The effect of pressure to several hundred kilobars has been measured on the lattice parameters of diamond, graphite, and hexagonal boron nitride. The linear compressibility of diamond is independent of pressure. The a axis of graphite (characteristic distance in the plane) is considerably more compressible than diamond at low pressure but above about 160 kbar is much less compressible than diamond. The c axis of graphite is much more compressible than the a axis. Assuming van der Waals forces between the graphite planes and using the lattice sums of Girifalco and Lad, it is possible to describe the compressibility of graphite to very high pressure from low-pressure data. Hexagonal BN is much like graphite, with both a and c axes somewhat more compressible than the corresponding parameters of graphite.

The room-temperature structure of A12TiOs has been re-examined and is compared with the 600°C structure. This material is isomorphous with pseudobrookite, crystallizing in space group Bbmm with cell dimensions a0= 9-429, b0= 9.636, and co = 3"591 A at room temperature and a0 = 9-481, b0 = 9.738, and c0=3"583 A at 600°C. Our results, as well as refinement of previously published data, indicate that there is complete disorder in the metal sites for this compound. Provided that allowances are made concerning electrostatic interactions, the differences in the structure at the two temperatures appear to support the suggestion that the coordination polyhedra about the metal ions tend towards more regular configurations with increasing temperatures. The importance of cleavage planes, resulting from edge-shared octahedra in the hysteresis of the thermal expansion of the ceramic, is pointed out.

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The effect of pressure up to several hundred kilobars has been measured on the lattice parameters of a number of hep metals and alloys including magnesium and six dilute alloys, cadmium and two alloys, two alloys of silver with indium and two with antimony, and the rare-earth metals holmium, erbium, and thulium. For magnesium and its alloys the x-ray data combined with resistance data give rather clear evi dence for a transition at high pressure from hep to dhep (ABABAB---to ABACABAO ••)• Electrical resistance measurements on pure cadmium indicate a similar event. No transition is noted for the cadmium alloys. All four silver alloys underwent a similar transition at high pressure. The rare earths show a highpressure transition accompanied by an increase in the lattice parameter с with increasing pressure, which may be related to those appearing in the group II metals and the silver alloys.T HE effect of pressure to several hundred kilobars has been measured on the lattice parameters of 17 metals and alloys having the hexagonal-close-packed structure (hep) at 1 atm and 25 °C. The materials studied include magnesium and six dilute alloys of mag nesium, cadmium and two alloys, four alloys of silver, and three rare-earth metals. The magnesium and alloys were obtained from C. S. Smith of Case Institute of Technology and from R. S. Busk of Dow Chemical Company. The cadmium, silver, antimony, and indium were 99.99% material from A. D. Mackay. The cad mium alloys with silver were prepared by melting mixed powders of the two in vacuo in a resistance furnace at 650°C. The alloys were annealed for 21 days at 300°C. The samples were filed and the filings reannealed for 48-72 h at 290°C. The silver alloys were prepared by melting in an argon atmosphere in an induction furnace, annealed in vacuo for 23 days at 550°C. The filings were reannealed for 72 h at 550°C. All alloy samples were checked for weight loss at each stage. The compositions of the alloys, the electron to atom ratios, and the one atmosphere lattice parameters for both pure metals and alloys are summarized in Table I.

Using X-ray diffraction techniques, thermal expansion and compressibility were measured on the orthorhombic compounds HfTiO,, Hf,.,,Tk,.,40,, and ZrTiO, (both quenched and cooled slowly from 1300°C). The thermal expansion of HfTiO, is highly anisotropic; the thermal expansion coefficients along the crystallographic axes are an= +(8.7+0.5) XlO-, "C-', ab= -(5.2+0.5)Xlo-B "C", and a c = +(5.3+0.5)X104 "CY. The thermal expansion of Hf,.,,Tio.,,04 was similar to that of HfTiO, but that of ZrTiO, was markedly less anisotropic. The compressibilities of HfTiO, and ZrTiO4 also differed markedly. All compounds investigated, however, behaved similarly in exhibiting a polymorphic transition to a high-pressure phase having the monoclinic baddeleyite (ZrO,) structure. The polymorphism can be explained qualitatively on the basis of crystal structure.