Superstructure of a phosphor material Ba 3MgSi 2O 8 determined by neutron diffraction data

Abstract

Ba 3MgSi 2O 8, 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 3 ¯ , Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) Å 3; R p/ R wp=3.79%/5.03%, χ 2=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 S 6 ( 3 ¯ ) symmetry, Ba2 in a nine-coordinate site with C 3 (3) symmetry, and Ba3 in a ten-coordinate site with C 1 (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.