\textit{In situ} X-ray Synchrotron Diffraction Study of the Synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$

The reaction path for the synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$ by nominally solid state reaction was studied by \textit{in situ} x-ray synchrotron diffraction technique and Differential Thermal Analysis (DTA) in the temperature interval 100\r{ }C $\le $ T $\le $ 1150\r{ }C. Starting materials were LaAs, Fe$_{2}$O$_{3}$, Fe and for the F containing materials LaF3. The results show that the synthesis is characterized by three temperature intervals: (1) below 400\r{ }C, Fe$_{2}$O$_{3}$ gradually transforms to Fe$_{3}$O$_{4}$. (2) In the temperature interval 400\r{ }C $<$ T $<$ 800\r{ }C, multiple intermediate reactions take place resulting in the formation of La$_{2}$O$_{3}$ and Fe --As compounds. (3) above 800\r{ }C, reaction leads to the formation of LaFeAsO. Possible reaction paths and the difference between F-free and F-doped samples will be discussed in the talk.