Crystal structure of magnesium selenate heptahydrate, MgSeO4·7H2O, from neutron time-of-flight data

The structure of MgSeO4·7H2O has been determined by single-crystal Laue diffraction methods using pulsed neutron radiation. The compound is isostructural with the sulfate analogue, MgSO4·7H2O.

coordination octahedron is elongated along one axis due to the tetrahedral coordination of the two apical water molecules; these have Mg-O distances of $2.10 Å , whereas the remaining four trigonally coordinated water molecules have Mg-O distances of $2.05 Å . The mean Se-O bond length is 1.641 Å and is in excellent agreement with other selenates. The unit-cell volume of MgSeO 4 Á7H 2 O at 10 K is 4.1% larger than that of the sulfate at 2 K, although this is not uniform; the greater part of the expansion is along the a axis of the crystal.

Chemical context
Since their discovery almost two hundred years ago, the heptahydrates of divalent metal selenates have received scant attention. This is in stark contrast with the M 2+ SeO 4 hexahydrates, which have been extensively characterized, including studies of their morphology and optical properties (Topsøe & Christiansen, 1874), their crystal structures (Stadnicka et al., 1988;Kolitsch, 2002), their formation of isomorphous solution series (e.g., Ojkova et al., 1990 and their dehydration properties (Nabar & Paralkar, 1975: Stoilova & Koleva, 1995. In part this may be due to the fact that the heptahydrates must be prepared at lower temperatures. Nevertheless, it is striking that the only information concerning their crystal structures, namely their apparent isomorphism with the M 2+ SO 4 heptahydrates, has remained largely unaltered since the observations made prior to 1830 by Berzelius and his student Mitscherlich, which is that MgSeO 4 Á7H 2 O forms deliquescent four-sided prismatic crystals below 288 K (e.g., Berzelius, 1818Berzelius, , 1829. The only known goniometric data relate to FeSeO 4 Á7H 2 O and CoSeO 4 Á7H 2 O (Wohlwill, 1860: Topsøe, 1870: Tutton, 1918, which are isomorphous with the monoclinic series of M 2+ SO 4 heptahydrates. It is worth stating that MgMoO 4 Á5H 2 O is isomorphous with both the sulfate, chromate and selenate analogues but is not isostructural with them [Bars et al., 1977; see also Lima-de-Faria et al. (1990) for further discussion of these nomenclature], so the occurrence of MgSeO 4 Á7H 2 O as acicular rhombic prisms is no guarantee that it is isostructural with the sulfate salt. Additional confusion arises from conflicting observations of the MgSeO 4 -H 2 O binary phase diagram (Meyer & Aulich, 1928: Klein, 1940, including our own recent ISSN 1600-5368 discovery of hitherto unknown hydrates (containing 9H 2 O and 11H 2 O) below 273 K (Fortes, 2014).
As part of a wider study into low-temperature crystal hydrates of MgSeO 4 and related compounds (Fortes et al., 2013) we synthesised the title compound and carried out a single-crystal neutron diffraction experiment in order to determine its structure.

Structural commentary
The crystal structure ( Fig. 1) (Table 2). This distortion was also noted in the sulfate by Baur (1964) and is manifested in subsequent neutron single-crystal and powder diffraction studies (Ferraris et al., 1973: Fortes et al., 2006. The difference is due to the tetrahedral coordination of OW2 and OW5; both of these water molecules (in addition to being Mg-coordin-ated) donate two hydrogen bonds and accept one hydrogen bond, from OW7 and OW6 respectively. The four 'equatorial' water molecules donate but do not accept any hydrogen bonds. In the sulfate at 2 K (Fortes et al., 2006), the average equatorial Mg-O distances were found to be 2.029 Å and the average axial Mg-O distances to be 2.100 Å (2.056 and Acta Cryst. Asymmetric unit of MgSeO 4 Á7H 2 O with anisotropic displacement ellipsoids drawn at the 50% probability level (75% for Mg and the selenate O atoms to aid visibility). Dashed rods indicate hydrogen bonds. The superscripts (i) and (ii) denote, respectively, the symmetry operations [1 À x, 1 2 + y, 3 2 À z] and [ 3 2 À x, 1 À y, 1 2 + z]. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Packing of the polyhedra and interstitial water in MgSeO 4 Á7H 2 O viewed down the c-axis. The polyhedral ions have been blurred in order to emphasize the location of the interstitial water molecules.

