A quantum revelation in century-old chemistry
Imagine pouring silvery chunks of lithium, sodium, or potassium into a vat of clear ammonia and watching the liquid turn a deep, vivid blue. Add more metal, and suddenly—like a chemical chameleon—it transforms into a shimmering golden bronze. This century-old chemistry demonstration isn't just visually stunning; it hides a quantum secret.
For decades, scientists believed the transition from non-metallic blue electrolyte to metallic bronze liquid was gradual and continuous. But groundbreaking research now reveals a hidden phase where the solution dances between these states at unimaginable speeds—switching roles up to 25 trillion times per second 1 5 .
This discovery, emerging from advanced simulations at the Institute of Organic Chemistry and Biochemistry in Prague, challenges textbook models and introduces a new physical phenomenon at the boundary of chemistry and physics.
The color change in alkali metal-ammonia solutions was first observed in 1864 by W. Weyl, but its quantum nature remained unexplained for over a century.
When alkali metals dissolve in ammonia, they release electrons that become trapped—or "solvated"—within cages of ammonia molecules. These cages, called cavity solvates, act like quantum prisons where electrons behave as localized particles. This state gives dilute solutions (<1 MPM, or mole percent metal) their iconic blue color and electrolyte properties 1 2 .
At concentrations above 10 MPM, something extraordinary happens. Solvated electrons escape their cages, delocalize, and form a shared "electron sea" akin to liquid mercury or copper. This transforms the solution into a disordered liquid metal with electrical conductivity rivaling copper 1 .
For solutions between 3–8 MPM, neither model fully applies. Historically, three theories competed to explain this transition:
Metallic regions form connected pathways like a circuit board.
Metallization occurs when electron density exceeds a threshold.
Sudden dielectric failure of the solvent 2 .
Scattering experiments hinted at nanoscale fluctuations in these intermediate solutions but couldn't determine whether they were static clusters or dynamic processes 1 2 .
Using ab initio molecular dynamics (AIMD), Prof. Pavel Jungwirth's team simulated lithium-ammonia solutions across critical concentrations. Their simulations treated all electrons—both bound and excess—quantum mechanically, capturing their behavior in real time. What they found was revolutionary:
"The system doesn't settle. At intermediate concentrations, it flips between metallic and non-metallic states every ~40 femtoseconds—faster than molecular vibrations." 5
| Phase | Concentration | State of Electrons | Key Feature |
|---|---|---|---|
| Electrolyte | <3 MPM | Localized in cavities | Deep blue color; insulating |
| Flipping Regime | 3–10 MPM | Rapidly switching | Dynamic gap opening/closing |
| Metallic | >10 MPM | Delocalized electron sea | Golden sheen; high conductivity |
Simulated boxes (~15 Å side) contained 64 ammonia molecules with varying Li⁺/e⁻ pairs to model 3.0–13.5 MPM concentrations 1 .
The revPBE38-D3 density functional tracked electron behavior, validated against many-body GW theory for accuracy 1 2 .
Snapshots were taken every 2 femtoseconds over 1-picosecond trajectories—enough to catch 25 flipping events 1 .
The team computed the electronic density of states (DoS) for each snapshot. In metallic states, electrons filled orbitals continuously across the Fermi level. In electrolyte states, a clear band gap appeared. At 6.0 MPM, the DoS revealed rapid "blinking":
Band gap closes; electrons delocalize.
| Time (fs) | DoS at Fermi Level | State Classification | Trigger |
|---|---|---|---|
| 0 | High | Metallic | Electron density fluctuation |
| 42 | Near zero | Electrolyte | Solvent reorganization |
| 86 | High | Metallic | Lithium ion clustering |
| 128 | Near zero | Electrolyte | Solvent cavity formation |
Flipping is driven by tiny structural shifts:
| Reagent/Tool | Function | Experimental Role |
|---|---|---|
| Lithium metal | Electron donor | Source of solvated electrons |
| Liquid ammonia | Solvent; forms electron-trapping cavities | Medium for dissolution and reaction |
| revPBE38-D3 functional | Quantum mechanical model for electron behavior | Simulates electron dynamics with hybrid accuracy |
| AIMD simulations | Tracks atomic/electronic motion over time | Captures femtosecond-scale state flipping |
| Photoelectron spectroscopy | Measures electron energy levels | Validates simulated DoS against experimental data |
This flipping phenomenon isn't just a curiosity—it represents a new class of transient quantum materials with potential applications in:
Switching between conductive/non-conductive states at femtosecond speeds.
Mimicking neuronal firing timescales (~10⁻¹³ seconds) 5 .
Enhancing electron-transfer reactions in Birch reduction or ammonia synthesis 1 .
The challenge? Confirming flipping experimentally. As PhD student Marco Vitek notes:
"Capturing a 40-fs event is like photographing a bullet in flight with a shutter speed of 1 second." 5
The team is now collaborating with synchrotron facilities to attempt time-resolved X-ray scattering using pulsed lasers—a technique that could freeze the flipping in action.
The dance between electrolyte and metallic states in ammonia solutions reveals nature's preference for dynamic ambiguity over static compromise. Like a traffic light blinking between green and red too fast for the eye to see, this third phase operates in a realm where quantum fluctuations reign. It's a reminder that even in century-old chemical systems, fundamental discoveries await—if we look fast enough.
"No one had realized a system might oscillate between states on such timescales. It simply hadn't been considered." —Prof. Pavel Jungwirth 5