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I~C16 crystallizes in complexly twinned triclinic crystals; space group Pi, one IuCI 6 per unit cell. The crystal structure was determined by the use of three-dimensional Patterson and Fourier syntheses. The final value of the disagreement factor R was 0-12. The bond lengths are subject to an uncertainty of 0.02 /~ and in addition to this to a standard deviation of 0.01 A. The structure consists of planar molecules with negligible deviations from the point group symmetr, y mmm. Each iodine atom is surrounded by two chlorine atoms at a distance of 2.38 and 2.39 A and by two others at 2.72 and 2.68 A.. A discussion of the twhanlng and of the atomic distances is given.

The crystal structure of CsI a has been determined by X-ray analysis. Space group: P21/a; 4CsI 4 per unit cell. The signs of 60 out of the 71 observed hOl structure factors could be calculated directly from inequalities. Inequalities were not useful in the [001] projection, but a first approximation could be obtained from Buerger's minimum function. The coordinates were refined by Fourier syntheses of the three projections and corrected for the termination of the series. The final value of the disagreement factor R is 0.085, and the standard deviation in the atomic distances is 0-02-0.03 A.The structure consists of Cs + ions and flat [I~. 12. I~-] ions, the latter showing a weak interaction between their constituents.

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