Caesium in high oxidation states and as a p-block element

Journal name:
Nature Chemistry
Volume:
5,
Pages:
846–852
Year published:
DOI:
doi:10.1038/nchem.1754
Received
Accepted
Published online

Abstract

The periodicity of the elements and the non-reactivity of the inner-shell electrons are two related principles of chemistry, rooted in the atomic shell structure. Within compounds, Group I elements, for example, invariably assume the +1 oxidation state, and their chemical properties differ completely from those of the p-block elements. These general rules govern our understanding of chemical structures and reactions. Here, first-principles calculations show that, under pressure, caesium atoms can share their 5p electrons to become formally oxidized beyond the +1 state. In the presence of fluorine and under pressure, the formation of CsFn (n > 1) compounds containing neutral or ionic molecules is predicted. Their geometry and bonding resemble that of isoelectronic XeFn molecules, showing a caesium atom that behaves chemically like a p-block element under these conditions. The calculated stability of the CsFn compounds shows that the inner-shell electrons can become the main components of chemical bonds.

At a glance

Figures

  1. Stability of CsFn compounds under pressure.
    Figure 1: Stability of CsFn compounds under pressure.

    a, Enthalpies of formation of CsFn under a range of pressures. Dotted lines connect data points, and solid lines denote the convex hull. Compounds corresponding to data points located on the convex hull are stable against disproportionating into other compositions. b, Predicted stable pressure ranges for CsFn compounds (orange). The blue segment represents the pressure range where Cs+[F3] is stable (see Fig. 2c for its structure), instead of CsF3. The enthalpies of CsF are calculated for the NaCl structure under ambient conditions and the CsCl structure at high pressures. The enthalpies of F solids are obtained for molecular crystals, which are in C12/m1 and C12/c1 space groups. Details of the stable structures for CsF and F solid are provided in Supplementary Section SIV.

  2. Crystal structures of CsFn compounds, and selected interatomic distances as functions of pressure.
    Figure 2: Crystal structures of CsFn compounds, and selected interatomic distances as functions of pressure.

    (Parameters of all structures are shown in Supplementary Tables SI–IV). a, CsF2 at 20 GPa in an I4/mmm structure. b, CsF3 at 100 GPa in a C2/m structure. c, CsF3 at 0 GPa with actual formula of Cs+[F3]. (For structure information see Supplementary Section SIII.) d, CsF4 at 100 GPa in a C2/m structure. e, CsF5 at 150 GPa in an Fdd2 structure. f, CsF6 at 200 GPa in a P1 structure. Green and pink spheres represent Cs and F atoms, respectively; dark blue spheres in b and c represent isolated F ions in CsF3 at 100 GPa (b), which become the central atoms in F3 at 0 GPa (c). gi, Interatomic distances (between atoms shown in the structure directly underneath) in CsF2 (I4/mmm), CsF3 (C2/m) and CsF5 (Fdd2) as a function of external pressure. The Cs–F bond in CsF and F–F in the F2 molecule are shown by dashed lines for comparison.

  3. Electronic structures and the nature of Cs–F bonds.
    Figure 3: Electronic structures and the nature of Cs–F bonds.

    a,b, Calculated PDOS (a) and COHP (b) for CsF2 at 20 GPa. c,d, PDOS (c) and COHP (d) for CsF3 at 100 GPa. e,f, PDOS (e) and COHP (f) for XeF2 at 20 GPa. The states are aligned at the Fermi level (vertical dashed lines). In all PDOS plots, green lines represent the 5p state of the Cs (Xe) atoms, and the pink lines represent the 2p state of the F atoms bonded with Cs. The overlap between the two indicates possible strong interaction between Cs and F atoms. The blue line in c represents the 2p state of the isolated F anion, showing a bonding feature very different to the F atoms covalently bonded with Cs. In the COHP plots, black and red lines represent the COPH between Cs (Xe) and its nearest neighbour F and those between Cs (Xe) and its next nearest neighbour F. They reveal that the Cs and its nearest neighbouring F form strong covalent bonds, whereas the covalent bonding between Cs and next nearest F is rather weak. gj, ELFs of CsF3 at 50 GPa [(100) plane] (g), CsF5 at 100 GPa [(3–10 3) plane] (h), XeF2 at 50 GPa [(1 0 0) plane] (i) and NH4XeF5 at 100 GPa [(0 0 4)] plane] (j). Red areas (ELF ≈ 0.8) around the Cs (Xe) and F atoms indicate the lone electron pairs, and green areas (ELF ≈ 0.5) between the Cs(Xe) and F atoms indicate strong covalent bonding. Blue areas in ELFs typically indicate non-covalent bonding, for example, delocalized electrons in metals. k, Bonding features of CsFn (n = 2–5), given in VSEPR notation.

  4. Mechanism of pressure-driven high oxidation states.
    Figure 4: Mechanism of pressure-driven high oxidation states.

    a, Calculated Bader charge of Cs in CsFn at 100 GPa. b, The 6s, 5p and 5s energy levels of the Cs atom in neutral, +1 and +2 charge states (black bars). For comparison, the same levels for Xe0, Xe+ and Xe2+ are shown by orange bars. c, Radial wavefunction of the Cs 6s (black), 5p (orange) and 5s (blue) states. Solid and dashed lines indicate the all-electron and pseudo-wavefunctions, respectively. It clearly shows that the Cs 5p wavefunction has large components outside the Cs+ radius of 1.81 Å. d, Energies of the outermost filled p levels of selected elements, including F, Cs, Rb, K and Ba, as a function of external pressure. The pressure effect is modelled by putting elements in a face-centred cubic (fcc) He matrix. An fcc supercell of 108 He (3 × 3 × 3) is used, in which one He is replaced by the tested atom. It shows that the Cs 5p energy becomes higher than the F 2p energy at pressures higher than 10 GPa, indicating that Cs can be oxidized by F beyond the +1 state under these conditions.

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Affiliations

  1. Materials Research Laboratory, University of California, Santa Barbara, California 93106-5050, USA and

    • Mao-sheng Miao
  2. Beijing Computational Science Research Center, Beijing 10084, China

    • Mao-sheng Miao

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