Missing X-Ray Peaks: Structure Factor | Crystallography 2.7

Bragg's law promises a copper peak at 35°, and iron should show one near 31°, yet the detector records nothing. This episode turns each atom in the cell into an arrow, adds them up to get the structure factor, and shows why body-centered cells cancel when h + k + l is odd and face-centered cells cancel when the indices are mixed. It reads simple, body-centered and face-centered lattices from simulated powder patterns, explains why silicon's (002) disappears, and returns to the 1913 puzzle of why potassium chloride looked like a simple cube. Chapters 0:00 Where did the (110) peak go? 1:14 Bragg, 1912 to 1922 1:45 Bragg's two salts 2:48 Iron's missing (100) peak 3:40 Adding up arrows: the structure factor 5:05 How strongly an atom scatters 5:39 Which peaks vanish: SC, BCC and FCC 7:26 A dark point on the Ewald sphere 8:04 Three simulated powder patterns 9:38 Silicon: a second atom erases (002) 11:23 Rock salt and potassium chloride 12:43 What decides a peak Reference: M. Martínez-Ripoll (with the late F. Hernández Cano), Crystallography-Cristalografia, web course (CSIC, CC BY-NC 4.0, https://www.xtal.iqf.csic.es/Cristalo..., and university crystallography and diffraction lecture notes. Historical sources: W. L. Bragg, Nobel lecture (1922): https://www.nobelprize.org/uploads/20... · W. L. Bragg, The structure of some crystals as indicated by their diffraction of X-rays, Proc. R. Soc. A 89, 248 (1913): https://doi.org/10.1098/rspa.1913.0083 · W. H. Bragg and W. L. Bragg, The reflection of X-rays by crystals, Proc. R. Soc. A 88, 428 (1913): https://doi.org/10.1098/rspa.1913.0040 Previous episode: 2.6 "Bragg and Ewald". Next episode: 2.8 "How Bright Is a Peak". Narration: AI voice (ElevenLabs); music: AI-generated (Lyria) and original synthesized tracks.