AB-INITIO CALCULATION OF THE STRUCTURE OF THE RANDOM ALLOYS SNXGE1-X

We present a theoretical analysis for the system of random alloys SnxG e1-x (x=0.25, 0.50, and 0.75). An ab initio pseudopotential total-ener gy calculation is performed to determine the structure of the system. It is found that the equilibrium lattice constant of this kind of allo y is close to a linear average of the lattice constants of its constit uent elements,i.e., Ge and alpha-Sn with composition x and the possibi lity of forming a SnGe zinc-blende-type compound is only slightly favo rable energetically. Both bond-length and bond-angle distributions are determined. The topological rigidity parameter a* of the alloy is fo und to be 0.69(-0.03)(+0.02) The average value of bond angle for each alloy calculated is approximately unchanged from 109.5 degrees, while the specific angles such as Ge-Ge-Ge, Sn-Sn-Sn, etc. are different fro m each other and increase with Sn composition x from one alloy to the other.