No 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.

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.

No abstract