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One of the unusual features of the M n+1 AX n phases ͑where M is a transition metal, A is a group A element, X is carbon or nitrogen, and n =1,2,3. . .͒ is that for a given M-A-X system, only certain values of n are found to occur and there is no systematic behavior between the different systems. Density-functional theory was used to verify the stability of the different phases by comparing their total energy to that of the appropriate competing phases. Five systems ͑Ti-Al-C, Ti-Si-C, Ti-Al-N, Ti-Si-N, and Cr-Al-C͒ were studied for n =1-4. Complete agreement with observed occurrences of these phases was found. Very small energy differences suggest that it may be possible to fabricate Ti 2 SiC, Ti 2 SiN, and Ti 3 AlN 2 as metastable phases. None of the M 5 AX 4 phases were predicted to occur and in all cases the ␣ phases were found to be more energetically favorable than the ␤ phases.

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Three activated carbon filters for point-of-use water treatment were tested in laboratory and field studies for chemical removal and microbiological effects on water. All removed free available chlorine in municipally treated water to below the limit of detection, but removed only about 50 to 70% of the total available chlorine and 4 to 33% of the total organic carbon. Standard plate count bacteria in the effluent increased steadily with time for 3 weeks and remained elevated over the 8-week period of the study. Total coliform bacteria were found to persist and proliferate on the filters for several days after transient contamination of the influent water. Silver-containing activated carbon filters suppressed total coliform but not total bacterial growth. Pseudomonas aeruginosa was recovered from the effluents of all filters at some time during the tests.

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The crystal structure of fl-beryllia has been determined from X-ray powder data collected at 2100 °C The structure is tetragonal with a = 4.75 and c--2.74 A, and contains four molecules of BeO in the unit cell. The density is 2.69 g.cm -3. The structure is related to rutile, TiO~, in that the oxygen arrangement is identical. Beryllium atoms are in tetrahedral sites. The space group is P4~./mnm, and the oxygen and beryllium atoms are in sites 4(f) and 4(g) respectively with xo =0.310 and XBe --0"336.

Space groups and atomic coordinates for the 4H, 6H, 8H, 10H, 15R, and 21R polytypes of diamond are presented. The systematic method used to determine the highest symmetry space group for diamond polytypes is described.

A WCI6 compound, isostructural with UCI6, has been identified by comparing calculated intensities with Debye-Scherrer patterns.During the examination of a commercial~ preparation of supposed WCIs, the material was submitted for X-ray analysis to verify the compound designation. Chemical analyses were inconclusive in determining the W:CI ratio because of difficulties with the reactivity of the compound. With the Debye-Scherrer techniques used, the pattern proved unidentifiable with any other tungsten-chlorine compound previously described (Reick, 1967). Other physical property measurements such as color and boiling point implied that the compound should be WCI6. The Debye-Scherrer pattern, however, did not conform to that of WCI6 described by Ketelaar & Oosterhout (1943).Needle-shaped single crystals of the unknown were examined on a Buerger precession camera. The diffraction symbol proved to be 3m P---with cell constants a0= 10.511+0.003, c0=5. for WC]6. Observed intensities are estimated visually from the DebyeScherrer pattern. The calculated intensities include an absorption correction for ~t=498 cm-1 and a 0-3-mm-dia. spindle. In space group P3ml, hk.l and kh.l are nonequivalent; however, no distinction between these reflections has been made in the assignment of indices.