Crystal structure and monoclinic distortion of glaserite-type Ba{sub 3}MnSi{sub 2}O{sub 8}
Highlights: • Glaserite-type Ba{sub 3}MnSi{sub 2}O{sub 8} is hexagonal P-3m1 at room temperature. • Below ~ 250 K the structure undergoes transition to monoclinic C2/c. • The transition is driven by in-plane displacement of the apical oxygen atoms. • No magnetic transition was observed down to 1.6 K but expected at lower temperature. - Abstract: Crystal structure and magnetic properties of glaserite-type Ba{sub 3}MnSi{sub 2}O{sub 8} 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 (√3a{sub hex}, a{sub hex}, 2c{sub hex}, 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 [SiO{sub 4}] and [MnO{sub 6}] 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.