Crystal structure and monoclinic distortion of glaserite-type Ba3MnSi2O8

Abstract

Crystal structure and magnetic properties of glaserite-type Ba3MnSi2O8 were investigated using variable temperature neutron powder diffraction and magnetometry. At room temperature the composition is hexagonal and the crystal structure is best described by the P-3m1 space group (a~ 5.7 Å, c~ 7.3 Å) with the apical oxygen atom modelled on a split site. On cooling below ~ 250 K the structure undergoes a phase transition into a monoclinic C2/c form (√3ahex, ahex, 2chex, β~ 90°). Analysing diffraction data in terms of symmetry-adapted distortion modes suggests that the transition is primarily driven by increasing in-plane displacements of O1, which in turn results in the coupled tilting of [SiO4] and [MnO6] octahedra and in-plane displacements of Ba1 atoms. Magnetic susceptibility measurements and neutron powder diffraction data show no evidence for long-range magnetic ordering down to 1.6 K, although the development of magnetic diffuse scattering suggests that a magnetic transition may take place at lower temperature.