Multielement NMR Studies of the Liquid–Liquid Phase Separation and the Metal-to-Nonmetal Transition in Fluid Lithium– and Sodium–Ammonia Solutions

Abstract

(1)H, (7)Li, (14)N, and (23)Na high resolution nuclear magnetic resonance (NMR) measurements are reported for fluid solutions of lithium and sodium in anhydrous liquid ammonia across the metal-to-nonmetal transition (MNM transition), paying particular attention to the phenomenon of liquid-liquid phase separation which occurs in the composition/temperature region close to the MNM transition. Our results are discussed in terms of the electronic structure of fluid metal-ammonia solutions at low temperatures (ca. 240 K). We find that the electronic phase transition to the metallic state in these solutions, especially at temperatures close to the liquid-liquid critical consolute temperature, occurs from a nonmetallic, electrolytic solution containing a predominance of electron spin-paired, (diamagnetic) charged bosonic states. The possible implications of these observations to the nature of the liquid-liquid phase separation are discussed, both from the views of N. F. Mott, regarding the MNM transition in sodium-ammonia solutions, and those of R. A. Ogg, regarding the possibility of high-temperature superconductivity in these solutions. Similarities between the electronic structure of metal-ammonia solutions and the high-temperature cuprate superconductors are also briefly emphasized.

The Role of Structural and Compositional Heterogeneities in the Insulator-to-Metal Transition in Hole-Doped APd3O4 (A = Ca, Sr)

“…This phenomena is well known in alkali-metal liquid NH 3 solutions, where the phase separation between more metallic, high alkali-content liquid from the insulating, less concentrated liquid can be physically observed, and has also been followed by NMR. 46 The phenomena of such microscopic phase separation was predicted …”

Electron Solvation and the Unique Liquid Structure of a Mixed‐Amine Expanded Metal: The Saturated Li–NH3–MeNH2 System

“…As confirmation of the homogeneity at 20 MPM, Figure 1 shows that only a single feature is witnessed in the corresponding 7 Li NMR spectrum. This has proven to be an extremely sensitive indication of homogeneity in previous metal–amine studies 8. Interestingly, the 7 Li peak appears at higher ppm than both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 ,8, 12, 25 suggesting a higher conduction electron spin‐density at the Li nucleus (or indeed larger molar spin susceptibility) in the mixed amine than in either of the parent amines.…”

“…Interestingly, the 7 Li peak appears at higher ppm than both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 ,8, 12, 25 suggesting a higher conduction electron spin‐density at the Li nucleus (or indeed larger molar spin susceptibility) in the mixed amine than in either of the parent amines. The temperature dependence of the 7 Li Knight shift8 and the Dysonian lineshape of the ESR are also indicative of a homogeneous metallic system 22, 26, 27…”

“…[6,7] Thec oncentration and temperature dependence of this liquid-liquid phase separation has been mapped through multi-element NMR spectroscopy. [8] Along with metal concentration, chemical tunability of the electronic properties of these systems can be achieved by varying the amine.L ithium will also dissolve in MeNH 2 to yield as ystem whereby solvated electrons transition to am etallic state. [3,[9][10][11][12] TheM IT in Li-MeNH 2 occurs at 15 MPM, an otably higher concentration than in Li-NH 3 .…”

“…This has proven to be an extremely sensitive indication of homogeneity in previous metal-amine studies. [8] Interestingly,t he 7 Li peak appears at higher ppm than both Li(NH 3 ) 4 and Li(MeNH 2 ) 4 , [8,12,25] suggesting ahigher conduction electron spin-density at the Li nucleus (or indeed larger molar spin susceptibility) in the mixed amine than in either of the parent amines.T he temperature dependence of the 7 Li Knight shift [8] and the Dysonian lineshape of the ESR are also indicative of ah omogeneous metallic system. [22,26,27] Neutron diffraction is au niquely powerful method for determining the liquid structure of metal amines,allowing for an umber of isotopically distinct samples to be measured (defined in the Experimental Section below).…”