tetragonal space group P42(1)m, a = b ≃ 7.86, c ≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7 at 10 K are presented and compared with the literature X-ray diffraction results at 130 and 170 K (low-temperature commensurate phase), as well as at ∼ 500 K (high-temperature normal phase).
authors.affiliation: Institute of Crystallography, RWTH Aachen University, Aachen, Germany.
authors.forename: Andrew
authors.initials: A
authors.lastname: Sazonov
authors.affiliation: Institute of Crystallography, RWTH Aachen University, Aachen, Germany.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Hutanu
authors.affiliation: Institute of Crystallography, RWTH Aachen University, Aachen, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Meven
authors.affiliation: Institute of Crystallography, RWTH Aachen University, Aachen, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Roth
authors.affiliation: Department of Physics, Budapest University of Technology and MTA-BME Lendület Magneto-optical Spectroscopy Research Group, Budapest, Hungary.
authors.forename: István
authors.initials: I
authors.lastname: Kézsmárki
authors.affiliation: Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), University of Tokyo, Tokyo, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Murakawa
authors.affiliation: Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), University of Tokyo, Tokyo, Japan.
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Tokura
authors.affiliation: École Polytechnique Fédérale de Lausanne, Laboratory of Nanostructures and Novel Electronic Materials, Lausanne, Switzerland.
authors.forename: Bálint
authors.initials: B
authors.lastname: Náfrádi
keywords: magnetoelectric coupling; multiferroic melilite; single-crystal neutron diffraction; twinning