Influence of Structural Effects on the Superconductivity of Ammonia-Doped NaK2C60.

The role of structural effects on the anomalous superconducting properties of (NH3)xNaK2C60 fullerides was investigated using 23Na and 2H NMR. In the metallic (NH3)xNaK2C60 (0.5 < x < 1) compounds, the 23Na quadrupolar splitting is observed to be independent from ammonia concentration x which, on the other hand, substantially affects the superconducting transition temperature. The marginal influence of sodium cation displacement in the superconducting properties is confirmed also by the absence of any electric field gradients in the recently synthesized nonmagnetic insulator (NH3)2NaK2C60. 2H NMR measurements on deuterated samples enabled us to study the ND3 reorientational dynamics, indicating a hampered ammonia rotation in the insulating (x = 2) case, where the Na+ ions become centered. [ABSTRACT FROM AUTHOR]