Silicates usually possess stable and rigid crystal structures. Thus, they make excellent host matrices for efficient luminescent materials with use in applications as lamps, cathode ray tubes, scintillators, etc. [1]. Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ is currently the best persistent luminescence material [2]. It can store energy from e.g. sun or lamp light and release it as visible light persisting up to 24 hours in the dark. The applications of persistent luminescence range from the traditional luminous paints to more sophisticated fields as radiation detection, structural damage and temperature sensing as well as medical diagnostics. The other stable composition in the Sr_nMgSi₂O_5+n series, Sr₃MgSi₂O₈:Eu²⁺,Dy³⁺, also shows persistent luminescence [3].
The crystal structure of Sr₃MgSi₂O₈ has been suggested to be orthorhombic with unit cell parameters a = 5.4, b = 9.6 and c = 7.2 Å, Z = 2 [4,5]. However, no space group or structural details have been reported. The structure has also been referred to as merwinite (Ca₃MgSi₂O₈) type with the monoclinic space group P2₁/a (No. 14), Z = 4 [6]. In this work, the crystal structure was investigated based on X-ray powder diffraction data.
The undoped and Eu²⁺/Dy³⁺ codoped Sr₃MgSi₂O₈ samples were prepared by solid state reactions. Stoichiometric amounts of strontium carbonate, magnesium nitrate hexahydrate, silicon dioxide, europium oxide and dysprosium oxide were heated at 1250 ^oC for 10 hours in a reducing N₂ + 12 % H₂ atmosphere. The X-ray powder diffraction patterns were collected at room temperature by a Huber 670 Guinier camera using monochromatic copper K_α1 radiation (λ = 1.5406 Å).
The unit cell was found to be of lower symmetry than that of the glaserite type trigonal Ba₃MgSi₂O₈ [7]. The previously proposed orthorhombic cell could not account for all the observed reflections, either, but a doubling of the unit cell c-axis was needed. This cell corresponds to the merwinite one, but the systematic absences suggest possible space groups Cc (No. 9) or C2/c (15). Even if no clear reflection splitting was observed, the broadening and intensities of the reflections suggest a unit cell belonging to the monoclinic crystal system with a = 9.442, b = 5.452, c = 13.860 Å and β = 90.2^o.

1. Shionoya S., Yen W.M. (Eds.), Phosphor Handbook, CRC Press, Boca Raton, FL, USA, 1999.
2. Lin T., Tang Z., Zhang Z., Wang X., Zhang J., J. Mater. Sci. Lett. 20 (2001) 1505.
3. Lin Y., Tang Z., Zhang Z., Nan C.W., J. Alloys Comp. 348 (2003) 76.
4. Klasens H.A., Hoekstra A.H., Cox A.P.M., J. Electrochem. Soc. 104 (1957) 93.
5. JCPDS, 1997, Powder Diffraction File, entry No. 10-0075.
6. Moore P.B., Araki T., Am. Miner. 57 (1972) 1355.
7. Aitasalo T., Hietikko A., Hölsä J., Lastusaari M., Niittykoski J., Piispanen T., Z. Kristallogr. Suppl. 26 (2007) 461.

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