Superstructure of a phosphor material Ba₃MgSi₂O₈ determined by neutron diffraction data

Ba₃MgSi₂O₈, a phosphor host examined for use in white-light devices and plant-growth lamps, was synthesized at 1225 °C in air. Its crystal structure has been determined and refined by a combined powder X-ray and neutron Rietveld method (P over(3, -), Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) ų; R_p/R_wp=3.79%/5.03%, χ²=4.20). Superstructure reflections, observed only in the neutron diffraction data, provided the means to establish the true unit cell and a chemically reasonable structure. The structure contains three crystallographically distinct Ba atoms-Ba1 resides in a distorted octahedral site with S₆ (over(3, -)) symmetry, Ba2 in a nine-coordinate site with C₃ (3) symmetry, and Ba3 in a ten-coordinate site with C₁ (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.