(3 + 1) dimensional modulated crystal structure of Cs3ScSi6O15

C. Hejny1, V. Kahlenberg1 and P. Dabić2

1University of Innsbruck, Institute of Mineralogy and Petrography, Innrain 52, A-6020 Innsbruck, Austria

2University of Belgrade, Laboratory of Crystallography, Djusina 7, 11000 Belgrade, Serbia


Single crystal X-ray diffraction experiments of Cs3ScSi6O15 showed intense Bragg reflections that can be indexed on an R-centred hexagonal cell with a = 13.861(1), c = 6.992(1) Å.  Inspection of the distribution histograms of the remaining unindexed reflections revealed them to be satellite reflections of up to the third order in c*-direction; the satellite reflections are present for hkl + m = 3n. The modulation wave vector was subsequently defined and refined to q = 0.14153(2)c*. This is close to, but distinctly different from 1/7 = 0.142857. The crystal structure of Cs3ScSi6O15 was solved in (3 + 1) dimensional superspace group Xm1(00g)0s0 from 1838 observed reflections by charge flipping with the program Superflip [1] and refined with the program JANA2006 [2]. Refinement of three modulation waves for positional and ADP values for all atoms converged to Ro values for all, main, and satellite reflections of first, second and third order of 0.0200, 0.0166, 0.0181, 0.0214 and 0.0303, respectively

Cs3ScSi6O15 has a mixed octahedral-tetrahedral framework structure with six-membered rings of silicon tetrahedra and regular ScO6 octahedra forming infinite chains along the [0001] direction. According to bond valence calculations ten interatomic distances up to 3.5 Å have to be considered as bonding for Cs. All atoms apart from Sc show very large positional modulations with maximum atomic displacements of up to 0.93 Å [ScO6]-octahedra and [SiO4]-tetrahedra remain rigid with minor variation of interatomic distances. However, as a function of t the [ScO6]-octahedra and [Si6O18]-rings rotate around the -axis by over 38 Å and inter-tetrahedral angles within the Si6O18 ring diversify greatly. The coordination environment of Cs is very complex with oxygen atoms moving in and out of the Cs coordination in order to maintain the bond valence sum around Cs at a constant level of ca. 1.075.

Cs3DySi6O15, a = 13.996(1), c = 7.1775(6) Å, Rm1 [3] has been described with the same basic structure. However, the statement of positional disorder for Si, O and Cs atoms without further specification leads to the suggestion, that Cs3DySi6O15 has a modulated structure similar to Cs3ScSi6O15.

The only another Cs-Sc-silicate know to date, Cs3ScSi8O19 [4], has a microporous framework structure with an unusual high [TO4]:[MO6] ratio of > 6:1.

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4. U. Kolitsch, E. Tillmanns, Mineral. Mag. 68 (2004) 677.