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Generalized structure factor expressions are derived for the zinc blende structure. Inclusion of dispersion corrections is shown to give rise to the breakdown of Friedel's law for all-even-index reflections, an effect due to the presence of bonding or anharmonicity. X-ray intensity measurements from an extended-face crystal are interpreted within the generalized structure factor formalism to yield the B factors BZn = 1.020 ± 0.005 Å2 and BSe = 0.739 ± 0.008 Å2 and an effective cubic anharmonic thermal parameter ZnSe = βZn/α3Zn - βSe/α3Se = --(6.8 ± 1.1) × 1036 J-2 Å3 at 296 K. These are compared with values for other zinc blende compounds and with theoretically calculated B factors.