(a) DTA curve, heating and cooling, of composition A: Fe and composition B: . [(b) and (c)] Heating curves only for compositions A (upper curve) and B (lower curve).
(a) Typical x-ray diffraction image recorded on the CCD detector for composition A at . (b) 1D plot of angle-integrated intensity vs for composition A at . (c) 1D plot of angle-integrated intensity vs for composition B at .
Surface plot of the x-ray diffraction patterns as a function of temperature for (a) composition A and (b) composition B. Three different temperature ranges discussed in text are labeled as I, II, and III, respectively. At the lowest temperature, phases observed in the two theta range are labeled as ◻ for , ○ for , and # for LaAs. At , hexagonal (◆) in (a) and LaOF (▼) in (b) are observed besides LaFeAsO.
Temperature dependence of diffraction peak intensity of all observed phases for (a) composition A: LaFeAsO and (b) the F-doped composition B. Three temperature intervals as described in the text are denoted with vertical dashed lines.
Selected diffraction patterns of sample A in the temperature interval highlighting the change of to and the development of diffuse scattering of cubic . Reflections with boxed indices belong to , while peaks with indices in parentheses belong only to starting . The dashed area highlights the increase in the background close to 2.2° where the (222) peak of cubic emerges at higher temperatures. The diffraction patterns were shifted vertically for clarity. The weak reflection labeled with a solid triangle is from from the capillary. Reflections from sample B show the same temperature dependence.
Selected diffraction patterns highlighting the formation of cubic and hexagonal in (a) composition A: LaFeAsO and LaOF in (b) composition B: F-doped LaFeAsO. The patterns are shifted vertically for clarity. Characteristic peaks shown are (100) of hexagonal , (222) of cubic , and (101) of LaOF. The red curves highlight the coexistence of cubic , hexagonal , and in F-doped composition B, LaOF.
GSAS fitting of the diffraction patterns of compositions A (a) and B (b) taken at . Liquid FeAs impurity is also present but its x-ray diffraction is too diffuse to be observable.
Ternary cut through the La–Fe–As–O quaternary phase diagram showing the solid solution , the two phase region (a), the two phase region (b), and the three phase region . The remainder of the cut consists of four-phase regions containing at least one phase not in the plane of the cut.
Phase fractions for starting composition A with no F, LaFeAsO. The values of the observed weight percentage of the detected phases based on Rietveld refinement of the diffraction pattern compared to the calculated phase fractions obtained by least squared fitting the phase fractions by varying for . The best fit to the relative phase fractions is for .
Phase fractions for starting composition B with F, . The values of the observed weight percentage of the detected phases based on Rietveld refinement of the diffraction pattern compared to the calculated phase fraction obtained by fitting the phase fractions by varying and for . The best fit value is and . Given the small value of , the data were also fit forcing which yielded .
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