Role of interstitial "caged" Fe in the superconductivity of FeTe₁₂Se₁₂

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 (Fe_int) at the 2c site increased from 5% for RTQ to 8% for 500Q. Sample 700Q crystallized in the Fe₇Se ₈ 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 (^Tmag) was observed at around 125 K for all the samples. The prominence of ^Tmag 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 FeTe₁₂Se₁₂ and thus both its superconducting and magnetic properties. Further, it clear that limited presence of interstitial Fe at 2c site is not completely contrary to the observation of superconductivity, because the 300Q sample possesses higher ^Tc (13 K) for higher Fe_int (6%) than the RTQ sample with relatively lower ^Tc (10 K) having lower Fe_int (5%). Further, the 500Q sample, with much higher Fe_int (8%), is non-superconducting.