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The crystal structure of XeF3+SbF6' has been accurately determined from three-dimensional X-ray counter data. Crystals are monoclinic with a = 5.394 (1) A, b = 15.559 (2) A, c = 8.782 (1) A, ß = 103.10 (1)°, V= 717.84 A3, Z = 4, and dc = 3.92 g cm-3. The structure has been refined in space group P2jn to a final conventional R factor of 0.048 for 1264 independent reflections with / > 3 ( ). The structure consists of XeF3+SbF6' units with a close contact of 2.485 (10) A between the Xe atom of the T-shaped cation and an F atom of the octahedral anion. The bridging F atom is coplanar with the cation with a bridge angle of 140.8 (4)°.

Neutron diffraction has been used to check the structure of polycrystalline orthorhombic lead(II) fluoride. The X-ray work of Bystr6m has been confirmed, and more accurate interatomic distances have been found.From an X-ray powder investigation Bystr6m (1947) found that orthorhombic lead(II) fuoride has the lead(lI) chloride structure. As the fluorine positions were derived from packing considerations only, some doubt has been expressed recently with regard to the correctness of this determination (van den Berg, 1964). Because of this, a neutron powder study was undertaken, the neutron scattering lengths of 0.96 and 0.55 x 10 -12 cm for lead and fluorine, respectively, being very favourable for a direct localization of fluorine.A polycrystalline sample was prepared by slowly adding PbO to concentrated hydrofluoric acid (39 % by weight) and drying the residue at 80-100°C in an inert atmosphere. An X-ray investigation showed the sample to be free from contamination by the cubic form of PbF2.Three different neutron diffraction patterns were obtained, all of which were used independently for refining the structure. For all three patterns monochromatic radiation was obtained from the 111 reflexion of copper; the other experimental details have been collected in Table 1.With the space group Pnma, the cell constants are: a= 6.440, b=3.899, c=7.651/~ (Sauka, 1951).The structure was refined by means of a full-matrix leastsquares program described by Rietveld (1966), taking the Table 1. Experimental details cq is the angular divergence of the collimator between the reactor and the monochromator, ~3 that of the collin-ator in front of the BF3 detector; it is the wavelength used and (sin 0)/), the range of observation. Patternoq cq 2 (sin0)/2 A 10" 10"

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