Neutron diffraction measurements were carried out at 25 °C for aqueous LiNO
3 heavy water solutions, (*LiNO
3)
x (D
2O)
1?x where
x = 0.1, 0.05 and 0.01, in which the
6Li/
7Li isotopic ratios were varied. Structural information on intermolecular nearest neighbor Li
+···D
2O interactions in the extensive concentration range was derived from the first-order difference function, ?
Li(
Q), obtained from the difference in scattering cross sections between
6Li- and
7Li-enriched sample solutions. The nearest neighbor Li
+···O distance and coordination number for sample solution with
x = 0.1 were determined to be
r LiO = 1.969 (8) Å and
n LiO = 4.12 (6), respectively, corresponding to the four-coordinated Li
+ ion in the solution. On the other hand, those obtained for the solution with
x = 0.01 are
r LiO = 2.00 (2) Å and
n LiO = 6.0 (2), respectively, indicating that hexaaqua Li
+ is dominant in the dilute solution. These results clearly indicate that a concentration dependence of the hydration number of Li
+ occurs in the aqueous solutions.
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