= Structure factor
{title2=$F(\mathbf Q)=\sum_jf_j(\mathbf Q)e^{i\mathbf Q\cdot\mathbf r_j}$}
The coherent <scattering amplitude> associated with a specified spatial arrangement, obtained by summing the site amplitudes with their relative <wave phases>. Distinguish the <crystal lattice structure factor>, which sums over lattice sites, from the <crystal basis structure factor>, which sums the <chemical atoms> inside a <unit cell>. In a periodic <crystal> the two factors multiply. A basis factor can vanish even at an allowed <reciprocal lattice> vector, causing an extinct reflection.
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