Long-range order and the electrical resistivity

Long range atomic ordering (LRO) is known to affect the effective number of conduction electrons n eff as a result of new energy gaps forming in the Fermi surface at the superlattice Brillouin zone boundaries. A simple expression is derived for this behaviour and combined with the effects of LRO on the relaxation time tau to give an overall dependence of resistivity with LRO of the form rho (S,T)=(1-S 2 )/(1-AS 2 ))+(T/T c )(1/(1-AS2)) where T is the measuring temperature, S the Bragg-Williams LRO Parameter, T c the critical order-disorder temperature and A a parameter whose sign and magnitude depend upon the relative positions of the Fermi surface and superlattice Brillouin zone boundaries. This is found to be in good agreement with experimental data for Cu 3 Au and Fe 3 Al.