X-ray powder diffraction analysis of two new magnesium selenate hydrates, MgSeO₄·9H₂O and MgSeO₄·11H₂O

Abstract

Several hitherto unknown hydrates of magnesium selenate have been formed by quenching aqueous solutions of MgSeO4 in liquid nitrogen. MgSeO4·11H2O is apparently isostructural with the mineral meridianiite (MgSO4·11H2O), being triclinic, $P{\rm \bar 1}$, Z = 2, with unit-cell parameters a = 6.779 00(8) Å, b = 6.965 16(9) Å, c = 17.4934(2) Å, α = 87.713(1)°, β = 89.222(1)°, γ = 63.121(1)°, and V = 736.15(1) Å3 at −25 °C. MgSeO4·9H2O represents a new hydration state in the MgSeO4–H2O system; it is monoclinic, space-group P21/c, Z = 4, with unit-cell parameters a = 7.270 24(6) Å, b = 10.510 94(9) Å, c = 17.4030(2) Å, β = 109.447(1)°, and V = 1254.02(1) Å3 at −22 °C. The heavy-atom structure of MgSeO4·9H2O has been determined by direct-space methods from X-ray powder diffraction data and consists of isolated Mg(H2O)62+ octahedra and SeO42− tetrahedra linked by hydrogen bonds. The remaining three water molecules occupy the space between the polyhedral ions, contributing to the H-bonded network, which comprises 4-, 5-, and 6-membered rings. A third phase has been observed to crystallise prior to the 11-hydrate upon warming of liquid-nitrogen-quenched glass, but this transforms rapidly to the meridianiite-structured 11-hydrate and the identity of this phase is unclear.

Structure of magnesium selenate enneahydrate, MgSeO4·9H2O, from 5 to 250 K using neutron time-of-flight Laue diffraction

“…One of these was extracted from solution, dried and powdered under liquid nitrogen before being subjected to X-ray powder diffraction analysis using a custom-made Peltier cold stage (Wood et al, 2012) mounted on a PANAlytical X'Pert Pro powder diffractometer. The X-ray powder diffraction pattern of this material was identical to that reported previously for MgSeO 4 Á9H 2 O obtained by quenching droplets of the solution in liquid nitrogen (Fortes, 2015).…”

“…Time-of-flight Laue data were then collected in a series of four orientations, counting each for $ 2 h (300 mA of beam current). The diffraction spots from both crystals in the beam were indexed with the unit cell obtained at 250 K (Fortes, 2015), after which the intensities were extracted using the three-dimensional profile-fitting method implemented in SXD2001 (Gutmann, 2005). Data were subsequently collected using the same measurement strategy at 100, 175 and 250 K.…”

“…atoms, is obliged to be ordered. However, the first ring is not so obliged and it was hypothesized by Fortes (2015) that occupational disorder of the H atoms could occur in ring #1 in a manner similar to that which occurs in the water tetramer in Na 2 SO 4 Á10H 2 O (cf. Brand et al, 2009, and references therein).…”

“…8. This work has revealed new examples of 11-hydrates that are isotypic with the sulfate compound, MgCrO 4 Á11H 2 O Fortes et al, 2013) and MgSeO 4 Á11H 2 O (Fortes, 2015), and shown that hydrates with n = 8 and 9 can be synthesized (Fortes et al, 2012a,b;Fortes, 2014). All of the M 2+ sulfate octa-and enneahydrates prepared thus far are metastable, being crystallized only by rapidly quenching small droplets of aqueous solution in liquid nitrogen, after which they typically undergo solid-state transformations over periods of minutes to hours into other stable hydrates.…”

“…All of the M 2+ sulfate octa-and enneahydrates prepared thus far are metastable, being crystallized only by rapidly quenching small droplets of aqueous solution in liquid nitrogen, after which they typically undergo solid-state transformations over periods of minutes to hours into other stable hydrates. However, this proved not to be the case for magnesium selenate, where it was determined that MgSeO 4 Á11H 2 O undergoes incongruent melting to form MgSeO 4 Á9H 2 O apparently in equilibrium with aqueous solution, and furthermore that MgSeO 4 Á7H 2 O crystals stored in a freezer formed pseudomorphs after the heptahydrate composed of the enneahydrate (Fortes, 2015).…”