Role of interstitial “caged” Fe in the superconductivity of FeTe1/2Se1/2

All samples are synthesized through a standard solid state reaction route and are quenched to room temperature systematically at 700 °C, 500 °C, 300 °C, and room temperature (RT); the samples are denoted 700Q, 500Q, 300Q, and RTQ, respectively. The structural, and magnetic properties are studied. Careful Rietveld analysis of the X-ray diffraction patterns revealed that all samples except 700Q crystallized in a single phase with space group P4/nmm; the amount of interstitial Fe (Feint) at the 2 c site increased from 5% for RTQ to 8% for 500Q. Sample 700Q crystallized in the Fe7Se8 phase. The magnetization result revealed that RTQ and 300Q are superconducting at 10 K and 13 K, respectively, while 500Q and 700Q are not superconducting. Magnetic ordering ( T mag ) was observed at around 125 K for all the samples. The prominence of T mag in terms of effective moment is sufficiently higher for 500Q and 700Q than for RTQ and 300Q. Summarily, it is found that quenching-induced disorder affects the occupancy of interstitial Fe in FeTe1/2Se1/2 and thus both its superconducting and magnetic properties. Further, it clear that limited presence of interstitial Fe at 2 c site is not completely contrary to the observation of superconductivity, because the 300Q sample possesses higher T c (13 K) for higher Feint (6%) than the RTQ sample with relatively lower T c (10 K) having lower Feint (5%). Further, the 500Q sample, with much higher Feint (8%), is non-superconducting.