Convective solution transport - An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent

An improved technique of convective solution transport using KCl as solvent at 840-790° C (where mass transport takes place across a vertical temperature gradient) is described for the growth of crystals of the recently discovered superconductor -FeSe _x (x 1-0.8). The crystals were annealed in situ at 400-350° C for 20-30 h to improve the superconducting properties. Hexagonal plate like crystals measuring 5-6 mm across and 0.25-0.5 mm thick were obtained. High resolution transmission electron microscopy (HRTEM) measurements show good crystallinity and the energy dispersive x-ray analysis (EDX) gives a composition very close to the starting powders. The zero resistance temperature of the crystals was found to increase from 6.5 to 9 K as the composition is decreased from x 0.95 to 0.9 and decrease thereafter. Similar behavior was also observed in the powder x-ray diffraction (XRD) patterns and Raman spectra with the main peak shifting to higher value until 0.9 and decrease thereafter. In addition the XRD patterns show reducing hexagonal phase reflections as x decreases to 0.9. Anisotropic magnetic behavior was observed when the magnetic field is applied parallel and perpendicular to the (101) face.