The crystal structure of charmarite - the first case of a 11 x 11 ? superstructure mesh in layered double hydroxides

Charmarite, Mn4Al2(OH)(12)CO3 & sdot;3H(2)O, is a hydrotalcite supergroup member (or layered double hydroxide, LDH) with a previously unknown crystal structure and a Mn2+-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, Qu & eacute;bec, Canada and are best processed in the space group P3 , a = 10.9630(4), c = 15.0732(5) & Aring; and V = 1568.89(12) & Aring;(3). The crystal structure has been solved by direct methods and refined to R-1 = 0.0750 for 3801 unique reflections with F-o > 2 sigma(F-o). The charmarite structure has long-range periodicity in the xy plane due to 2 root 3a' x 2 root 3a' scheme (or 11 x 11 & Aring;) determined for LDHs for the first time (where a' is a subcell parameter approximate to 3.2 & Aring;). This periodicity is produced by the combination of two superstructures formed by: (1) Mn2+ and Al3+ ordering in the metal-hydroxide layers [Mn4Al2(OH)(12)](2+) according to the root 3a' x root 3a' pattern and (2) the (CO3)(2-) ordering according to the 2a' x 2a' pattern in the [CO3(H2O)(3)](2-) interlayer sheet in order to avoid close contacts between adjacent carbonate groups. The 2 root 3a' x 2 root 3a' superstructure is an example of the adaptability of the components of the interlayer space to the charge distribution of the metal-hydroxyl layers. The Mn2+ and Al3+ 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 (2T or 2H) with d(00n) approximate to 7.57 & Aring;. 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.