Synthesis and structural analysis of nonstoichiometric ternary fulleride $K_{1.5}Ba_{0.25}CsC_{60}$

The existence of cation-vacancy sites in fullerides might lead to long-range ordering and generate a new vacancy-ordered superstructure. The purpose of this work is to search whether or not long-range ordering of vacant tetrahedral sites, namely superstructure emerges in nonstoichiometric$K_{1.5}Ba_{0.25}CsC_{60}$ fulleride. Therefore, $K_{1.5}Ba_{0.25}CsC_{60}$ with cation-vacancy sites is synthesized using a precursor method to avoid inadequate stoichiometry control and formation of impurity phases within the target composition. For this purpose, first, phase-pure $K_6C_{60}, Ba_6C_{60} and Cs_6C_{60}$ precursors are synthesized. Stoichiometric quantities of these precursors are used for further reaction with C60 to afford $K_{1.5}Ba_{0.25}CsC_{60}$. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data of the precursors and $K_{1.5}Ba_{0.25}CsC_{60}$ confirms that $K_6C_{60}, Ba_6C_{60} and Cs_6$ C60$ are single-phase and they crystallize in a body-centeredcubic structure (Im3‒) as reported in the literature. The analysis also shows that $K_{1.5}Ba_{0.25}CsC_{60}$phase can be perfectly modeled using a face-centered cubic structure. No new peaks appear which could have implied the appearance of a superstructure. This suggests that there is no long-range ordered arrangement of vacant tetrahedral sites in $K_{1.5}Ba_{0.25}CsC_{60}$

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