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It is a pleasure to thank Dr M. B. Wilk for valuable discussions of the standard deviation in the integrated intensity, Mrs G. Hansen for assistance with the FAP section of PEXRAD INPUT, Mr L. J. Cirincione for writing the IBM 1011 paper tape to magnetic tape program, and Mr J. L. Bernstein for running many calculations with these programs on the IBM 7090.Note added in proof.--The K-values of N¢ are no longer necessarily used in calculating I(hkl) and a~ (I(hkl)). Now, the program integrates from the first ten 'good' background points on one side of the principal maximum to the first ten 'good' background points on the other side of the same maximum. If this criterion is not met, the program then uses all K points. The structure of a-Ga2S 3 has been determined from single-crystal and powder diffraction data. The unit cell is monoclinic, space group Cc, a = 11.140, b = 6.411, c = 7.03 s/~, fl = 121.22 °, and Z = 4. The structure, which is based on wurtzite with 4 cation vacancies per cell, differs from that proposed by Hahn & Frank mainly in the disposition of the vacant sites. Shrinkage of sulphur tetrahedra around these sites leads to some distortion from ideal close packing.

The crystal structure of AIESe3 has been determined from X-ray powder diffraction data. The unit cell is monoclinic, being centred on the C face, with parameters a = 11.680, b = 6"733, c = 7"329 -~ and fl = 121.1 o z • The space group is Cc and the structure is similar to that of e-Ga2S3.

The crystal structure of 0c-CdP2 is orthorhombic with a=9"90, b=5"40s and c=5"171/~. The space group is Pna21 with four molecules in the unit cell. The phosphorus atoms are closely bonded in chains parallel to the c axis, the chains being linked together by tetrahedrally coordinated cadmium atoms. The average P-P bond length (2.22/~) in a chain is much less than that (4.14/~) in a Cd-P tetrahedron.