TWINNING IN LANTHANUM OXYSULFATE. Christer Svensson and Bengt Aurivillius+, Dept. of Inorganic Chemistry 2, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
There has long been some confusion about the structure of the lanthanide oxysulfates, Ln2O2SO4. The intensity weighted reciprocal lattice can easily be mistaken for that of a tetragonal I-centered unit cell. Fahey [1] managed a partial structure solution, based on powder data, in the orthorhombic space group I222. Similarly, a single crystal study [2] in space group Immm indicated disordered sulfate oxygen atoms. In 1991 Kampf [3] succeeded in solving the related structure of grandreefite, Pb2O2SO4, in space group A2/a but concluded that "... it is highly unlikely that any of the lanthanide oxide sulfates have the grandreefite structure".
We have prepared single crystals of La2O2SO4 and refined the structure in space group I2/c to an R-factor of 0.027 based on 1198 observed reflections in a hemisphere of reciprocal space. All atoms are ordered, and the structure is indeed isomorphous with grandreefite.
There were two problems with the La2O2SO4 structure solution:
(1) There is a set of weak superstructure reflections that are not seen at all in powder diffraction. These lower the symmetry to monoclinic. The doubled cell has space group I2/c with dimensions a = 14.352(2), b = 4.288(1), c = 8.391(1) Å and ß = 106.92(2)o. A very similar C2/c cell is reached by the transformation (1 0 1/0 -1 0/0 0 -1) : a = 14.362(2), b = 4.288(1), c = 8.391(1) Å and ß = 107.06(2)o.
(2) The crystals are twinned on (1,0,-2) on either cell such that the twin reflections of, say, the I2/c cell are found at positions expected for the C2/c cell oriented as the I2/c cell. Structurally, the twinning only means different orientations, and slight displacemens, of one half of the sulfate ions.
References:
[1] J.A. Fahey (1976) in C.E. Lundin, Ed., Proceedings of the 12th Rare Earth Research Conference, Vail, Colorado, p. 762-771.
[2] J.M. Haschke (1988) J. Solid State Chem. 73, 71-79.
[3] A.R. Kampf (1991) Amer. Mineral. 76, 278-282.