The crystal structure of the low-temperature phase of methylammonium alum

Abstract

At 170 °K methylammonium alum undergoes a phase change from the room-temperature structure, which is cubic with unit-cell edge of length 12-50/~, to a low-temperature structure which is orthorhombic with unit-cell edges of length a = 12.57, b---12.33 and c--12.38 /~. The change from the symmetry of the non-polar class m3, space group Pa3, to that of the polar class ram2, space group Pca21 is consistent with the onset of ferroelectricity which is known to occur at a temperature in the region of 170 °K; in conformity also with the conditions associated with a ferroelectric transition are the absence of interchange of atoms between different sites and, with the exception of the methylammoniurn group, the absence, too, of large displacements of atoms from the positions occupied in the structure of higher symmetry. In the low-temperature phase the methylammoninm group is shown to be fixed and not to simulate, either by statistical or by rotational means, the possession of the centre of symmetry required of the room-temperature structure.

Vibrational spectra of hexaaquacomplexes. XII. On the possible anion disorder in selenate alums: prediction of the crystal structure and vibrational spectra of KAl(SeO4)2·12H2O and related alums

“…Bacon and Gardner [12] for the first time get insight into the geometry of the hydrogen bonds in KCrSD and the positions of the four crystallographically different H-atoms in the structure. Fletcher and Steeple [13] confirm Lipson"s notion about dimorphism in methylammonium alum and refine the structure of the low-temperature phase [14].…”

Structures of dimethylammonium metal(III) sulfate hexahydrates (metal = Al, Cr)

“…The Cr and A1 atoms positioned on the symmetry centres are octahedrally coordinated by the Ow atoms of the water molecules. The weighted means of A1--Ow and Cr--Ow bond lengths [1.880 (6) and 1.960 (4) A, respectively] are in accord with the corresponding averaged bonds from related literature data: 1.875 (Schein, Lingafelter & Stewart, 1967), 1.87 (Lipson, 1935), 1.86 (at 170 K;Fletcher & Steeple, 1964), 1.868 (2) (neutron data; , 1.851 (5) and 1.897 (2) (neutron data; Abdeen, Will, Sch/ifer et al, 1981) and 1.89/~ (Cameron, Chute, Owen, Aherne & Linden, 1990) for the A1--Ow bonds and 1.954 (Schein et al, 1967) and1.962 A (Chesick &Doany, 1981) for the Cr--Ow bonds. The mean deviations from 90 ° of Ow---A1--Ow and Ow---Cr--Ow bond angles are 2.2 (2) and 2.5 (4) ° , respectively.…”

“…Trivalent metal cations (most often A13+ and Cr 3+) form not only a variety of double sulfates with univalent alkaline and ammonium cations, but also with univalent guanidinium and 0108-2701/92/020256-03503.00 substituted ammonium cations such as (NH3OH) + and (NH3CH3) + . Also, the corresponding alums have been studied extensively (Haussiihl, 1961;Fletcher & Steeple, 1964;Ledsham & Steeple, 1969;Ledsham, Steeple & Hughes, 1970;Bol'shakova, Zalogina & Selivanova, 1971;Abdeen, Will, Sch~ifer, Kirfel, Bargouth, Recker & Weiss, 1981;Serezhkin, 1984;and references therein).…”

The crystal structure of monomethylammonium perchlorate

“…The two independent N-C distances in the methylammonium ions are 1-54 and 1-51 fi. These values are somewhat larger than the expected normal single-bond lengths between carbon and quaternary nitrogen atoms: 1.49, 1.50 and 1.51 fi in spermine phosphate (Iitaka & Huse, 1965), 1.47 fi in tetramethylammonium perchlorate (McCullough, 1964), 1.47 fit in monomethylammonium chloride (Hughes & Lipscomb, 1946), 1.51 fi in monomethylammonium alum (Fletcher & Steeple, 1964); but the difference seems not to be significant considering their standard deviations.…”

“…Only two of these compounds, namely the tetragonal monomethylammonium chloride (Hughes & Lipscomb, 1946) and the monomethylammonium aluminum sulphate alum in the room temperature cubic phase (Okaya, Ahmed, Pepinsky & Vand, 1957) and in the low temperature orthorhombic phase (Fletcher & Steeple, 1964), have been examined by X-ray methods during the last twenty years.…”