Reaction Kinetics of the Formation of SmFeAsO$_{1-x}$F$_x$ High \textit{T}$_{\rm c}$ Superconductor Probed by High Energy X-ray Diffraction

The recent discoveries of superconductivity with $T_{\rm c}$ up to 55 K in a new class of compounds $R$FeAsO$_{1-x}$F$_x$ ($R$ = rare earth), have attracted much attention. It is difficult to synthesize single phase high quality samples that are needed to study the intrinsic properties of these materials. The samples reported are made at high temperatures $T$ $\sim$~1100~$^{\circ}$C and usually contain impurity phases along with the $R$FeAsO$_{1- x}$F$_x$ phase. Information on the kinetics of formation of the superconducting phase would be helpful. Here we report the results of high-energy x-ray diffraction measurements versus temperature, carried out at the Advanced Photon Source. We determined the kinetics of the reaction between the appropriate mixtures of SmAs, ${\rm Fe_2O_3}$, Fe, and ${\rm SmF_3}$ to give nominal compositions of SmFeAsO and SmFeAsO$_{1-x}$F$_x$, respectively. Our results indicate that the SmFeAsO$_{1-x}$F$_x$ phase starts forming at $T~\sim$~750~$^{\circ}$C and its formation ceases by $\sim$~1000~$^{\circ}$C\@. \noindent $^*$Supported by the USDOE under Contract Nos.\ DE- AC02-07CH11358 and DE-AC02-06CH11357.