Table 2
Selected bond lengths (Å ). (8) 2.102 Å at room temperature; Ferraris et al., 1973;Calleri et al., 1984). The [SeO 4 ] 2À tetrahedron exhibits a similar property in that the bond lengths are influenced by the hydrogen-bond coordination. Two of the oxygen atoms (O1 and O3) accept two hydrogen bonds and have mean Se-O bond lengths of 1.631 Å , whereas the other two oxygen atoms (O2 and O4) accept three hydrogen bonds and have a mean Se-O bond length of 1.652 Å . This distinction is not readily apparent in any of the data pertaining to the sulfate, but it is worth observing that the neutron scattering cross-section of selenium is almost three times greater than that of sulfur so our result should be considered more accurate. The mean Se-O bond length of 1.641 Å is in excellent agreement with other similar high-precision analyses of selenate crystals (Kolitsch, 2001(Kolitsch, , 2002Weil & Bonneau, 2014).
Overall, the unit-cell volume of the selenate at 10 K is 4.1% larger than the sulfate analogue (deuterated) at 2 K. This expansion is not isotropic, however, with the greatest proportion being along the a axis of the crystal. We find that the a axis is 2.7% longer, the b axis 1.0% longer, and the c axis 0.3% longer in the selenate than the sulfate. It is not readily apparent from examination of the structure why this should be so. The magnitude of the volumetric strain is virtually identical to that found in MgSeO 4 Á11H 2 O (4.1% larger than the sulfate analogue; Fortes, 2014) and somewhat less than is observed in, for example, CuSeO 4 Á5H 2 O (5.1% larger than the equivalent sulfate; Kolitsch, 2001) or MgSeO 4 Á6H 2 O (5.2%; Kolitsch, 2002).

Synthesis and crystallization
In our initial attempts to make MgSeO 4 we employed the widely cited method of reacting basic Mg-carbonate with aqueous selenic acid (e.g., Stoilova & Koleva, 1995), but this was found to leave a substantial amount of acid in solution, giving a pink-coloured viscous liquid with a sour odour, which yielded an intimate mixture of MgSeO 4 Á6H 2 O and Mg(HSeO 3 ) 2 Á4H 2 O crystals (cf., Kolitsch, 2002;Mička et al., 1996) even after repeated re-crystallization and treatment with aqueous H 2 O 2 . Consequently, we prepared an aqueous solution of magnesium selenate by stirring MgO into a solution of H 2 SeO 4 (Sigma-Aldrich 481513, 40% wt diluted further in its own weight of distilled water) heated to 340 K. This reaction is much less dramatic than is the case when Mg-carbonate is used and the only clear indication that it has run to completion is the pH of the solution, which changed from 0.11 to 8.80. After a period of evaporation in the open air, the solution precipitates cm-sized crystals of MgSeO 4 Á6H 2 O. After a further round of recrystallization from distilled water the phase purity of the hexahydrate was verified both by X-ray powder diffraction and Raman spectroscopy.
Finally, crystalline MgSeO 4 Á6H 2 O was dissolved in distilled water to a concentration of 35% wt MgSeO 4 at 333 K, and this liquid was left to evaporate in a refrigerated workshop at 269 K. After two days, slender prismatic crystals indis-tinguishable in habit from MgSO 4 Á7H 2 O, appeared. One of these was removed from the liquid, dried on filter paper and cut into a pair of fragments each with dimensions 1 x 1 x 4 mm. The two fragments were placed side-by-side in an aluminium foil pouch suspended inside a standard thin-walled vanadium sample can (6 mm inner diameter). The lid of the can was sealed with indium wire and was then transported to the ISIS neutron source immersed in liquid nitrogen.
The sample can was screwed onto a standard centre stick and inserted into a pre-cooled Closed-Cycle Refrigerator (CCR) already mounted on the SXD beam-line (Keen et al., 2006). Initial data collection as the sample was cooled from 200 K down to 10 K revealed strong reflections from both crystals that could be indexed with an orthorhombic unit cell of similar shape but roughly 4% larger than that of MgSO 4 Á7H 2 O. After cooling to 10 K data were collected with the crystals in four discrete orientations with respect to the incident beam, optimizing the coverage of reciprocal space, with integration times of 1600 mAhr each (roughly 10 h per frame at typical ISIS beam intensity). The peaks were indexed and integrated using the instrument software, SXD2001 (Gutmann, 2005)   Upon completion of the experiment, crystals of the title compound that had been stored in a glass vial at 253 K for ten days were analysed by means of X-ray powder diffraction. This measurement, carried out on a custom Peltier cold stage (Wood et al., 2012) at 253 K, revealed that the heptahydrate had transformed completely to the newly reported MgSeO 4 Á9H 2 O (Fortes, 2014), thus providing some initial insight into the relative stability of the two compounds.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Structure refinement with SHELXL using the model obtained at 2 K for the deuterated MgSO 4 analogue (Fortes et al., 2006) based on earlier work (Baur, 1964: Ferraris et al., 1973: Calleri et al., 1984 yielded a good fit with no density residuals larger than 4.5% of the nuclear scattering density due to a hydrogen atom. No restraints were used and all anisotropic temperature factors were refined independently. Extinction correction: SHELXL2014 (Gruene et al., 2014), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0026 (3) Absolute structure: All f" are zero, so absolute structure could not be determined

Special details
Experimental. For peak integration a local UB matrix refined for each frame, using approximately 50 reflections from each of the 11 detectors. Hence _cell_measurement_reflns_used 550 For final cell dimensions a weighted average of all local cells was calculated Because of the nature of the experiment, it is not possible to give values of theta_min and theta_max for the cell determination. The same applies for the wavelength used for the experiment. The range of wavelengths used was 0.48-7.0 Angstroms, BUT the bulk of the diffraction information is obtained from wavelengths in the range 0.7-2.5 Angstroms. The data collection procedures on the SXD instrument used for the single-crystal neutron data collection are most recently summarized in the Appendix to the following paper Wilson, C. C. (1997