Problem: Structure of A C

       

Fleming and co-workers describe the structure of various alkaline metal-C compounds in Fleming et al., Nature, 352, 701 (1991). Read the paper and answer the following questions:

(a) In Figure 2a of the paper an fcc structure of A C is indicated. In the usual representation of the fcc structure the C molecules would be placed on the corners, e.g. at positions , and the face centers of a unit cube. The Figure certainly does not look like a cube. What is a possible choice for the ``conventional" (x,y,z) coordinates of the C molecule in the middle of Figure 2a from this paper?

(b) Assume you perform a powder X-ray diffraction measurement on a Rb doped C material with Å X-rays. You want to compare your results to Fleming's to see what stoichiometry your compound is. What are the positions , in degrees) of the first five diffraction peaks for the three observed structures (doping = 3, 4, and 6 in Table 2 of the paper)?