Molecular dynamics studies on molten alkali hydroxides. I. Static properties of molten LiOH

Molecular dynamics calculation for molten LiOH has been executed using a set of effective pair potentials obtained by a small modification of ab initio ones. Some experimental data on static and dynamic properties of the melt were successfully reproduced by the calculation. Anisotropic coordination structures of Li+ and OH− ions around an OH− ion have been elucidated from two-dimensional pair correlation functions, three-particle angular distribution functions, and four-particle distance distribution functions. The most probable location of a Li+ ion is on the O atom side of the OH− ion, being somewhat different from the pair potential minimum position. The correlation is strikingly large irrespective of the libration of the OH− ion. The OH− ion was found to coordinate to the neighboring OH− ion in two ways. The first one consists of a direct contact between ions on their H atom sides in spite of the large pair repulsion. The second is an indirect one on the O atom side through intervening Li+ ions which have strong Coulombic attraction to the OH− ion. This feature has a good correspondence to that of the LiOH crystal. These anisotropic structures result in an approximately isotropic coordination of the O atom around the OH− ion and give rise to a gOO(r) with a considerably sharp and large first peak and long range oscillatory correlations. This is the origin of the sharp and high first peak of the neutron structure function characteristic of the LiOH melt. It was also found that, on the average, OH− ions around a Li+ ion form a regular tetahedron, whereas the Li+ ions are located around the apexes of a square which is slightly folded downward along a diagonal just under the O atom of the OH− ion. These short range framework structures of the liquid are nearly the same with those of the crystal except that the four Li+ ions just under the OH− ion form only one plane in the crystal. Space–time correlation functions indicate that the above structures dissipate substantially in about 1 ps.