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Acta Cryst. (2013). E69, i1    [ doi:10.1107/S1600536812048921 ]

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

A. M. T. Bell and C. M. B. Henderson

Abstract top

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.

Comment top

The title compiound, Sr2TiO(Si2O7), is the Sr analogue of the mineral fresnoite, Ba2TiO(Si2O7). It is of interest as a potential storage medium for radioactive strontium from nuclear waste (Park & Navrotsky, 2010). An incommensurately modulated structure of Sr-fresnoite has been determined from room-temperature single crystal data and refined in the 5D superspace group P4bm (-α, α, 1/2; α, α, 1/2) with α = 0.3 and with lattice parameters a = 8.312 (2) Å and c = 10.07 (1) Å (Höche et al., 2002). However, ICDD PDF card 39–228 (ICDD, 1989) states that at room temperature this material is tetragonal with space group P4bm and lattice parameters a = 8.3218 (2) Å and c = 5.0292 (2) Å. The crystal structure of the mineral fresnoite has been decribed in the same space group with lattice parameters a = 8.5159 (6) Å and c = 5.2184 (4) Å. Solid solutions with composition Ba2-xCaxTiO(Si2O7) (x = 0.0, 0.2, 0.4, 0.8, 1.0; Barbar & Roy, 2012) adopt the same structure. The ordered crystal structure of Sr2TiO(Si2O7) in space group P4bm and a halved c parameter in comparison with the single crystal study is reported in the present communication.

The mean Si—O and Ti—O distances in the titanosilicate layer of Sr-fresnoite are respectively 1.64 Å and 1.92 Å. The corresponding Si—O and Ti—O distances are 1.64 Å and 1.93 Å in fresnoite. The respective distances in the structures of the solid solutions Ba2-xCaxTiO(Si2O7) are: 1.59 Å and 2.01 Å (x = 0.2); 1.65 Å and 2.03 Å (x = 0.4); 1.66 Å and 2.02 Å (x = 0.8); 1.71 Å and 1.95 Å (x = 1.0) (Barbar & Roy, 2012). Due to the distortion of the crystal structures by the partial replacement of Ba by Ca these distances are less comparible with those in Sr-fresnoite.

The mean Sr—O distance in the title structure is 2.62 Å, which is comparible with the mean Sr—O distance of 2.61 Å in Sr4Ti5O8(Si2O7)2 (Miyajima et al., 2002).

The O—Si—O angles deviate significantly from the ideal tetrahedral angle of 109.5°, indicating a strong distortion due to the presence of the TiO5 polyhedra in the titanosilicate layer.

Fig. 1 shows the Rietveld difference plot for the present refinement. The crystal structure of Sr2TiO(Si2O7) is displayed in Fig. 2 and consists of layers of corner-sharing SiO4 and TiO5 polyhedra extending parallel to the ab plane. These layers are separated along the c axis by layers of distorted SrO6 octahedra.

Related literature top

For the crystal chemistry of fresnoites, see: Barbar & Roy (2012); Höche et al. (2002); ICDD (1989). For properties of Sr–fresnoites, see: Park & Navrotsky (2010). Atomic coordinates as starting parameters for the Rietveld refinement (Rietveld, 1969) of the present phases were taken from Ochi (2006); Goldschmidt & Thomassen (1923); Machida et al. (1982); Mitchell et al. (2000). For related strontium titanosilicates, see: Miyajima et al. (2002). For synchrotron data analysis, see: Hammersley (1997); Hammersley et al. (1996).

Experimental top

A synthetic sample of Sr-fresnoite was made by melting a stoichiometric mixture of SrCO3, TiO2 and SiO2 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 CeO2 (NIST SRM 674a) standard was added to this powdered sample to act as an internal standard.

Refinement top

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 Sr2TiO(Si2O7) 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. SrTiO3 and SrSiO3 were formed as by-products during preparation.

Initial lattice parameters for the three Sr-containing phases were refined using local software. The CeO2 (NIST SRM 674a) standard was used to calibrate the sample to detector distance. The CeO2 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 (Ba2TiO(Si2O7); Ochi, 2006) was used as a starting model for the Rietveld refinement (Rietveld, 1969) of the structure of Sr2TiO(Si2O7). The crystal structures of SrSiO3 (Machida et al., 1982), SrTiO3 (Mitchell et al., 2000) and CeO2 (Goldschmidt & Thomassen, 1923) were used for the impurity phases in the refinement. Isotropic atomic displacement parameters were used for all phases. For the Sr2TiO(Si2O7) phase the Si—O and Ti—O distances in the SiO4 and TiO5 polyhedra were soft-constrained to those for Ba2TiO(Si2O7) (Ochi, 2006). The Uiso factors for all O sites were constrained to be the same. 81 (1) wt.% of Sr-fresnoite was present in this sample with 4.6 (3) wt.% CeO2, 7.0 (6) wt.% SrTiO3 and 7.4 (8) wt.% of SrSiO3 present as impurities.

