A method for the selective synthesis of sulfide clusterfullerenes Dy₂S@C_2n is developed. Addition of methane to the reactive atmosphere reduces the formation of empty fullerenes in the arc-discharge synthesis, whereas the use of Dy₂S₃ as a source of metal and sulfur affords sulfide clusterfullerenes as the main fullerene products along with smaller amounts of carbide clusterfullerenes. Two isomers of Dy₂S@C₈₂ with C_s(6) and C_3v(8) cage symmetry, Dy₂S@C₇₂-C_s(10528), and a carbide clusterfullerene Dy₂C₂@C₈₂-C_s(6) were isolated. The molecular structure of both Dy₂S@C₈₂ isomers was elucidated by single-crystal X-ray diffraction. SQUID magnetometry demonstrates that all of these clusterfullerenes exhibit hysteresis of magnetization, with Dy₂S@C₈₂-C_3v(8) being the strongest single molecule magnet in the series. DC- and AC-susceptibility measurements were used to determine magnetization relaxation times in the temperature range from 1.6 K to 70 K. Unprecedented magnetization relaxation dynamics with three consequent Orbach processes and energy barriers of 10.5, 48, and 1232 K are determined for Dy₂S@C₈₂-C_3v(8). Dy₂S@C₈₂-C_s(6) exhibits faster relaxation of magnetization with two barriers of 15.2 and 523 K. Ab initio calculations were used to interpret experimental data and compare the Dy-sulfide clusterfullerenes to other Dy-clusterfullerenes. The smallest and largest barriers are ascribed to the exchange/dipolar barrier and relaxation via crystal-field states, respectively, whereas an intermediate energy barrier of 48 K in Dy₂S@C₈₂-C_3v(8) is assigned to the local phonon mode, corresponding to the librational motion of the Dy₂S cluster inside the carbon cage.

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