Unexpected stable stoichiometries of sodium chlorides (1211.3644v1)

Abstract: At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-free-electron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl2 molecules. Sodium chloride, due to the large electronegativity difference between Na and Cl atoms, has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt (B1-type) crystal structure in accordance with Pauling's rules. Up to now, Na-Cl was thought to be an ultimately simple textbook system. Here, we show that under pressure the stability of compounds in the Na-Cl system changes and new materials with different stoichiometries emerge at pressure as low as 25 GPa. In addition to NaCl, our theoretical calculations predict the stability of Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and electronic properties. The bandgap is closed for the majority of these materials. Guided by these predictions, we have synthesized cubic NaCl3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.

• The paper demonstrates the emergence of novel Na-Cl stoichiometries under pressures as low as 25 GPa, overturning conventional NaCl assumptions.
• Using ab initio crystal structure prediction and high-pressure experiments, the study validates phase transitions and dynamic stability of unexpected compounds.
• The identified compounds exhibit diverse electronic properties, with some phases showing metallic behavior and others remaining semiconducting, opening new material design avenues.

Na-Cl System Under High Pressure: Emergence of New Stoichiometries

The paper presented examines the high-pressure behavior of the sodium (Na) and chlorine (Cl) system, a classic example in chemistry. Historically, NaCl is perceived as a simple ionic compound with a well-known 1:1 stoichiometry governed by charge balance and characterized by the NaCl-type rock salt structure. This paper challenges that classical notion by demonstrating the emergence of stable sodium chloride compounds with unexpected stoichiometries—Na$_3$Cl, Na$_2$Cl, Na$_3$Cl$_2$, and Na$_7$Cl—under pressures as low as 25 GPa. This research contributes significantly to the broader understanding of chemical bonding and structural behaviors under extreme conditions.

Summary of Findings

• New Stoichiometries and Structures: The results reveal that, contrary to traditional expectations, additional stable phases in the Na-Cl system emerge under high pressure. Using ab initio evolutionary crystal structure prediction methods like USPEX, previously unexplored stoichiometries were predicted and experimentally validated. NaCl$_3$ and NaCl$_7$, for example, exhibit cubic structures at elevated pressures.
• Pressure-Induced Phase Transitions: The paper maps phase transitions and the appearance of new compounds through a comprehensive pressure-composition phase diagram. Notably, NaCl undergoes a phase transformation from the B1 to B2 structure around 30 GPa, but new stoichiometries like NaCl$_3$ become stable beyond 48 GPa, contrasting with NaCl's expected behavior according to classical chemistry principles.
• Dynamical Stability: The thermodynamic stability of these new compounds at high pressures is supported by phonon calculations indicating dynamical stability for these stoichiometries. Notably, these newly discovered structures deviate from ionic characteristics, lacking discrete Cl$_2$ molecules, emphasizing the role of covalent interactions.
• Electronic Properties: The NaCl$_3$ and NaCl$_7$ compounds display unique electronic properties with metallic characteristics, linking their phase stability with electron-phonon coupling. Interestingly, NaCl$_2$ remains a semiconductor at lower pressures (25-48 GPa) before transitioning to metallic states.
• Experimental Validation: Laser heating experiments using a diamond anvil cell confirmed the synthesis of these novel stoichiometries at pressures up to 60 GPa. The agreement between experimental results and theoretical predictions for lattice parameters and symmetric structures validates the computational approach.

Implications and Future Directions

The revelation that the simplest ionic compound, NaCl, can form several stable phases under pressure challenges well-known chemistry principles like the octet rule and charge balance in ionic systems. These findings open avenues for exploration in other similar binary systems and more complex multicomponent systems, potentially transforming planetary science, which involves extreme pressure conditions found in planetary interiors.

Future work may explore additional systems with implications for materials science and geophysics. A similar methodology could foster the discovery of novel materials with unique properties beneficial for technological applications, including superconductivity, catalysis, and electronic devices. Moreover, extending such studies to the exploration of ternary or quaternary compounds could further enrich the field's understanding of pressure-driven chemical evolution.

Overall, the presented paper offers deep insights into the behavior of chemical compounds under high pressure, fundamentally altering how the chemical stability of materials is perceived when subjected to the extreme environments typically encountered in planetary and materials sciences.