Direct evidence for an order/disorder phase transition at x≅0.3 in single-crystal metastable (GaSb)_(1-x)(Ge₂)_x alloys: High-resolution x-ray diffraction measurements

High-resolution triple-crystal x-ray diffractometry has been used to investigate long-range order in single-crystal (100)-oriented metastable (GaSb)_(1-x)(Ge₂)_x alloys. Measurements of the integrated intensities of superstructure and fundamental reflections provide the first direct evidence of an order/disorder, zinc blende to diamond, structural transition in (III-V)_(1-x)(IV₂) _x alloys. The transition in (GaSb)_(1-x)(Ge₂)_x was found to occur at x≅0.3. The long-range order parameter S and the probability r that a Ga or Sb atom will occupy a site on its corresponding sublattice were determined as a function of composition from the normalized ratio of the integrated intensities of the (200) superstructure to the (400) fundamental reflections. S(x), which is unity for pure zinc blende structure GaSb, initially decreased slowly with increasing Ge concentration, but decreased ever more rapidly as x approached 0.3. S(x) was essentially zero for alloys with x>0.3, indicating a "disordered" diamond structure. Correspondingly, r decreased from 1 at x=0 to 0.5 at x≥0.3.