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M. H. FRANCOMBE AND B. LEWIS703 extent, and may well account for the similarity in the c o parameter.The Pb ion apparently plays an important role in maintaining strong linkages between Nb06 octahedra in the (001) planes. Removal of more than a small amount of Pb from the lattice seems to result in a breakdown of these linkages, and of the co-operative ionic displacements associated with ferroelectricity. I~lolanite is hexagonal, probably P63mc, with a ----5.85, c = 9.29 A, and formula approximately Fp+2V+3V+4(~ The structure, which was refined by c-and a-axis projections, consists of a close-~2.5--1.5--6 ~16" packed hexagonal (ABAC) framework of 16 oxygen anions, with metal ions in some of the interstices. The tetravalent vanadium ions occupy a 6-fold position of octahedral coordination, while the iron and trivalent vanadium ions are distributed among two 2-fold positions (on the 3-fold axes), one of octahedral and one of tetrahedral coordination. Results obtained with different radiations suggest that trivalent vanadium has a strong, but not exclusive, preference for tetrahedral coordination. References

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angle. It seems likely then that the 'double' bonds in the complex are intermediate in length to a normal single and a normal double bond.We feel that we have obtained all of the information on the ring geometry which is possible with the present visual intensity data. We agree with Baenziger et al. (1961) that in this type of problem better data are probably required if significant information on ring geometry is to be obtained. (1961). and [110] at temperatures ranging from --70 to --110 °C. The crystal structure was determined by heavy-atom methods, and refined with the aid of two-dimensional Fourier syntheses. The molecule has the expected form of a trigonal bipyramid, but is slightly distorted in such a way as to allow closer packing.