The crystal structure of sodium chromium alum and caesium chromium alum

Abstract

1287 similarity probably no conclusions can be made with regard to the angles of twist existing in solution.The spectral change going from the planar biphenyl (253 mp) and non-planar o-diphenylbenzene (237 mp) (Dale, 1957) to C6Ph6 is rather surprising.All the calculations were performed on an Elliott 803B computer using the programming system devised by Daly, Stephens & Wheatley (1967 (Received 5 December 1967) Sodium chromium alum with a= 12.40/~ and caesium chromium alum with a= 12-50/~ both have space group Pa3. Refinement of the structure of the sodium alum has resulted in a residual of 0.165 but no attempt has been made to refine the structure of the caesium alum. Sodium chromium alum, which has the ~ structure, does not fit into the classification scheme of Lipson, whereas caesium chromium alum, with the fl structure does. In view of earlier work by Ledsham & Steeple it is concluded that the class of an alum structure is governed by the particular combination of monovalent and trivalent ions that is involved, and that there is no infallible rule for the pre-determination of a given structure.

In Search of the True Structure of the Sodium Chromium Alum: Crystal Growth and Structure of the Double Salt NaCr(SO4)2(H2O)6

“…The existence of a compound of this composition has been postulated from the results of thermodynamic investigations [12]. The powder diffractogram of our monoclinic crystals as well as a theoretical diffractogram calculated from the single-crystal structure both match the experimental PDF-2 pattern which -erroneously 6 . The scale covers a distance of 2.5 mm.…”

“…1) which turned out to belong to the monoclinic crystal system. A structure analysis showed them to consist of the title compound, NaCr(SO 4 ) 2 (H 2 O) 6 , with a composition very similar to that of the expected alum but containing only 6 water molecules per formula unit, instead of 12. The existence of a compound of this composition has been postulated from the results of thermodynamic investigations [12].…”

“…One of the numerous LPAs retrieved affects the cubic lattice parameter of the alkali chromium alum series ACr(SO 4 ) 2 (H 2 O) 12 (A = Na, K, Rb, Cs) ( Table 1). The published single-crystal value for a(Na-Cr alum) [6] is by 0.17Å larger than that of a(K-Cr alum) [7]. (Note: in this work the term "alum" is used exclusively for the dodecahydrates of the corresponding double salts).…”

“…A closer look at the different A-Cr alum structures in the ICSD [3b] revealed that the structure model of NaCr(SO 4 ) 2 (H 2 O) 12 [6], obtained from 2D Weissenberg data, shows, besides the large lattice parameter comparable to the one of the Cs-Cr alum (Table 1), some shortcomings (according to modern standards) like high R value (0.165), missing standard deviations and severely distorted octahedral coordination for the Cr hexaquo complex (O-Cr-O (cis) = 78 and 102 • ). Thus, before trying to explain the lattice parameter anomaly, we decided to first re-determine the structure of the Na-Cr alum which meant, in the first instance, to grow crystals of the compound.…”

“…No information regarding the synthesis and crystal growth is given for the single-crystal structure analysis of the Na-Cr alum [6]. In the powder case the standard procedure for the synthesis of chromium alums had been used: reduction of A 2 CrO 4 in sulfuric acid, usually by ethanol, in this case by SO 2 at a temperature below 5 • C, and with A = Na [17].…”