Zn this paper we briefly show the results of joined studies of the free electron concentration and X-ray diffuse scattering (detected by HR XRD - reciprocal space maps) as a function of Te atoms concentration in GaAs and controlled annealing of samples at high temperatures 700-1200 degrees C. The free electron concentration can be reversible changed between low and high values merely by a proper annealing of GaAs samples with Te concentration higher than similar to 4 x 10(18) cm(-3). Strong X-ray diffuse scattering is observed in annealed samples, if electron concentration is lowered. Detailed features of X-ray scattering in the reciprocal space can be very well understood within Krivoglaz microscopic model of scattering due to local correlations of impurity positions in a solid solution (local order) and related crystal lattice distortions.