The structure of molten salts.

Abstract

B y R. L. M c G r e e v y 1 a n d L. P u s z t a i2 1 Clarendon L a b o r a t o r y The three-dim ensional stru ctu res of th irteen MX and M X 2 m olten salts (M, m e ta l; X, halide) have been modelled from neutron diffraction d a ta by using the Reverse M onte Carlo m ethod. A lthough th e stru ctu res are highly disordered the dom inant stru c tu ra l sym m etries of th e short-range order can be determ ined by using bond angle correlations and spherical harm onic invariants. I t is found th a t cations in the alkali chlorides tend to be octahedrally coordinated by anions (and vice versa), although the large num ber of ' vacancies ' leads to coordination num bers lower th an six. CuCl has a tetrah ed ral coordination of anions about cations. In M X2 melts small cations are octahedrally coordinated except in ZnCl2 where the coordination is tetrah ed ral. As th e cation size increases the coordination tends to cubic. In all cases the local stru ctu ra l sym m etries in the m elt are sim ilar to those in the corresponding crystals. I t is shown th a t the 'p rep e a k ' which occurs in the p artial stru ctu re factor i MM(^) for M X 2 m elts w ith small cations, and which has been associated with interm ediate range ordering, arises from a local density fluctuation due to cation 'clu sterin g '. This is the first tim e th a t a visual picture of such interm ediate-range order has been obtained.