Semiconductor physicsby K. Seeger

Revision of the Mg(ClO4)2·4H2O crystal structure

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A revision is presented of the restrained Rietveld analysis of the crystal structure of magnesium perchlorate tetrahydrate, Mg(ClO 4 ) 2 Á4H 2 O, recently published by Robertson & Bish [(2010), Acta Cryst. B66, 579-584].

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“…Cryst. 37, 743-749], anisotropically for all non-H atoms and isotropically for two independent H atoms without restraints.In the X-ray powder diffraction (XRPD) crystal structure analysis of Mg(ClO 4 ) 2 Á4H 2 O recently reported by Robertson & Bish (2010), the authors applied the charge-flipping algorithm and the restrained Rietveld refinement that gave a structure model in the C2 space group. The resultant difference between the observed and calculated XRPD patterns was enormous, making the results quite dubious.…”

“…The resultant difference between the observed and calculated XRPD patterns was enormous, making the results quite dubious. A revision of the structure using the diffraction data from the supplementary materials of Robertson & Bish (2010) reveals that the actual symmetry of the material is C2/m. The C2/m structure model is refined without restraints using the derivative difference method (DDM; Solovyov, 2004) that gave a perfect fit of the calculated XRPD profile to the experiment (Fig.…”

“…The structural disordering revealed might be the reason for the erroneous space-group assignment by Robertson & Bish (2010) as the charge-flipping algorithm is normally started from the lowest symmetry. The example presented demonstrates that structure solutions involving the restrained Rietveld refinement must be validated with special care and all possible attempts should be made at the unrestrained refinement since the application of restraints disables the use of geometric and displacement parameters as the structure reliability indicators.…”