Crystal chemistry of natural layered double hydroxides. 5. Single-crystal structure refinement of hydrotalcite, [Mg6Al2(OH)16](CO3)(H2O)4

Hydrotalcite, ideally [Mg6Al2(OH)16](CO3)(H2O)4, was studied in samples from Dypingdal, Snarum, Norway (3R and 2H), Zelentsovskaya pit (2H) and Praskovie–Evgenievskaya pit (2H) (both Southern Urals, Russia), Talnakh, Siberia, Russia (3R), Khibiny, Kola, Russia (3R), and St. Lawrence, New York, USA (3R and 2H). Two polytypes, 3R and 2H (both ‘classical’), were confirmed on the basis of single-crystal and powder X-ray diffraction data. Their chemical composition was studied by electron-microprobe analysis, infrared spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The crystal structure of hydrotalcite-3R was solved by direct methods in the space group R3m on three crystals (two data collections at 290 K and one at 120 K). The unit-cell parameters are as follows (290/290/120 K): a = 3.0728(9)/3.0626(3)/3.0617(4), c = 23.326(9)/23.313(3)/23.203(3) Å and V = 190.7(1)/189.37(4)/188.36(4) Å3 . The crystal structures were refined on the basis of 304/150/101 reflections to R1 = 0.075/0.041/ 0.038. Hydrotalcite-2H crystallises in the P63/mmc space group; unit-cell parameters for two crystals are (data collection at 290 K and 93 K): a = 3.046(1)/3.0521(9), c = 15.447(6)/15.439(4) Å, V = 124.39(8)/124.55(8) Å3 . The crystal structures were refined on the basis of 160/142 reflections to R1 = 0.077/0.059. This paper reports the first single-crystal structure data on hydrotalcite. Hydrotalcite distribution in Nature, diagnostic features, polytypism, interlayer topology and localisation of M2+–M3+ cations within metal hydroxide layers are discussed

Structural complexity of barium uranyl arsenates: Synthesis, structure, and topology of Ba4[(UO2)2(As2O 7)3],Ba3[UO2)2(AsO4)2(As207)], and Ba 5Ca[(UO2)8(As04)4O 8]

Three new barium uranyl arsenates, Ba-4[(UO2)(2)(As2O7)(3)] (1), Ba-3[(UO2)(2)(AsO4)(2)(As2O7)] (2), and Ba5Ca[(UO2)(8)(AsO4)(4)O-8] (3), with mono-, pyro-, and mixed arsenate oxoanions have been prepared by high-temperature solid-state reactions. The crystal structures have been solved by direct methods and refined using the least-squares method: 1 - orthorhombic, Pbc2(1), 5.6010(6), b = 13.344(1), c = 31.751(2) angstrom, V = 2373.1(3) angstrom(3), Z = 4, R-1 = 0.044; 2 - monoclinic, C2/c, a = 19.246(2), b = 9.536(1), c = 9.619(1) angstrom, beta = 95.585(6)degrees, V = 1757.0(3) angstrom(3), Z = 4, R-1 = 0.049; 3 - monoclinic, P2(1)/c, a = 12.073(1), b = 12.838(1), c = 13.472(1) angstrom, beta = 116.969(3)degrees, V = 1861.0(3) angstrom(3), Z = 2 R-1 = 0.0584. The compounds demonstrate unprecedented structural topologies (one-dimensional chains in 1, three-dimensional framework in 2, two-dimensional layers in 3), which, however, can be related to previously known structures by topological analysis. The novelty of the compounds obtained can be attributed in part to the synthesis method, which implies the absence of water in the system at the reaction temperature. In turn, the higher ionic strength of divalent cations defines enhanced structural complexity and the deviation of the observed structural topologies from those known for similar systems with monovalent cations