The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7

In the antiferromagnetic ground state, belowTN≃ 5.7 K, Ca2CoSi2O7exhibits 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 Ca2CoSi2O7at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space groupP21212 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 P\overline {4}2_{1}m,a=b≃ 7.86,c≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7at 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).