THE DETERMINATION OF THE RADIAL-DISTRIBUTION FUNCTIONS GLI-LI(R), GLI-O(R) AND GLI-N(R), IN MOLTEN LITHIUM-NITRATE FROM NEUTRON-DIFFRACTION

The structure of molten lithium nitrate at 573 K has been investigated by applying the technique of neutron diffraction to five isotopically enriched samples: (LiNO3,)-Li-N-N-N, (LiNO3)Li-0-N-N, (LiNO3)-Li-7-N-15 and (LiNO3)-Li-0-N-15. The ion-ion and ion-counterion partial structure factors relating to g(Li-Li)(r), g(Li-N)(r), g(Li-O)(r) partial pair correlation functions have been successfully extracted from the experimental data by the isotopic difference methods. Some general conclusions are made about the detailed structure of molten lithium nitrate. Comparison is also made with earlier X-ray and neutron diffraction and molecular dynamics (MD) simulation studies. In particular, the nearest-neighbour Li-O and Li-Li distances of 1.86 angstrom and 3.86 angstrom reported in MD study are significantly less than the values of 2.1 angstrom and 4.1 angstrom, respectively found in the present study. This we feel is a result of the uncertainty in the effective pair potentials used in the MD simulation. The nearest-neighbour distances r(Li-O) = 2.1 +/- 0.1 angstrom and r(Li-N) = 2.8 +/- 0.1 angstrom obtained in the present study on a stable nuclear reactor source compare well with those obtained by Kameda et al. [1] from a recent time-of-flight neutron diffraction study on a spallation source. The lithium monovalent cation is surrounded on an average by four nitrate (NO3-) ions; one oxygen atom in each of these NO3- ions facing towards Li+. Although, these results are found to be consistent with those of Kameda et al., certain discrepancies between the two results become apparent in the next-nearest neighbour shells. Additionally, the closest Li+-Li+ approach r(Li-Li) = 4.1 +/- 0.2 angstrom is smaller than that for NO3--NO3-, r(N-N) = 4.8 +/- 0.2 angstrom [1]. The space and time averaged local structure of LiNO3 in the molten state is found to be appreciably different from that in its crystalline state.