Rietveld refinement of the crystal structures of Rb2 XSi5O12 (X = Ni, Mn)

Rietveld refinements show that the crystal structures of synthetic leucite silicate framework mineral analogues Rb2 XSi5O12 (X = Ni, Mn) are isostructural with the Pbca cation-ordered structure of Cs2CdSi5O12.

The synthetic leucite silicate framework mineral analogues Rb 2 XSi 5 O 12 {X = Ni [dirubidium nickel(II) pentasilicate] and Mn [dirubidium manganese(II) pentasilicate]} have been prepared by high-temperature solid-state synthesis. The results of Rietveld refinements, using X-ray powder diffraction data collected using Cu K X-rays, show that the title compounds crystallize in the space group Pbca and adopt the cation-ordered structure of Cs 2 CdSi 5 O 12 and other leucites. The structures consist of tetrahedral SiO 4 and XO 4 units sharing corners to form a partially substituted silicate framework. Extraframework Rb + cations sit in channels in the framework. All atoms occupy the 8c general position for this space group. In these refined structures, silicon and X atoms are ordered onto separate tetrahedrally coordinated sites (T-sites). However, the Ni displacement parameter and the Ni-O bond lengths suggest that for the X = Ni sample, there may actually be some T-site cation disorder.

Chemical context
Synthetic analogues of the silicate framework minerals leucite KAlSi 2 O 6 (Mazzi et al., 1976) and pollucite CsAlSi 2 O 6 (Dimitrijevic et al., 1991) can be prepared with the general formulae ABSi 2 O 6 and A 2 CSi 5 O 12 . A is an alkali metal cation (K, Rb, Cs), B is a trivalent cation (Al, B, Fe 3+ ) and C is a divalent cation (Be,Mg,Mn,Fe 2+ ,Co,Ni,Cu,Zn,Cd). The title compounds are leucite analogues with A = Rb and C = Ni and Mn, these structures are in the space group Pbca and are isostructural with Cs 2 CdSi 5 O 12 .
These leucite structures all have the same topology, a silicate framework structure with B or C cations partially substituting on the tetrahedrally coordinated silicon sites (Tsites). A cations sit in the extraframework channels, these extraframework cations can be removed by ion exchange which makes them of technological interest as a possible storage medium for radioactive Cs from nuclear waste (Gatta et al., 2008).

Structural commentary
For the X = Ni refinement, the Ni site isotropic temperature factor was larger than expected [B iso = 7.5 (9) Å 2 ]. Also the mean Ni-O bond length for the NiO 4 tetrahedron is 1.90 (2) Å , shorter than that seen in tetrahedrally coordinated NiO 4 units. NiCr 2 O 4 has the cubic spinel structure with Ni in tetrahedral coordination. A single-crystal structure refinement (Crottaz et al., 1997) gives the Ni-O distance as 1.967 (3) Å .
An EXAFS/XANES study (Farges et al., 2001) Fig. 2 and consists of a framework of corner-sharing tetrahedral SiO 4 and NiO 4 units, and Rb + cations sitting in the extraframework channels.
For the X = Mn refinement, unlike in the X = Ni sample, the mean Mn-O distance is 2.02 (1) Å . This is in agreement with the mean Mn-O distance for Cs 2 MnSi 5 O 12 (1.98 (3) Å ; Bell & Henderson, 1996). This would suggest that this structure has complete T-site cation ordering. However, the isotropic temperature factors for the Mn site and the O sites could not be refined to chemically sensible positive values, so these were both fixed at B iso = 0.1 Å 2 . This refinement was done assuming that Mn was present as Mn 2+ and that all sites were fully occupied. If some or all of the Mn atoms were present with a higher oxidation state, then this could account for the problem with refining these temperature factors. However, a higher quality neutron/synchrotron X-ray powder diffraction study may also be needed for a more precise determination of the state of Mn in this structure. Fig. 3 shows the Rietveld difference plot for Rb 2 MnSi 5 O 12 . The crystal structure of Rb 2 MnSi 5 O 12 is displayed in Fig. 4 and consists of a framework of corner-sharing tetrahedral SiO 4 and MnO 4 units; Rb cations sit in the extraframework channels. Note how inclusion of the larger Mn cation in the silicate framework compared to Ni causes the central channel of the crystal structure to be slightly more distorted for Rb 2 MnSi 5 O 12 (Fig. 4) compared to Rb 2 NiSi 5 O 12 (Fig. 2).

Database survey
Many different leucite analogue crystal structures are known at ambient temperature.       groups, lattice parameters, and references for some known ambient temperature leucite crystal structures. In addition, a high-temperature structure for Cs 2 ZnSi 5 O 12 in the space group Pa3 has been reported above 566 K (Bell & Henderson, 2012).

Synthesis and crystallization
The samples were made from stoichiometric mixtures of Rb 2 CO 3 , SiO 2 and NiO (X = Ni) or MnO (X = Mn). These mixtures were ground together and then heated overnight at 873 K to decompose the carbonates, then melted in platinum crucibles at 1573 K for 1.5 h (X = Ni) or 1673 K for 2 h (X = Mn) before quenching to form glasses. The glasses were drycrystallized at ambient pressure and 1193 K for 12 d.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For each sample, a small amount of powder was ground and mounted on a low-background silicon wafer with a drop of acetone. These were mounted in flat-plate mode on a PANalytical X'Pert Pro MPD diffractometer. X-ray powder diffraction data were collected at 293 K using CuK X-rays over the range 10-80 /2 using a PANalytical X'Celerator area detector. The powder diffraction data collection time for each sample was 8 h 20 min.
All Bragg reflections in both of the powder diffraction patterns could be indexed in the space group Pbca with similar lattice parameters to that for the Cs 2 CdSi 5 O 12 leucite . The crystal structures (Bell & Henderson, 1996) of Cs 2 NiSi 5 O 12 (X = Ni) and Cs 2 MnSi 5 O 12 (X = Mn) were respectively used as starting models for Rietveld (1969) (1)

Figure 4
The crystal structure of Rb