Melilite-related compounds of general formula X2T1T22O7 (with X = large mono- to trivalent cations; T1 and T2 = small di- to tetravalent cations) are widespread over many classes of minerals and materials. Melilites are typical constituents of a some ultramafic rocks (e.g. alnöite and melilitites) and also occur in extra terrestrial materials (e.g. in carbonaceous chondrites). Their synthetic analogues have attracted considerable interest in different technological fields: ceramics, glasses, luminescent materials, low-dimensional magnets, etc. The (average) crystal structure of melilite-type compounds can be described in the P-421m space group. The presence of an incommensurately modulated (IC) structure in some melilite compositions (e.g. åkermanite) has received much attention and many studies have been devoted to its determination and to transitions from IC to normal (N) or lock-in phases as a function of temperature, pressure and chemistry. Much less attention has been paid to the occurrence of compounds having melilite stoichiometry but with reduced symmetry and, more important, different sheet topology compared to melilites . Ba2MgSi2O7 was suggested as a compound which could not crystallize with a melilite-type structure  but only with a Ba2CuSi2O7-type  monoclinic structure. This suggestion was later confuted by a single-crystal refinement of Ba2MgSi2O7 in the s.g. P-421m . Nevertheless, the same dichotomy was found for Ba2CoSi2O7 where opposing claims for a P-421m  or C2/c  structure exist. Recently, monoclinic structure refinements of Eu:Ba2MgSi2O7 have been also reported [7, 8]. However, none of the above reports attempted to solve the dichotomy, based on the different sheet topology. Aim of this contribution is to provide a better understanding of the relationships between the two polymorphs of Ba2MgSi2O7 starting from a new Rietveld refinement of the monoclinic form. During a systematic synthesis and characterization work of Co-doped Sr- and Ba-melilites, we obtained a nearly monophasic sample of Ba2MgSi2O7 whose structure was successfully Rietveld refined in the C2/c space group starting from the model of , with Mg replacing Co. Similarly to the melilite-type polymorph the monoclinic structure consists of Si2O7 dimers connected by MgO4 tetrahedra to form tetrahedral sheets parallel to the ac plane (vs. the ab plane in the P-421m structure). The first major difference between the two polymorphs is in the coordination polyhedron of Ba: a distorted square antiprism (4 ligands above + 4 below) in tetragonal and a 3 + 5 configuration in the monoclinic forms. More important is the difference in the sheet topology: only five-membered rings of tetrahedra in the tetragonal form while four- and six membered rings in the monoclinic one. This implies that the mechanism to switch between the two polymorphs must be of reconstructive nature and is not related to any IC-N or lock-in transition. We suggest that the misfit between the interlayer and tetrahedral sheets in Ba2MgSi2O7 is reversed compared to that causing the IC structure in the tetragonal form. The monoclinic structure is the most stable form at low-T, always found in powders from solid state reactions (as in our case), while the tetragonal polymorph is a metastable form at low-T, quenched from high-T, usually found in single crystals obtained by a melt. References.  Armbruster T, Rothlisberger F, Seifert F, Am. Miner., 75, 847-858, 1990;  Rothlisberger F, Seifert F, Czank M., Eur. J. Miner., 2, 585-594, 1990;  Malinovskii Yu. A. Sov. Phys. Dokl., 29, 706, 1984;  Shimizu M, Kimata M, Lida I, N. Jahr. Miner.-Mon., 1, 39-47, 1995;  El Bali B., Zavalij P. Y. Acta Cryst., E59, i59-i61, 2003;  Adams R. D., Layland R., Payen C., Datta T. Inorg. Chem., 35, 3492-97, 1996;  Komeno A., Uemastu K., Toda K., Sato M. J. All. Comp., 408-412, 871-874, 2006;  Aitasalo T, Holsa J, Laamanen T, Lastusaari M, Lehto L, Niittykoski J, Pelle F Zei.t Kristall Suppl. 23, 481-486, 2006.
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