DETERMINATION OF THE ANHARMONICITY CONSTANT OF GAAS BY MEANS OF THE BIJVOET RELATION OF THE WEAK (666) REFLECTION

As a result of the influence of anomalous dispersion the weak (hhh) an d (hhhBAR) reflections of the zinc blende structure differ from each o ther. At large scattering vectors this difference, described by the Bi jvoet relation B, depends solely on the size of the anharmonic force c onstant beta. It can be determined by measuring B near the K-absorptio n edge of any constituent. This experiment was performed for the (666) and (666BAR) reflections of GaAs between lambda = 0.90 and 0.97 angst rom using synchrotron radiation. Outside the extended X-ray absorption fine-structure spectroscopy region the integrated intensities decreas e in a different manner with increasing lambda for both reflections me asured at 'umweganregung' free azimuthal positions. Under the assumpti on Of beta(Ga) = -beta(As) and using the measured wavelength dependenc e of B, the anharmonicity constant is evaluated to beta = - 1.75 (0.15 ) x 10(-17) J angstrom-3 which is nearly the same as that for germaniu m.