X-ray powder diffraction analysis of a new magnesium chromate hydrate, MgCrO₄·11H₂O

Abstract

A new hydrate of magnesium chromate is synthesized by quenching aqueous solutions of MgCrO4 in liquid nitrogen. MgCrO4·11H2O is isostructural with the rare mineral meridianiite (MgSO4·11H2O) being triclinic, $P{\bar 1}$, Z = 2, with unit-cell parameters a = 6.811 33(8) Å, b = 6.958 39(9) Å, c = 17.3850(2) Å, α = 87.920(1)°, β = 89.480(1)°, γ = 62.772(1)°, and V = 732.17(1) Å3 at −15 °C. The difference in unit-cell parameters between SO4- and CrO4-bearing species is only partially accounted for by the difference in S–O and Cr–O bond lengths; the remainder of the difference (over 90% in the cell volume) is attributed to weakening of the interpolyhedral hydrogen-bond network.

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

“…Neither the top or bottom diffraction patterns shown in Figure 1 resembled any phase seen previously. However, the middle pattern closely resembles (both in terms of peak positions and intensities) the powder diffraction patterns of MgSO 4 ·11H 2 O and MgCrO 4 ·11H 2 O reported previously (see figures in Wood et al , 2012, and Fortes and Wood, 2012). Since this structure is substantially strained with respect to the analogous sulphate and chromate, it proved necessary to index the Bragg peaks using DICVOL06 (Boultif and Louër, 2004); the unit cell was subsequently refined by the LeBail method to yield the unit-cell parameters listed in Table I.…”

“…% of the sample, the remainder being water ice, Rietveld refinement of the structure proved possible only with extremely rigid bond-distance and bond-angle restraints and fixed isotropic displacement parameters. Whilst this demonstrated to my satisfaction that there are no significant differences between the heavy atom structures of MgSeO 4 •11H 2 O and its chromate analogue, the accuracy of the atomic coordinates given in Table II is likely to be considerably poorer than for the previously reported MgCrO 4 •11H 2 O and MgSO 4 •11H 2 O structures (Fortes et al, 2008(Fortes et al, , 2013Fortes and Wood, 2012).…”

“…FOX was initialised with the unit-cell parameters obtained by DICVOL06 and these were refined, along with peak profile coefficients, diffractometer zero-shift and TABLE II. Refined atomic coordinates (x, y, and z) and fixed isotropic displacement parameters (U iso ) for the heavy atoms in the meridianiite-structured MgSeO 4 •11H 2 O (cf., Section III.A) compared to those obtained previously for meridianiite-structured MgCrO 4 •11H 2 O by neutron time-of-flight diffraction methods (Fortes and Wood, 2012). This X-ray powder refinement yielded R p = 0.0845 and χ 2 = 2.897 for 57 variables.…”

“…In a series of papers, my colleagues and I reported the results of work to form structural analogues of MgSO 4 •11H 2 O in which the cation was replaced by Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ or Zn 2+ (Fortes et al, 2012a(Fortes et al, , 2012b and the oxyanion was replaced by CrO 4 2− (Fortes and Wood, 2012;Fortes et al, 2013). These efforts provide information on the response of the structure to chemically induced "internal" stress that are complementary to other means of generating strain in the crystal, such as variable temperature and pressure (cf., Fortes et al, 2008Fortes et al, , 2009Fortes et al, , 2012c.…”

A compact, transportable, thermoelectrically cooled cold stage for reflection geometry X-ray powder diffraction

“…Meridianiite and related compounds are currently of interest because of their possible roles in the near-surface geology of Mars and in the crust and mantles of the icy moons of the outer solar system (see e.g. Fortes & Wood, 2012;Fortes et al, 2012a,b), and the cold stage described in this note has been designed primarily with the aim of studying materials of this kind. Its performance is modest in terms of the temperature range that it covers, but it has the great advantage that it may very easily be demounted from the X-ray diffractometer and taken into a cold room; the entire stage may then be chilled to the cold-room temperature before the sample is loaded.…”