Low Porosity FeSe Preferred Orientation Crystal Growth by Bridgman Method

The new kind of iron-based superconductor FeSe which has being widely studied recently is arsenic-free and has about 8 K of Tc. The less poisonous and simple binary FeSe is interesting for both scientific studies and engineering applications, even though its Tc is lower than that of other iron-based compounds. Recently, the single crystal and thin film technology of FeSe has been studied to realize the anisotropic property. In our study, the highly preferred orientation poly-crystal was grown by the Bridgman method. The vacuum condition during growth affects the porosity of FeSe. It is difficult to produce high density under 1 atmosphere even when maintained at high temperature for a long time. When the pressure is controlled at less than
$10^{-1}\ {\rm torr}$
, ultra low porosity (and high density) crystal can be grown. Polarized optical microscope images show the single crystal domain up to about 5 mm with (110) preferred orientation by XRD analysis. A XRD peak shift is observed between samples grown by furnace-cooling and the Bridgman method, which is supposed to be due to residual stress during growth. It also induced a Tc decrease to 5.9 K in the Bridgman sample, but recovered to 6.8 K after annealing. From the microstructure, considerable columnar precipitation is observed in the Bridgman sample, which is identified as iron-rich phase by energy dispersive spectrum (EDS) analysis.