X-ray reflexions from dilute solid solutions

Abstract

The effect on X-ray reflexion of deviations of the atoms from the ideal lattice sites caused by the presence of randomly distributed foreign atoms in a dilute solid solution is investigated quantitatively. The form of the function used to describe these deviations is suggested by the distortions produced in an elastic medium by a number of spherically symmetric point centres of distortion. Hence one is led to two types of effects, exactly analogous to the thermal effects: (i) a weakening of the ordinary interference maxima; (ii) the presence of 'diffuse maxima' associated with the ordinary maxima. The change of lattice constant appears naturally in the analysis. It is used to determine the magnitude of the effects (i) and (ii). By applying the theoretical formulae to the solid solutions Au-Cu, we find that it should be possible to detect (i) experimentally; the thermal effect is secondary and cannot mask the distortion effect. But the effect (ii) mixes with the thermal diffuse maxima and is found to be very much smaller at ordinary temperatures. This conclusion is, however, not regarded as general, especially in view of the anisotropic nature of the thermal effect.