Crystal-structure investigations of LaF 3 crystals twinned by merohedry have been carried out with X-ray and neutron diffraction data. The twin law can be described by any one of the additional symmetry elements of point group 6/mmm compared to 3ml. The superimposed intensities were used for structure refinements. Structure parameters as well as the volume ratios for the two twin domains were determined. These structure refinements resulted in R (I) = 0.092 for 666 neutron data and R(I) = 0.059 for 964 X-ray data for the superimposed and observed [I-> 3o'(I)] intensities. Twin-volume ratios ranged from 1:1 (crystal Q 2801~m)to 1.7:1 (Q 85 ixm). The single-crystal domains have a centre of symmetry and crystallize in space group P3cl. Similar results were found for PrF3. Inspection of earlier structure investigations of LaF3 crystals in the light of our results makes it very probable that these investigations have been carried out with twinned crystals with different twin-volume ratios. Furthermore, it is shown that a recently published structure refinement of LaF3 based on a hexagonal structure model is incorrect because the diffraction data were measured from a LaF3 twin with balanced volume ratio.

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