The P43 enantiomorph of Sr2As2O7

The crystal structure of strontium diarsenate has been reinvestigated from single-crystal X-ray diffraction data. In contrast to the previous determinations of this structure [Weil et al. (2009 ▶). Solid State Sci. 11, 2111–2117; Edhokkar et al. (2012 ▶). Mater. Sci. Eng., 28, 012017] and to all isotypic A 2 B 2O7 compounds that crystallize in the space group P41, the current redetermination revealed the P43 enantiomorph of Sr2As2O7 with a purity of 96.3 (8)%. The crystal structure is made up from two eclipsed As2O7 diarsenate groups (symmetry 1) with characteristically longer As—O bridging bonds [1.756 (4)–1.781 (4) Å] than the terminal As—O bonds [1.636 (4)–1.679 (4) Å] and four Sr2+ sites with coordination numbers ranging from seven to nine. The building units are arranged in sheets parallel to (001).


Related literature
The crystal structure of Sr 2 As 2 O 7 has previously been refined from X-ray powder diffraction data (Weil et al., 2009) in the space group P4 1 and was later reinvestigated (Edhokkar et al., 2012). For isotypic structures crystallizing in space group P4 1 , see: Baglio & Dann (1972)
We took the opportunity to have obtained single crystals of good quality of Sr 2 As 2 O 7 to improve the geometrical characteristics of this structure by a redetermination. Interestingly, it was found that in contrast to all above mentioned various A 2 B 2 O 7 structures (space group P4 1 ), the space group determined for the re-investigated material is P4 3 , with only a minor contribution of 3.7 (8)% for the P4 1 enantiomorph. In comparison with the two previous studies, the precision in terms of bond lengths and angles is significantly higher for the current redetermination.
The structure of Sr 2 As 2 O 7 is characterized by the presence of two independent eclipsed As 2 O 7 diarsenate groups, both with site symmetry 1 (Fig. 1). The As-O bridging bonds are characteristically longer (Weil & Stöger, 2010) than the terminal As-O bonds. The brindging As-O bonds range from 1.756 (4) to 1.781 (4) Å, the terminal bonds from 1.636 (4) to 1.679 (4) Å. This trend is also observed in the closely related structures of β-Ca 2 P 2 O 7 and Sr 2 V 2 O 7 but to a lesser extent in La 2 Si 2 O 7 . This can be understood in terms of cationic repulsion since the X 5+ ···X 5+ (X = P, As, V) repulsion is stronger than that of Si 4+ ··· Si 4+ . The As-O-As bridging angles, viz. 126.8 (2)° and 129.3 (2)°, are slightly greater than the corresponding V-O-V angles, 123.04° and 123.53°, in Sr 2 V 2 O 7 .
The crystal packing is based on discrete Sr 2+ cations and isolated (As 2 O 7 ) 4anions arranged in sheets parallel to (001) (Fig. 2). The Sr 2+ cations are divided into four independent atomic sites and exhibit coordination numbers from seven to nine, with irregular coordination polyhedra and Sr-O distances spreading over the range 2.458 (4) -3.228 (5) Å.

Experimental
Single crystals of the title compound were synthesized in a solid state reaction by reacting As 2 O 5 with SrCO 3 in an alumina boat. A mixture of these reagents in the molar ratio 30:70 was used for the synthesis. The mixture was heated at 823 K for 24 h. After grinding, the reacting mixture was heated up to 1173 K and maintained at this temperature for 48 h.
Then the mixture was cooled to room temperature by switching off the furnace power. Translucent single crystals of Sr 2 As 2 O 7 were extracted from the batch.

Refinement
Reflections (0 0 4), (0 0 4), (0 1 2) and (0 1 1) were omitted from the refinement due to large differences between calculated and measured intensities. With regard to the anisotropic refinement of the atomic displacement parameters it should be mentioned that a first attempt carried out from the initial data collection routinely recorded using an exposure time of 10 s per frame resulted in some large ADP max/min ratios and either prolate or oblate displacement ellipsoids for some oxygen atoms. The corresponding value of θ max for this data collection was 29.51°. Then a new data collection was carried out with an exposure time of 20 s per frame. The corresponding refinement lead to more homogeneous and acceptable values of the principal mean square atomic displacements U. For this data collection the θ max value was also increased up to 33.22° and resulted in 4593 intensities with I>2σ(I) versus 3324 in the preceding data collection. The highest residual peak in the final difference Fourier map was located 0.70 Å from the Sr2 site and the deepest hole was located 0.65 Å from the As4 site.

Figure 1
A view of a part of the structure of Sr 2 As 2 O 7 . Displacement ellipsoids are drawn at the 50% probability level. [Symmetry

Figure 2
Projection along [100] of the Sr 2 As 2 O 7 structure showing the stacking of (As 2 O 7 ) 4sheets parallel to (001).

Strontium diarsenate
Crystal data  (8) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refined as a 2-component inversion twin.