The crystal structure of hexagonal barium titanate has b~en determined by X-ray methods. The unit cell, which contains six units of BaTiOs, ha: %= 5.735 A. and co= 14.05 A. In the space group C6Jmmc all the ions occur at special positions: 2 Ba at (b), 4 Ba at (f), 2 Ti at (a), 4 Ti at (f), 6 O at (h), and 12 O at (k). The structure is built up of six close-packed layers of Ba and O ions, each layer consisting of one Ba to three O ions, having the packing sequence ABCACB. The Ti ions are located in the oxygen octahvdral holes between the layers. The structure is remarkable in that two-thirds of the Tie e octahedra occur in pairs which share a face to form Ti209 co-ordination groups. The compensating distortion which occurs in the Ti209 groups increases the Ti-Ti distance by 0.33 A. while the O-0 distance in the shared face is decreased by 0.38 A. The existence of Ti809 groups in hexagonal barium titanate, an unusually stable substance under these circumstances, constitutes a notable exception to certain of Pauling's rules for complex ionic structures.

A technique has been developed for handling the extremely reactive interhalogen compounds which makes it possible to investigate their structures by x-ray diffraction at low temperatures. As the first step in a program of study of the interhalogen group the crystal structure of chlorine trifluoride has been determined at −120°C. The chlorine trifluoride molecule is planar with the point group symmetry mm. The Cl atom is bonded to one F atom at 1.621A and to two F atoms at 1.716A. The F–Cl–F bond angle is 86°59′. In the succeeding pages of this journal, a parallel investigation of the vapor phase by microwave spectroscopy is presented by Dr. D. F. Smith. The molecular configuration is identical in the two cases.

To maintain intensity measurements on a fixed scale an identical and constant spectral distribution must be integrated in either the (o or 20 scan. Both the vertical and horizontal dimensions of the detector aperture are critical for either scan. With the mechanical arrangements currently in use for upper level o~ scans both aperttwe dimensions are dependent on the mosaic spread of the crystal as well as the spectral dispersion. The optimum type of scan for a given set of conditions is that which yields the true integrated intensity plus the least amount of thermal diffuse scattering. This will be the scan which defines the smallest illuminated volume in reciprocal space. When other factors, such as anomalous dispersion, are equal, a long characteristic wavelength is desirable in that it will give rise to a smaller illuminated volume at a given point, in reciprocal space than will a short wavelength.

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