Specifichyphen;heat measurements were made on a series of alloys of the pseudobinary system MgCu2minus;xSixbetween 1.5deg; and 4deg;K. The compositions studied ranged from MgCu2to MgCu1.25Si0.75and included 12 alloys having the MgCu2structure, two having the MgZn2structure and three involving mixtures of phases. The variation of ggr; with electron concentration shows maxima at 1.44 and 1.58 electron/atom. These are identified as marking contact between the Fermi surface and the 311 and 222 Brillouin zone faces, respectively. The peaks in the curve of ggr; vs concentration are more pronounced in the MgCu2minus;xSixalloys than those which were observed in MgCu2minus;xAlxalloys reported earlier, suggesting that the distortion of the Fermi surface is greater in the alloys containing silicon.
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