REINVESTIGATION OF THE MIXED-METAL PERIODATES M'MIO6 (M' = ALKALI-METAL, M = GE, SN, PB)

The periodate compounds M'MIO6 (M'=K, Rb, NH4, Cs, sometimes Na; M=Ge, Sn, Pb) have been studied by vibrational spectroscopy, X-ray powder diffraction and EXAFS. The structures are based upon an h.c.p. array of oxide ions with one third of the octahedral holes in every second layer filled with M', and in the other layers one third of the holes are filled with I and one third with M. The Ge compounds adopt the space group P312, but the tin and lead compounds belong to P6(3)22, corresponding to a doubling of the axis perpendicular to the layers as a result of alternate stacking of IO6 and MO6 octahedra. The M-O, M'-O and I-O distances have been determined by a combination of X-ray powder diffraction and EXAFS and the results are discussed.