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

Abstract

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 R$ {\bar 3} $m 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.

Crystal Chemistry of Chlormagaluminite, Mg4Al2(OH)12Cl2(H2O)2, a Natural Layered Double Hydroxide

“…The problem of the presence of inconsistent reflections is not entirely new for hydrotalcite-supergroup minerals due to crystal imperfections, e.g., [31]. However, in the present study, even the best-quality crystal produced an essential number of such reflections, which was also typical for all studied samples.…”

“…Thus, the presence of the reflection with d = 4.57 Å in the powder pattern of pure LDHs having M 2+ :M 3+ = 2:1 is a reliable indicator of cation ordering similar to the Li 2 Al LDHs [47][48][49][50]. In contrast to Cl-bearing hydrotalcite-supergroup members, the carbonate species have been structurally characterized in more details including recent single-crystal or Rietveld structure refinements of hydrotalcite [31], pyroaurite [51][52][53], quintinite [34,[53][54][55][56][57][58], takovite [59], and zaccagnaite [60]. It has been found that C atoms of CO 3 groups occupy central parts of the interlayer prisms (Figure 7b), whereas O atoms of CO 3 groups and H 2 O molecules are disordered around the middle points of the prism edges.…”

Akopovaite, Li2Al4(OH)12(CO3)(H2O)3, a new Li member of the hydrotalcite supergroup from Turkestan Range, Kyrgyzstan

“…Results of three measurements provide a ratio of Al/Li = 9.36, which was used to obtain an Al(OH) 3 content of 10.2 wt.% (Table 2). It should be noted, that normalised values of CO 2 and H 2 O are in a good agreement with calculated ones and correspond to the stoichiometry of the hydrotalcite-supergroup minerals (Mills et al ., 2016; Zhitova et al ., 2018, 2019b). The empirical formula calculated on the basis of Li+Al+Fe = 6 apfu is Li 1.94 (Al 4.05 Fe 0.02 ) Σ4.07 (OH) 12 (CO 3 ) 1.03 (H 2 O) 3.03 .…”

“…The difficulty in the structure refinement of akopovaite is in the presence of light elements and statistic disorder of interlayer C and O atoms (also characteristic for single-crystal structure refinements of other hydrotalcite-supergroup minerals) (Krivovichev et al ., 2010a,b; Zhitova et al ., 2010, 2017a,b, 2018, 2019b). Therefore, atom coordinates were refined with soft restraints applied for the interlayer atoms.…”

Synthesis and Characterization of New Layered Double Hydroxide-Polyolefin Film Nanocomposites with Special Optical Properties

“…These anions are weakly bound to the structure and ion exchange reactions are suitable for the modification of LDHs [15,16,17]. LDHs composed of Mg and Al hydroxyl sheets with carbonate ions as counter anions and M 2+ :M3 + = 3:1 are called hydrotalcite, and quintinite when the ratio M 2+ :M 3+ = 2:1 [18,19,20]. LDHs have attracted the attention of the scientific community due to their unique properties such as anion exchange ability, thermal stability and easy production [21,22,23] and the promising applications in different fields of adsorption [24,25,26], sensors [27,28,29,30], biomedicine [31,32,33], polymer additives [34,35,36], among others.…”