Herzfeld instability versus Mott transition in metal–ammonia solutions

Abstract

Although most metal-insulator transitions in doped insulators are generally viewed as Mott transitions, some systems seem to deviate from this scenario. Alkali metal-ammonia solutions are a brilliant example of that. They reveal a phase separation in the range of metal concentrations where a metal-insulator transition occurs.Using a mean spherical approximation for quantum polarizable fluids, we argue that the origin of the metal-insulator transition in such a system is likely similar to that proposed by Herzfeld a long time ago, namely, due to fluctuations of solvated electrons. We also show how the phase separation may appear : the Herzfeld instability of the insulator occurs at a concentration for which the metallic phase is also unstable. As a consequence, the Mott transition cannot occur at low temperatures. The proposed scenario may provide a new insight into the metal-insulator transition in condensed-matter physics. keywords : Solvated electrons ; Mott transition ; Phase separationIn the first theory of metallization, Herzfeld [1] has considered dipolar fluctuations of neutral atoms as the origin of a metal-insulator transition (MIT). He emphasized that the restoring force of an electron bound to an atom collapses and the substance becomes metallic at increased densities due to local field effects. Could the Herzfeld idea be applied to describe the MIT in real systems ? Despite some successful examples of such applications [2,3,4,5], serious doubts arose because most of the experiments gave evidence in favor of the Mott scenario [6]. The latter focused on the screening of the long-range Coulomb potential by a stable electron gas, which prevents the formation of bound electrons. As far as this metal

Nature of the metal–nonmetal transition in metal–ammonia solutions. I. Solvated electrons at low metal concentrations

“…The solution remains nonmetallic ͑elec-trolytic͒ at these low concentrations and has an intense blue color independent of the type of alkali metal, while the optical absorption spectrum does not change up to concentrations of about 0.1 mol % of metal ͑MPM͒. We note 27 that at the experimental densities of the NMM transition ͑ϳ4 MPM͒ the Debye length is estimated to be 1 Å that is lower than the mean distance between the ions ͑ϳ12 Å͒ by an order of magnitude. The conductivity of MAS reaches that of liquid mercury at concentrations of about 20 MPM.…”

“…[5][6][7][8][9][10][11][12][13][14] Once an alkali metal is dissolved in liquid ammonia, it immediately dissociates to give separated entities with unlike charges: the solvated ions and electrons. We have recently shown 27 that the MNM transition is rather driven by the old Goldhammer-Herzfeld ͑GH͒ mechanism of metallization: 28,29 the quantum fluctuations of the electrons localized in cavities induce a polarization catastrophe resulting in metallization of the solution. 15 Path integral simulations [16][17][18][19] and density functional approaches, 20,21 pro-viding an evaluation of the microscopic structure around solvated electrons, indicate also the possibility of solvated dielectrons, i.e., paired electrons trapped in a cavity, eventually as metastable states.…”

“…15 Path integral simulations [16][17][18][19] and density functional approaches, 20,21 pro-viding an evaluation of the microscopic structure around solvated electrons, indicate also the possibility of solvated dielectrons, i.e., paired electrons trapped in a cavity, eventually as metastable states. 33 We emphasized the prominent role of the dipolar-dipolar interactions, 27 i.e., van der Waals interactions in the sense of the London dispersion forces in the MNM transition. 22 For metal concentrations exceeding 7 MPM, the MAS is a liquid metal with a typical bronze coloration.…”

Nature of metal–nonmetal transition in metal–ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure

“…In our recent papers 1,2 we have explained that these interactions are mainly due to the dispersion attractions between solvated electrons at metal concentrations in the range of 1-5 MPM. 1, this seems to forbid the Mott-like scenario for the MNM transition in MAS. Considering the role of the induced dipolar interactions between solvated electrons, we have restricted ourselves to the nonmetallic phase.…”

Comment on “Model of saturated lithium ammonia as a single-component liquid metal” [J. Chem. Phys. 124, 074702 (2006)]

“…However, such consideration should take into account the peculiarities of MAS, i.e., the solvent polarizability and dif-ferent energy scales for the ion-ion and the electron-electron interactions. Due to these peculiarities, metallic phases of MAS remain stable up to values of r s about 11, 8 …”