Melilitetype and meliliterelated compounds stru

Authors: Matteo Ardit Michele Dondi Marco Merlini Giuseppe Cruciani

Publish Date: 2011/12/23

Volume: 39, Issue: 3, Pages: 199-211

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Abstract

The structural variations along the solid solution Sr2−x Ba x MgSi2O7 0 ≤ x ≤ 2 combined to the highpressure characterization of the two endmembers have been studied A topological change from the tetragonal melilitetype to the monoclinic meliliterelated structure along the join Sr2MgSi2O7 eg Pbar42 1 m –Ba2MgSi2O7 eg C2/c occurs with a Ba content higher than 16 apfu Favored in the crystallization from a melt the tetragonal form has a tetrahedral sheet topology exclusively based on fivemembered rings which provide a regular “4 up + 4 down” ligand arrangement In contrast the meliliterelated structure favored by solidstate reaction synthesis is made by alternating six and fourmembered tetrahedral rings which give an asymmetric arrangement of alternated “5 up + 3 down” and “3 up + 5 down” ligands around Sr or Ba This latter configuration is characterized by an additional degree of freedom with Ba polyhedra hosted in the interlayer with a more irregular and compact coordination and longer Ba–O bond distances Further insights into the relationships between the two melilite typologies were achieved by investigating the in situ highpressure behavior of these systems The synchrotron highpressure experiments allowed to calculate the elastic moduli for the Sr melilitetype endmember and for the Ba monoclinic polymorph Sr2MgSi2O7 K T0 = 107 K a=b  = 121 and K c  = 84 GPa mBa2MgSi2O7 K T0 = 85 K a  = 96 K b  = 72 and K c  = 117 GPa and compare them with those reported in the literature for åkermanite Ca2MgSi2O7 The results show that although the volume of Ba polyhedron in tetragonal polymorphs is larger than in the monoclinic forms the interlayer compressibility is significantly lower in the former structures due to the occurrence of very short Ba–O distances This unfavored Ba environment also makes tetragonal Ba2MgSi2O7 a metastable phase at room conditions possibly favored by high pressure However no phase transition occurs from monoclinic to tetragonal form due to kinetic hindrance in reconstructing the sheet topologyAccess to ID27 beamline at the European Synchrotron Radiation Facility ESRF was provided under the public beamtime program exp no HS3936 We thank Pierre Bouvier ESRF and Michael Hanfland ESRF for advices and technical support In addition we are grateful to Luca Bindi and an anonymous referee for their constructive reviews as well as to the criticisms raised by the editor Milan Rieder which greatly improved the quality of this article

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