Encyclopedia of Crystallographic Prototypes

α-Alum [KAl(SO$_{4}$)$_{2}$ $\cdot$ 12H$_{2}$O, $H4_{13}$] Structure: AB24CD28E2_cP224_205_a_4d_b_2c4d_c-001

• The alums have the general formula AB(XO$_{4}$)$_{2}$ · 12H$_{2}$O, where A is a monovalent ion, B is a trivalent ion, and X is a chalcogen. In most cases atom B is aluminum and atom X is sulfur, leading to the name alum.
• All alums have their room-temperature form in space group $Pa\overline{3}$ #205, but the bonding between the A and B ions and the XO$_{4}$ complex can be quite different.
• (Lipson, 1935ab) described three general forms of alum based on the sizes of the monovalent ions. Each of these forms was given a Strukturbericht designation by (Gottfried, 1937):
  • $\alpha$-alum, with intermediate sized ions, prototype KAl(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{13}$ (this structure),
  • $\beta$-alum, with large ions, prototype (NH$_{3}$CH$_{3}$)Al(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{14}$, and
  • $\gamma$-alum, with small ions, prototype NaAl(SO$_{4}$)$_{2}$ · 12H$_{2}$O, $H4_{15}$.
• This classification scheme is not compete, e.g., (Ledsham, 1968) points out that NaCr(SO$_{4}$)$_{2}$ · 12H$_{2}$O does not fit into any of these categories, and that the actual structure depends on the combination of monovalent and trivalent ions.
• As noted above, the $Pa\overline{3}$ structures of alum are the room temperature form. As the temperature decreases the alum structure may transform. For example, in the temperature range 150-170K the $\beta$-alum (NH$_{3}$CH$_{3}$)Al(SO$_{4}$)$_{2}$ · 12H$_{2}$O transforms into an orthorhombic structure with fully ordered NH$_{3}$CH$_{3}$ ions.
• (Ewald, 1931) designated the $\alpha$-alum structure determined by (Cook, 1927) as $H4_{2}$, but when the superior structural determination of (Beevers, 1934) appeared this type was abandoned, and the new structure was given the designation $H4_{13}$. (Nyburg, 2000) were able to determine the positions of the hydrogen atoms, so we use their improved structure as our prototype.
• The oxygen atoms around the sulfur are statistically distributed in two ways: the probability of the O-I and O-III sites being occupied is 78.7%, while the probability of the O-II/O-IV combination being occupied is 21.3%.

Simple Cubic primitive vectors

References

• S. C. Nyburg, J. W. Steed, S. Aleksovska, and V. M. Petrusevski, Structure of the alums. I. On the sulfate group disorder in the α-alums, Acta Crystallogr. Sect. B 56, 204–209 (2000), doi:10.1107/S0108768199014846.
• P. P. Ewald and C. Hermann, eds., Strukturbericht 1913-1928 (Akademische Verlagsgesellschaft M. B. H., Leipzig, 1931).
• J. M. Cork, Crystal structure of certain of the alums, Phil. Mag. 4, 688–698 (1927).
• C. Gottfried and F. Schossberger, eds., Strukturbericht Band III 1933-1935 (Akademische Verlagsgesellschaft M. B. H., Leipzig, 1937).
• C. A. Beevers and H. Lipson, Crystal Structure of the Alums, Nature 134, 327 (1934), doi:10.1038/134327a0.
• H. Lipson, The Relation between the Alum Structures, Proc. Roy. Soc. London 151, 347–356 (1935), doi:10.1098/rspa.1935.0154.
• A. H. C. Ledsham and H. Steeple, The crystal structure of sodium chromium alum and caesium chromium alum}, Acta Crystallogr. Sect. B 24, 1287–1289 (1968), doi:10.1107/S0567740868004188.
\bibitem{Fletcher_Acta_Cryst_17_290_1964.bibR. O. W. Fletcher and H. Steeple, The crystal structure of the low-temperature phase of methylammonium alum}, Acta Cryst. 17, 290–294 (1964), doi:10.1107/S0365110X64000706.\bibAnnoteFile{Fletcher_Acta_Cryst_17_290_1964.bib

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