Optimization of Thermal Processing for Enhancing Superconducting Phase Fractions in Ternary Fullerides via a Drop-Quenching Approach

This study explores an optimized thermal protocol for synthesizing ternary superconducting fullerides, focusing on enhancing the face-centered-cubic (fcc) phase fraction while mitigating the formation of competing thermodynamically stable phases. Unlike previous studies, which primarily investigated annealing conditions, this work introduces a drop-quenching approach—rapid cooling in ice/water baths (-3°C to 1°C)—as a potential means to influence phase formation. The expanded ternary fulleride, K0.3Cs2.7C₆₀, was synthesized through a precursor-based solid-state synthetic route. The impact of two annealing temperatures (430°C and 480°C) and two cooling methods (drop-quenching vs. ambient air cooling) was systematically examined. X-ray powder diffraction confirms that K0.3Cs0.7C₆₀ adopts a cubic structure with fcc symmetry. Magnetization measurements reveal that the material exhibits superconductivity with a critical temperature (Tc) of 29.9 K. Structural characterizations shows that increasing the annealing temperature from 430°C to 480°C, combined with intermittent regrinding and pelletization, improved fcc-phase fractions and lattice expansion. However, the difference between quench cooling and natural cooling in air is found to be minimal. These findings suggest that increasing the annealing temperature to 480°C could be beneficial for the synthesis of expanded ternary fullerides by enhancing the superconducting fcc-phase fractions and addressing challenges posed by the increased size mismatch of the substituted cations relative to the interstitial sites in the fcc structure.