Vegard's law is an empirical rule that's approximately true for many alloys, and it says that so long as changing concentration of an alloying element does not result in a change in crystal structure, the lattice parameter is a linear function of composition (at fixed temperature).
So, for instance, if I know the lattice parameter of GaAs and that of AlAs, then I can linearly interpolate to find the approximate lattice parameter of any Ga1-xAlxAs.
PS: See, for instance, Denton and Ashcroft, Phys Rev A 43, 3161 (1991)
http://prola.aps.org/pdf/PRA/v43/i6/p3161_1