The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7
In the antiferromagnetic ground state, below T-N similar or equal to 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 x 3 x 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) angstrom] compared with the high-temperature normal state [tetragonal space group P (4) over bar2(1) m, a = b similar or equal to 7.86, c similar or equal to 5.03 angstrom]. 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 similar to 500 K (high-temperature normal phase).