Unexpected Stable Stoichiometries of Sodium Chlorides

Abstract

At ambient pressure, sodium, chlorine, and their only known compound NaCl, have well-understood crystal structures and chemical bonding. Sodium is a nearly-freeelectron metal with the bcc structure. Chlorine is a molecular crystal, consisting of Cl 2 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 Na 3 Cl, Na 2 Cl, Na 3 Cl 2 , NaCl 3 and NaCl 7 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 NaCl 3 at 55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.

Metal‐to‐Semiconductor Transition and Electronic Dimensionality Reduction of Ca2N Electride under Pressure

“…With the increase of pressure, the resistance‐temperature curves have a negative slope, indicating a typical semiconducting character. Generally, high pressure leads to insulator‐to‐metal or semiconductor‐to‐metal transition because of reduction of interatomic distances and strengthening of interatomic interactions 16. The observed metal‐to‐semiconductor transition in Ca 2 N should be attributed to the structural transitions starting at 13.5 GPa, in agreement with in situ XRD results and theory by Zhang et al14 (Figure 1b).…”

High-Energy Density and Superhard Nitrogen-Rich B-N Compounds

“…A recent example is the prediction of NaCl 3 and Na 3 Cl compounds, which have been confirmed experimentally [27]. Experiments have also demonstrated that molecular H 2 can interact readily with closed-shell molecules at easily accessible pressures (<20 GPa) in the formation of van der Waals compounds, such as SiH 4 -H 2 [28] and H 2 S-H 2 [29].…”

A fingerprint based metric for measuring similarities of crystalline structures

“…[1][2][3][4][5][6][7] On the other hand, ab initio structure predictions [8][9][10][11][12][13][14][15] can produce a huge number of new structures that have either not yet been found experimentally or are metastable. [16][17][18][19][20][21] In both cases, it is essential to quantify similarities and dissimilarities between structures in the data sets, requiring a configurational distance that satisfies the properties of a metric. Databases frequently contain duplicates and insufficiently characterized structures which need to be identified and filtered.…”