Inadequacy of the Clapp and Moss approximation in extracting pair ordering potentials from diffuse-scattering data: LiMg, a case point

The Dagens--Rasolt--Taylor (Phys. Rev. B , 1975, 11, 2726) first-principles pair potentials are used to generate the diffuse-scatterred intensity from a Li--10 at.% Mg alloy by means of the Tahir--Kheli (Phys. Rev. , 1969, 188, 1142) intermediate-temperature formulation of short-range order in binary alloys. When the intensity is subsequently used to extract the pair ordering potenital by means of the high temperature approximation of Clapp and Moss (Phys. Rev. , 1966, 142, 418), it is found that the potenital so obtained bears little resemblance to that generating the intensity. The difference between them is of the order of 40% at the first nearest-neighbour position, even as far away from the critical temperature as T/T sub c > 1.6. The Clapp and Moss approximation appears to distort the potential in a selective way for non-equiatomic alloys, depending upon whether the alloy presents short-range order of the "ordering" or "clustering" type. 23 ref.--AA