Optimization and doping of 112 Fe pnictide single crystals

The recent discovery of Ca$_{1-x}$La$_{x}$FeAs$_{2}$, which when doped with Sb has a T$_{c}$ of 43K, has led to an increased interest in Fe pnictides in the 112 structure [1]. We have grown plate-like single crystals of LaFe$_{0.6}$Sb$_{2}$ from a self flux. These form in a tetragonal 112 structure with many Fe vacancies, as measured by single crystal x-ray diffraction. The crystal growths were optimized in two ways. Arc melting elemental Fe granules before use resulted in larger ($\sim$ 1 cm$^{2}$) crystals, and including a rapid cool-down during the growth avoided the formation of a parasitic phase, thus increasing the yield. Doping Ni into the structure resulted in a change in the lattice constants from a = 4.4026 {\AA}, c = 10.0341 {\AA} for undoped LaFe$_{0.6}$Sb$_{2}$ to a = 4.4343 {\AA}, c = 9.8911 {\AA} for LaNiSb$_{2}$. Energy dispersive x-ray spectroscopy showed that Ni replaces Fe and also occupies the vacancies, and at 89\% Ni doping, there are no vacancies in the structure. Due to the many vacancies in undoped LaFe$_{0.6}$Sb$_{2}$, the Sb residing near the vacant sites is strongly anharmonic in character; the electronic structure changes with doping and this is seen in the parameter becoming harmonic. [1] Kudo et al. arXiv:1311.1269 (2013).