CRYSTAL-STRUCTURE OF BA2MGSI2O7 MELILITE - THE LONGEST TETRAHEDRAL MG-O DISTANCE

The crystal structure of synthesized Ba2MgSi2O7 was determined by sing le crystal X-ray work: the tetragonal cell: P ($) over bar 42(1)m (No. 113), a = 8.2036(4) Angstrom, c = 5.4058(4) Angstrom, V = 363.81(4) A ngstrom(3), Z = 2, Rw = 0.024 for 578 reflections and 35 variable para meters. The underlying structure of Ba2MgSi2O7 comprises a magnesiosil icate of tetrahedral sheets between barium coordinated to eight oxygen s conforming the square antiprisms. This solid-state structure is call ed as of the melilite family, containing isolated, eclipsed Si2O76- gr oups and their apical-bonded MgO4 tetrahedra. The most characteristic is the additional longer Mg-O bonds, 1.966(4) Angstrom, and the wider Si-O-Si angle, 144.3(5)degrees, contributed by the size effect of inte rlayer cations, in comparison with polyhedral properties in Ca2MgSi2O7 and Sr2MgSi2O7. Inasmuch as incommensature reflections observed in Ca 2MgSi2O7 with extremely underbonded CA and O(2) atoms cannot be detect ed in Ba2MgSi2O7 with nearly balanced bonds, they are considered to be modestly ascribed to stretching interaction between Ca and O(2) cause d by their extremely underbonding.