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.

No abstract

It is shown that the structures of rubidium chromium alum and of thallium chromium alum belong to the ct class of the alums. Crystals of methylammonium chromium alum grown in the absence of alcohol have the B structure and thus establish that this alum is dimorphous. The unit-cell dimensions of the three alums are: rubidium, 12.30A; thallium, 12.24 A; methylammonium, 12.52 A. Summaries are given of the classifications of the known alums from which it is concluded that the structural class is influenced to some extent by the nature of the trivalent ion.

No abstract