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Register a new userUnexpected stable stoichiometries of sodium chlorides
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Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. Due to the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding, with stoichiometry 1:1 dictated by charge balance, and B1-type crystal structure. Here, by combining theoretical predictions and diamond anvil cell experiments we show that new materials with different stoichiometries emerge at pressure as low as 20 GPa. Compounds such us Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 are theoretically stable and have unusual bonding and electronic properties. To test this prediction, at 55-80 GPa we synthesized cubic and orthorhombic NaCl3 at 55-70 GPa and 2D-metallic tetragonal Na3Cl. This proves that novel compounds, violating chemical intuition, can be thermodynamically stable even in simplest systems at non-ambient conditions.
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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 Paulings 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.
By combining first-principles electronic-structure calculations with the model Hamiltonian approach, we systematically study the magnetic properties of sodium superoxide (NaO2), originating from interacting superoxide molecules. We show that NaO2 exhibits a rich variety of magnetic properties, which are controlled by relative alignment of the superoxide molecules as well as the state of partially filled antibonding molecular pi_g-orbitals. The orbital degeneracy and disorder in the high-temperature pyrite phase gives rise to weak isotropic antiferromagnetic (AFM) interactions between the molecules. The transition to the low-temperature marcasite phase lifts the degeneracy, leading to the orbital order and formation of the quasi-one-dimensional AFM spin chains. Both tendencies are consistent with the behavior of experimental magnetic susceptibility data. Furthermore, we evaluate the magnetic transition temperature and type of the long-range magnetic order in the marcasite phase. We argue that this magnetic order depends on the behavior of weak isotropic as well as anisotropic and Dzyaloshinskii-Moriya exchange interactions between the molecules. Finally, we predict the existence of a multiferroic phase, where the inversion symmetry is broken by the long-range magnetic order, giving rise to substantial ferroelectric polarization.
By applying a genetic algorithm in a cascade approach of increasing accuracy, we calculate the composition and structure of MgMOx clusters at realistic temperatures and oxygen pressures. The stable and metastable systems are identified by ab initio atomistic thermodynamics. We find that small clusters (M <= 5) are in thermodynamic equilibrium when x > M. The non-stoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory (DFT) with a hybrid exchange-correlation (xc) functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor DFT with (semi)local xc functionals are sufficient for even a qualitative prediction.
Sodium ion ordering on in situ cleaved NaxCoO2 (x=0.84) surface has been studied by ultra high vacuum scanning tunneling microscopy (UHV-STM) at room temperature. Three main phases, with p(3x3), (root 7 x root 7), and (2 root 3 x 2 root 3) hexagonal unit cells and surface Na concentration of 1/3, 3/7, 1/2, respectively, were identified. One surprising finding is that Na trimers act as the basic building blocks that order in long range. The stability of Na trimers is attributed to the increased Na coordination with oxygen as indicated by ab initio calculations, and possibly at finite temperature by configuration entropy.
Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.
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