NEW ANALYTICAL METHODS FOR CALCULATION OF THE PAIR CORRELATION FUNCTION IN TWO-COMPONENT CRYSTAL STRUCTURES.
I. GENERAL THEORY

Roman V. Chepulski and Vladirnir N. Bugaev

Dept. af Solid State Theory, Inst. for Meta1 Physics, National Academy of Sciences of Ukraine, 252680, Kiev-142, Ukraine

In the context of a modified thermodynamic pertubation theory within the grand canonical ensemble, by the use of the thermodynarnic fluctuation rnethod, a nurnber of new analytical approximations for calculations of the grand thermodynamic potential as well as of the pair correlation function and íts Fourier transform in disordered. two-componet crystal structures with a Bravais crystal lattice are elaborated.

A crystal structure of an arbitrary cornposition is considered. in the frarnework of the lattice gas rnodel with pair site interactions af arbitrary effective radius of action. The inverse effective radius of interactions, inverse temperature as well as the concentration of the impurity component are used as a small parameter of expansion. The developed approach may be also followed in studies of magnetics within the framework of the Izing model, of fluids and amorphous materials within the lattice gas model as well as in investigations of the low-dirnension lattice systenns.