Sr–fresnoite determined from synchrotron X-ray powder diffraction data

The fresnoite-type compound Sr2TiO(Si2O7), distrontium oxidotitanium disilicate, has been prepared by high-temperature solid-state synthesis. The results of a Rietveld refinement study, based on high-resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the space group P4bm and adopts the structure of other fresnoite-type mineral samples with general formula A2TiO(Si2O7) (A = alkaline earth metal cation). The structure consists of titanosilicate layers composed of corner-sharing SiO4 tetrahedra (forming Si2O7 disilicate units) and TiO5 square-based pyramids. These layers extend parallel to the ab plane and are stacked along the c axis. Layers of distorted SrO6 octahedra lie between the titanosilicate layers. The Sr2+ ion, the SiO4 tetrahedron and the bridging O atom of the disilicate unit are located on mirror planes whereas the TiO5 square-based pyramid is located on a fourfold rotation axis.

The fresnoite-type compound Sr 2 TiO(Si 2 O 7 ), distrontium oxidotitanium disilicate, has been prepared by high-temperature solid-state synthesis. The results of a Rietveld refinement study, based on high-resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the space group P4bm and adopts the structure of other fresnoite-type mineral samples with general formula A 2 TiO(Si 2 O 7 ) (A = alkaline earth metal cation). The structure consists of titanosilicate layers composed of corner-sharing SiO 4 tetrahedra (forming Si 2 O 7 disilicate units) and TiO 5 square-based pyramids. These layers extend parallel to the ab plane and are stacked along the c axis. Layers of distorted SrO 6 octahedra lie between the titanosilicate layers. The Sr 2+ ion, the SiO 4 tetrahedron and the bridging O atom of the disilicate unit are located on mirror planes whereas the TiO 5 square-based pyramid is located on a fourfold rotation axis.

Experimental
A synthetic sample of Sr-fresnoite was made by melting a stoichiometric mixture of SrCO 3 , TiO 2 and SiO 2 to form a glass. This glass was then quenched to 293 K, reground and then heated for 7 days at 1323 K. A small amount of CeO 2 (NIST SRM 674a) standard was added to this powdered sample to act as an internal standard.

Refinement
The powdered sample was loaded into a 0.7 mm diameter quartz capillary, prior to synchrotron X-ray powder diffraction data collection using the P02.1 high resolution powder diffraction beamline at the PETRA-III synchrotron. The beam on the sample was 0.8 mm wide and 1.27 mm high. Powder diffraction data were collected using a PerkinElmer XRD 1621 flat panel image plate detector, which was approximately 1.4 m from the sample. One powder diffraction dataset was collected at 293 K out to approx. 11.9°/2θ, the data collection time was 30 s. Powder diffraction data were converted to a list of 2θ and intensity using FIT2D (Hammersley et al., 1996, Hammersley, 1997. Powder diffraction data in the range 1-11.7°/2θ were used for the Rietveld refinement. Data below 1°/2θ were excluded due to scatter from the beam stop and as there were no Bragg reflections in this region. Data above 11.7°/2θ were excluded as this corresponded to the edge of the image plate detector where the Bragg peaks were weaker. The main Bragg reflections of the powder diffraction pattern could be indexed in space group P4bm with similar lattice parameters to those of PDF card 39-228 (ICDD, 1989). The unit cell of the incommensurately modulated structure (Höche et al., 2002) corresponds to a doubled c axis compared to that given on the PDF card. The doubled c axis does not match with some of the low-angle Bragg reflections for the Sr 2 TiO(Si 2 O 7 ) sample used in the present study, therefore this incommensurate structure was not used for Rietveld refinement. Bragg reflections for three impurity phases could also be identified in the powder diffraction data. SrTiO 3 and SrSiO 3 were formed as by-products during preparation.
Initial lattice parameters for the three Sr-containing phases were refined using local software. The CeO 2 (NIST SRM 674a) standard was used to calibrate the sample to detector distance. The CeO 2 lattice parameter was fixed at 5.4111 Å so as to calibrate the wavelength as 0.207549 Å.
The P4bm crystal structure of the mineral fresnoite (Ba 2 TiO(Si 2 O 7 ); Ochi, 2006) was used as a starting model for the Rietveld refinement (Rietveld, 1969) of the structure of Sr 2 TiO(Si 2 O 7 ). The crystal structures of SrSiO 3 (Machida et al., 1982), SrTiO 3 (Mitchell et al., 2000) and CeO 2 (Goldschmidt & Thomassen, 1923) were used for the impurity phases in the refinement. Isotropic atomic displacement parameters were used for all phases. For the Sr 2 TiO(Si 2 O 7 ) phase the Si-O and Ti-O distances in the SiO 4 and TiO 5 polyhedra were soft-constrained to those for Ba 2 TiO(Si 2 O 7 ) (Ochi, 2006).

Computing details
Data collection: local software; cell refinement: local software; data reduction: local software; program(s) used to solve structure: coordinates taken from a related compound; program(s) used to refine structure: FULLPROF (Rodriguez-Carvajal, 2001); molecular graphics: VESTA (Momma & Izumi, 2008); software used to prepare material for publication: