Anharmonic temperature factors, anomalous‐dispersion effects and bonding charges in gallium arsenide

The introduction of dispersion corrections in the generalized structure-factor expression for the zinc blende structure leads to the breakdown of Friedel's law for all even-index reflections, owing to bonding or anharmonicity in these crystals. A systematic and precise X-ray diffraction intensity measurement has been carried out using a spherical single-crystal of gallium arsenide at 300 K. Least-squares refinement of the experimental data yields the Debye–Waller factors BGa = 0.62 (2), BAs + 0.49 (2) A2 for the harmonic model and BGa = 0.670 (3), BAs = 0.470 (2) A2 and individual anharmonic thermal factors BGa = 3.15 (59) × 10−18 and BAs= 8.92 (182) × 10−18 J A−3 for the anharmonic model. The residual index for the discrepancy between measured and calculated structure factors in the present work is R = 1.237% for the harmonic model and R = 1.229% for the anharmonic model. The present work reveals excellent agreement in both sign and magnitude of the Bijvoet inequalities for a large number of reflections. Covalent charge transfer has also been deduced from the experimental measurements on a few quasi-forbidden Bragg reflections showing a net transfer of charge from Ga to As.