Volume changes can be only attributed to changed crystal structure.

Thanks for your answer.

But, I should tell you that both crystal structures (in order and dis-order conditions) are BCC. Therefor, the crystal structure doesn't change by ordering transformation in FeCo alloys.

With Bests

When you get ordered structure, volume of this new structure is larger than unordered one, due to difference in atom size. After structure orders, Co will be spread evenly through it.    

Thanks for your attention.

But, I should tell you that in order and dis-order structure occur no change in number of atoms. So that, every unit-cell has a Fe atom and cobalt similar to dis-order condition. But, in order condition against dis-order condition, location of atoms is specified. You  can see it in Fig. 2 in attached article.

With bests

Saeed