Atomic Structure and Correlation in Liquid Binaries by X-Ray and Neutron Diffraction with Application to NaK

Abstract

For a liquid or amorphous binary three independent diffraction patterns are required to completely characterize the atomic structure, including the correlation of the two atom types. However if the distribution of atomic density is the same about either atom type only two patterns are required. This two-pattern method, as developed by Keating, was applied to 50 at. % liquid sodium—potassium using x-ray and neutron diffraction. The results indicated a very nearly random mixing of atom types, but further analysis of the data showed that the assumption of identical distributions about the Na and K atoms is invalid. Indeed, consideration of the factors which may determine the liquid structure makes this assumption highly questionable for most binaries. A new method of combining the x-ray and neutron-diffraction patterns was developed which can yield qualitative information on the correlation and serve as a test for theoretical structural models. The new approach utilizes the ratio of the two weighted atomic distributions obtained separately from the x-ray and neutron patterns. For the 50% NaK system the neutron pattern alone gives the true atomic distribution with nearest-neighbor concentrations at 4.4, 7.7, and 11.2 Å, and the comparison with the x-ray distribution indicates that at the first three nearest-neighbor distances a given atom tends to be paired with atoms of the opposite type, with some concentration of Na–Na pairs at 5.0 Å and K–K pairs at 6 to 7 Å.