Abstract
The local and intermediate range ordering in Ca–NH3 solutions in their metallic phase is determined through H/D isotopically differenced neutron diffraction in combination with empirical potential structure refinements. For both low and high relative Ca concentrations, the Ca ions are found to be octahedrally coordinated by the NH3 solvent, and these hexammine units are spatially correlated out to lengthscales of ∼7.4–10.3 Å depending on the concentration, leading to pronounced ordering in the bulk liquid. We further demonstrate that this liquid order can be progressively disrupted by the substitution of Ca for Na, whereby a distortion of the average ion primary solvation occurs and the intermediate range ion–ion correlations are disrupted.
Highlights
As a divalent metal, shares many of the fascinating properties commonly associated with the group I metals when dissolved in NH3, forming an electrolytic system of solvated electrons at low concentrations and itinerant, metallic liquids as the concentration of Ca is increased.[1,3]
As with other metal−ammonia systems, these different electronic states are not miscible and are separated by a forbidden region around a metal−insulator transition whereby a bulk liquid−liquid phase separation occurs, commonly referred to as the immiscibility region in the literature. This immiscibility region is extremely pronounced in the Ca−NH3 phase diagram, with the electrolytic state only surviving to a maximum Ca concentration of 1.68 mole percent metal (MPM) at the consolute point or a critical temperature of 290 K with the onset of phase separation at
The experimental neutron total structure factors, F(Q), for metallic Ca−NH3 solutions at 4 and 20 MPM are presented in Figure 1 for the deuterated, proteated, and mixed proteated−
Summary
The dissolution of electropositive group I and group II metals in liquid NH3 yields a rich series of electronic liquids, whose properties are chemically tunable through both the concentration and nature of the parent metal.[1,2] Ca, as a divalent metal, shares many of the fascinating properties commonly associated with the group I metals when dissolved in NH3, forming an electrolytic system of solvated electrons at low concentrations and itinerant, metallic liquids as the concentration of Ca is increased.[1,3] As with other metal−ammonia systems, these different electronic states are not miscible and are separated by a forbidden region around a metal−insulator transition whereby a bulk liquid−liquid phase separation occurs, commonly referred to as the immiscibility region in the literature This immiscibility region is extremely pronounced in the Ca−NH3 phase diagram, with the electrolytic state only surviving to a maximum Ca concentration of 1.68 mole percent metal (MPM) at the consolute point or a critical temperature of 290 K with the onset of phase separation at
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