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 h – k – l + 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.