A MOLECULAR-DYNAMICS STUDY OF MEDIUM-RANGE ORDER IN MOLTEN TRIVALENT METAL CHLORIDES

Extensive molecular-dynamics (MD) simulations are performed for a rigid-ion model of molten metal trichlorides (MCl3), in different regimes of metal radius, density and temperature. The cases of molten YCl3 and AlCl3 are examined in more detail in order to ascertain the presence of medium-range order (MRO) in these materials. For the YCl3 case, the experimental neutron structure factor, signalling the presence of MRO in the system, is fairly well reproduced by MD. The overall structural information available from the simulation indicates that order at intermediate distances basically consists of local 'layers' of comer-sharing (YCl6)3- octahedra. These roughly planar formations may represent a remnant of the real YCl3 crystalline structure, constituted by layers of edge-sharing (YCl6)3- octahedra; the internal topology of the 'planes' is discussed in relation to the assumed model potential and to global charge ordering. The effect of metal size on the MRO features is then investigated by adjusting the model parameters to the case of molten AlCl3, in which the Al3+ effective radius is significantly smaller than that of Y3+. In this case well defined tetrahedral (AlCl4)- units tend to be formed, and these link to each other in a highly connected network responsible for MRO. The presence in the melt of bound pairs of tetrahedra, interpretable as Al2Cl3 'dimers', is also established, in essential agreement with the prediction of recent theoretical work by other authors.