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 P21212 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, 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).The crystal structure of the multiferroic melilite Ca2CoSi2O7 is determined by means of single-crystal neutron diffraction at 10 K.