Computing details top

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: publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. Rietveld difference plot for the multi-phase refinement of Sr2TiO(Si2O7), CeO2, SrTiO3 and SrSiO3. The blue crosses, and red and black lines show respectively the observed, calculated and difference plots. Calculated Bragg reflection positions are indicated by triangles for the four phases.
[Figure 2] Fig. 2. The crystal structure of Sr2TiO(Si2O7). Purple polyhedra show TiO5 units, blue polyhedra show SiO4 units, green polyhedra show distorted SrO6 units. Green spheres represent Sr atoms, pink spheres represent Ti atoms, blue spheres represent Si atoms and red spheres represent O atoms.
Distrontium oxidotitanium disilicate top
Crystal data top
Sr2TiSi2O8Dx = 3.890 (1) Mg m3
Mr = 407.31Synchrotron radiation, λ = 0.207549 Å
Tetragonal, P4bmµ = 0.43 mm1
Hall symbol: P 4 -2abT = 293 K
a = 8.3200 (3) ÅParticle morphology: powder
c = 5.0239 (2) Åwhite
V = 347.77 (2) Å3cylinder, 20 × 0.7 mm
Z = 2Specimen preparation: Prepared at 1323 K and 100 kPa
Data collection top
In-house design
Data collection mode: transmission
Radiation source: SynchrotronScan method: continuous
Laue DCM diamond(111) & Si(111) monochromator2θmin = 0.053°, 2θmax = 11.915°, 2θstep = 0.008°
Specimen mounting: capillary
Refinement top
Rp = 0.052Excluded region(s): 0-1 and 11.7-12.0 degrees 2θ
Rwp = 0.073Profile function: T-C-H Pseudo-Voigt function
Rexp = 0.03171 parameters
RBragg = 0.0935 restraints
χ2 = 31.068
1476 data points
Crystal data top
Sr2TiSi2O8Z = 2
Mr = 407.31Synchrotron radiation, λ = 0.207549 Å
Tetragonal, P4bmµ = 0.43 mm1
a = 8.3200 (3) ÅT = 293 K
c = 5.0239 (2) Åcylinder, 20 × 0.7 mm
V = 347.77 (2) Å3
Data collection top
In-house design
Scan method: continuous
Specimen mounting: capillary2θmin = 0.053°, 2θmax = 11.915°, 2θstep = 0.008°
Data collection mode: transmission
Refinement top
Rp = 0.052χ2 = 31.068
Rwp = 0.0731476 data points
Rexp = 0.03171 parameters
RBragg = 0.0935 restraints
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
Sr10.3282 (2)0.8282 (2)0.017 (2)0.0070 (8)*
Ti10.000000.000000.558 (3)0.007 (2)*
Si10.1305 (6)0.6305 (6)0.535 (3)0.019 (2)*
O10.000000.500000.651 (5)0.017 (3)*
O20.1292 (15)0.6292 (15)0.191 (3)0.017 (3)*
O30.2985 (12)0.5984 (15)0.678 (3)0.017 (3)*
O40.000000.000000.209 (3)0.017 (3)*
Geometric parameters (Å, º) top
Sr1—O1i2.733 (18)Ti1—O3viii1.961 (12)
Sr1—O22.499 (13)Ti1—O3ix1.961 (13)
Sr1—O2ii2.676 (13)Ti1—O41.75 (2)
Sr1—O2iii2.676 (13)Si1—O11.642 (11)
Sr1—O3iv2.572 (15)Si1—O21.73 (2)
Sr1—O3v2.572 (14)Si1—O31.594 (14)
Ti1—O3vi1.961 (12)Si1—O3x1.594 (15)
Ti1—O3vii1.961 (13)
O3xi—Ti1xii—O3xiii84.6 (9)O3x—Ti1xii—O4xii107.9 (12)
O3xi—Ti1xii—O3xiv144.2 (10)O3xiv—Ti1xii—O4xii107.9 (12)
O3xiv—Ti1xii—O3xiii84.6 (8)O1—Si1—O2110.3 (16)
O3xi—Ti1xii—O4xii107.9 (12)O1—Si1—O3x108.0 (9)
O3xiv—Ti1xii—O3x84.6 (8)O1—Si1—O3108.0 (10)
O3xiii—Ti1xii—O3x144.2 (11)O2—Si1—O3117.1 (15)
O3xiii—Ti1xii—O4xii107.9 (12)O2—Si1—O3x117.1 (15)
O3x—Ti1xii—O3xi84.6 (9)O3x—Si1—O395.2 (11)
Symmetry codes: (i) y+1, x+1, z1; (ii) y+1, x+1, z; (iii) y, x+1, z; (iv) x, y, z1; (v) y1/2, x+1/2, z1; (vi) x+1/2, y1/2, z; (vii) y1/2, x1/2, z; (viii) x1/2, y+1/2, z; (ix) y+1/2, x+1/2, z; (x) y1/2, x+1/2, z; (xi) x+1/2, y+1/2, z; (xii) x, y+1, z; (xiii) y+1/2, x+3/2, z; (xiv) x1/2, y+3/2, z.

Experimental details

Crystal data
Chemical formulaSr2TiSi2O8
Crystal system, space groupTetragonal, P4bm
Temperature (K)293
a, c (Å)8.3200 (3), 5.0239 (2)
V3)347.77 (2)
Radiation typeSynchrotron, λ = 0.207549 Å
µ (mm1)0.43
Specimen shape, size (mm)Cylinder, 20 × 0.7
Data collection
DiffractometerIn-house design
Specimen mountingCapillary
Data collection modeTransmission
Scan methodContinuous
2θ values (°)2θmin = 0.053 2θmax = 11.915 2θstep = 0.008
R factors and goodness of fitRp = 0.052, Rwp = 0.073, Rexp = 0.031, RBragg = 0.093, χ2 = 31.068
No. of data points1476
No. of parameters71
No. of restraints5

Computer programs: local software, coordinates taken from a related compound, FULLPROF (Rodriguez-Carvajal, 2001), VESTA (Momma & Izumi, 2008), publCIF (Westrip, 2010).

Acknowledgements top

Thanks to Dr Hanns-Peter Liermann for help with data collection on P02.1.