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Yes—at least in computer predictions, extreme pressure can stabilize sodium–chlorine compounds and bonding patterns that look very different from familiar salt. A 2016 study by Gabriele Saleh and Artem R. Oganov predicted sodium-rich chlorides including Na4Cl3 and new phases of Na3Cl, with chlorine assigned an unusual −2 oxidation state in sodium subchlorides. These were theoretical predictions, not reports of experimentally synthesized samples.
What did the study predict?
Saleh and Oganov used evolutionary crystal-structure prediction calculations to investigate alkali subhalides—compounds with less halogen than a conventional alkali-halide formula would suggest. Their paper, “Alkali subhalides: high-pressure stability and interplay between metallic and ionic bonds,” reports an updated phase diagram for sodium-rich chlorides.
- Na4Cl3: a predicted sodium-rich chloride phase.
- Na3Cl: two previously undiscovered phases were predicted.
- Chlorine oxidation state: the calculations assigned chlorine a −2 oxidation state in sodium subchlorides, unlike the familiar −1 state in ordinary sodium chloride.
Chemistry World’s 23 February 2016 report says the calculations considered pressures up to 350 GPa. That is the modeled upper pressure reported in the coverage, not a general threshold at which chemistry changes or a statement that every predicted result occurs at precisely that pressure. The paper describes the work as involving pressures of hundreds of gigapascals.
Why does chlorine at −2 seem surprising?
In common chemical examples, chlorine is often treated as an anion with a −1 oxidation state. Oxidation states are a bookkeeping framework for assigning electrons in compounds; they are not direct measurements of an atom carrying an isolated, literal charge. The predicted −2 assignment is striking because it reverses that familiar pattern in a sodium-rich compound.
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At extreme compression, atoms are forced into much closer arrangements. The structures that can become stable, and the balance between ionic and metallic bonding, may therefore differ from what familiar low-pressure examples lead us to expect. The result is not that valency or oxidation-state concepts have become useless; rather, familiar rules alone may not anticipate which structures are stable in these conditions.
What model did the researchers propose?
The paper proposes a model to rationalize the high-pressure stability of alkali subhalides and the interplay between metallic and ionic bonds. The authors also explored its predictive ability with calculations on A3Y compounds, where A was lithium, sodium, or potassium and Y was fluorine, chlorine, or bromine. This computational check broadens the test beyond the sodium-chloride examples, but it does not turn the predicted phases into experimental observations.
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Does this mean the compounds were made in a laboratory?
No experimental synthesis of these specific predicted phases is established by the cited report and paper record. The central claims discussed here come from computational structure and electronic-structure calculations. They identify candidate structures and properties for extreme-pressure chemistry; experimental confirmation is a separate question.
What does “rewriting the textbooks” really mean?
The phrase captures how far the predictions depart from familiar examples, but it is more dramatic than the scientific conclusion. The work suggests that pressure can expose stable stoichiometries and bonding arrangements that ordinary conditions do not make familiar. It refines how chemists think about the limits of simple expectations; it does not erase the usefulness of chemical principles across the conditions where they already work well.
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Professor Paul McMillan of UCL Chemistry said the findings “go beyond the principles of valency established for over 200 years.” Saleh told Chemistry World, “High pressure chemistry is unpredictable,” and, “Most chemical rules break down at high pressure.” Those remarks refer to the unusual behavior being explored under extreme compression, not to chemistry in general.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where the report and paper fit in time
The Royal Society of Chemistry record says the article was first published on 2 December 2015 and appeared in volume 18 of Physical Chemistry Chemical Physics (2016), pages 2840–2849. Chemistry World’s coverage followed on 23 February 2016, and UCL published a related item on 6 April 2016.
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- Read the paper record at the Royal Society of Chemistry.
- Read Chemistry World’s report.
- Read UCL’s related report.
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