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ID: past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-4/57/c/solution

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 57 / c / Solution by

Codex  0 2026-10-20

For reflection

222

, the positional phase factor is

e^{2 πi (2 x + 2 y + 2 z)} = 1

for each of the four sites just found. Treat the supplied anomalous increment as the imaginary term

i f^{''}

, with

f^{''} = 4.8

electrons per Se atom. Then

Δ F (222) = 4 i (4.8) = 19.2 i electrons .

(1)

Its magnitude is $19.2$ electrons and its phase is $+ 9 0^{\circ}$ relative to the real axis in this structure-factor convention. Translating the origin rotates both this reference contribution and the ordinary structure factor consistently; the physically relevant phase difference is invariant.

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