X‐Ray Peak Broadening as a Result of Twin Formation in Some Oxides Derived by Dehydration

X‐ray diffraction peaks recorded from hematite derived from goethite by a dehydration reaction and its isostructural system corundum from diaspore exhibit systematic peak broadening for l ≠ 3n, which corresponds to the reflections belonging to either the obverse or reverse variant of hematite. The peak width of l = 3n lines, which are common to both twin components remains as sharp as that of goethite. Estimation of the size of the scatterer from half peak widths shows that the product of the topotactic transformation from an individual particle behaves like a single crystal to common reflections and like an agglomerate of hematite crystallite pieces to non‐common reflections. This implies a markedly good coherence at the twin boundaries. A decrease in peak width of non‐common reflections with dehydration temperature is observed. It is due to an increase of crystallite size in the grain growth process by surface diffusion and coalescence following transformation. The difference in peak broadening is interpreted in terms of the fact that the structure of the scatterer is the same, i.e. the final reaction product hematite, but the shape factor is different for common and non‐common reflections and after dehydration at different temperatures. The different sensitivity of X‐rays and electrons to the incoherency at twin boundaries is also considered in relation to their wavelengths.