and therefore lower Tc values generally, and this is indeed found experimentally [15.30-32], Also because of their smaller size, multiple alkaline earth metal ions can enter the larger octahedral sites for the case of the fee structure. In fact, the stoichiometrics that exhibit superconductivity from magnetic susceptibility and microwave measurements are Ca5C60 [15.30], Ba6C60 [15.31], and Sr6C60 [15.33], and neither Ba6C60 nor S^Qo exhibits the fee structure. Shown in Fig. 15.4 are measurements of the magnetization vs. temperature for Ca5C60 in a field-cooled (FC) and zero-field-cooled (ZFC) situation, with the intersection between the two curves yielding the Tc value.
Two aspects of the superconducting alkaline earth compounds bear special attention. The first relates to the difference in crystal structures relative to the alkali metal compounds and the greater variety of crystal structures
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