Single crystal growth of CaMg2(SO4)3 via solid‐ / gas‐phase reactions and its Nasicon‐related crystal structure

Crystals of the double sulfate CaMg₂(SO₄)₃ have been obtained by solid‐state reactions of stoichiometric amounts of anhydrous CaSO₄ and MgSO₄ in sealed and evacuated silica tubes with chlorine gas as mineraliser. The crystal structure was determined from single crystal X‐ray diffractometer data P 6₃/m, Z = 2, a = 8.3072(4), c = 7.3057(8) Å, R F² > 2σ (F²)] = 0.0317, wR (F² all) = 0.0785, 476 structure factors, 33 variable parameters] and consists of distorted CaO₆] octahedra (3 symmetry), MgO₆] octahedra (3 symmetry) and SO₄ tetrahedra (m symmetry) as single building units. The structure is made up of ¹_∞CaO_6/2] chains of face‐sharing CaO₆] octahedra that extend parallel to 001], alternating with columns of face‐sharing MgO_3/1O_3/2]₂ dimers. Both types of chains are linked via corner‐sharing with SO₄ tetrahedra into a three‐dimensional framework structure. Although the compound crystallizes in a new structure type, it is topologically related to the NaZr₂(PO₄)₃ (Nasicon) structure, and a comparative discussion between both structural arrangements is given. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)