The crystal structure of charmarite – the first case of a 11 × 11 Å superstructure mesh in layered double hydroxides

Charmarite, Mn 4 Al 2 (OH) 12 CO 3 ⋅3H 2 O, is a hydrotalcite supergroup member (or layered double hydroxide, LDH) with a previously unknown crystal structure and a Mn 2+ -analogue of quintinite (commonly erroneously reported as ‘2:1 hydrotalcite’). The single-crystal X-ray diffraction (XRD) data were obtained from the specimen from Mont Saint-Hilaire, Québec, Canada and are best processed in the space group P 3̄ , a = 10.9630(4), c = 15.0732(5) Å and V = 1568.89(12) Å 3 . The crystal structure has been solved by direct methods and refined to R 1 = 0.0750 for 3801 unique reflections with F o > 2σ( F o ). The charmarite structure has long-range periodicity in the xy plane due to 2√ 3 a ’ × 2√ 3 a ’ scheme (or 11 × 11 Å) determined for LDHs for the first time (where a ’ is a subcell parameter ≈ 3.2 Å). This periodicity is produced by the combination of two superstructures formed by: (1) Mn 2+ and Al 3+ ordering in the metal-hydroxide layers [Mn 4 Al 2 (OH) 12 ] 2+ according to the √ 3 a ’ × √ 3 a ’ pattern and (2) the (CO 3 ) 2– ordering according to the 2 a ’ × 2 a ’ pattern in the [CO 3 (H 2 O) 3 ] 2– interlayer sheet in order to avoid close contacts between adjacent carbonate groups. The 2√ 3 a ’ × 2√ 3 a ’ superstructure is an example of the adaptability of the components of the interlayer space to the charge distribution of the metal-hydroxyl layers. The Mn 2+ and Al 3+ cations have a large difference in size, which apparently leads to the considerable degree of their order as di- and trivalent cations resulting in a higher degree of statistical order of the interlayer components. Both powder and single-crystal XRD data show that the samples studied belong to the hexagonal branch of two-layer polytypes (2 T or 2 H ) with d 00 n ≈ 7.57 Å. The chemical composition of the samples studied is close to the ideal formula. The Raman spectrum of charmarite is reported and the band assignment is provided.