Practical considerations on the determination of the accuracy of the lattice parameters measurements from digital recorded diffractograms.

The influence of magnification and defocus on the precision Delta d/d of lattice parameter measurements have been studied by evaluating the diffraction vector length and the signal-to-noise ratio (SNR) of the diffraction peaks in diffractograms calculated by fast Fourier transformation (FFT) of digitally recorded images. Examples are perfect crystals (Si and SiC) and defective crystals (two- and three-dimensional defects within SiC). It is shown that the precision is independent of the diffuseness of the FFT reflections caused by defocus changes or by two-dimensional defects. For the case of a perfect and/or a defective but unstrained crystal, the precision Delta d/d is linearly dependent on the diffraction vector length. For optimal conditions, the highest precision is 0.002. For a defective crystal containing precipitates (nanocrystals), it was found for both the matrix and the nanocrystals that the precision of the measurement is determined by the contribution of strain value (Delta d/d)(strain) of the defective crystal and accuracy Delta d/d for the perfect matrix crystal.