In situ high energy x-ray synchrotron diffraction study of the synthesis and stoichiometry of LaFeAsO and LaFeAsO[subscript 1-x] F[subscript y]

The reaction path for the synthesis of LaFeAsO and LaFeAsO{sub 1-x}F{sub y} by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100 C - T - 1150 C. Starting with LaAs, Fe{sub 2}O{sub 3}, Fe, and LaF{sub 3} as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500 C the sequential reduction of Fe{sub 2}O{sub 3} and Fe{sub 3}O{sub 4} takes place through the oxidization of LaAs. Below 400 C, Fe{sub 2}O{sub 3} is reduced to Fe{sub 3}O{sub 4} by LaAs and then at 400 C < T < 500 C Fe{sub 3}O{sub 4} is further reduced to Fe. (2) In the temperature interval 500 C < T < 800 C, multiple intermediate reactions take place resulting in the formation of FeAs and La{sub 2}O{sub 3}. (3) The formation of LaFeAsO based phase could be unambiguously resolved above 800 C. For both LaFeAsO and LaFeAsO{sub 1-x}F{sub y}, FeAs is a primary impurity at high temperatures that melts at {approx} 1040 C. Possible reaction pathways and the difference between F-free and F-doped samples are discussed.