Research ArticleSOLID-STATE CHEMISTRY

Quantum and isotope effects in lithium metal

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1254-1259
DOI: 10.1126/science.aal4886
  • Fig. 1 Observed stable and metastable crystal structures of 6Li and 7Li measured along the identified P-T paths.

    (A) Isobaric results for 6Li. Isobaric cooling paths are connected by gray lines as guides to eye. Data points collected during isothermal compression or isobaric warming are labeled in numerical order. We used mineral oil (crossed symbols) or He (dotted and solid symbols) as pressure-transmitting media. Blue dotted lines show the onset of bcc → close-packed transitions upon cooling. The dot-dashed line shows the tentative bcc-fcc boundary based on the limited data available for this region. (B) Isobaric results for 7Li. Open symbols are data from previous studies that used either mineral oil or no pressure medium during isothermal compression and isobaric cooling (7, 23, 42). Points 3 and 7 are very close in P and T but were approached via different thermal paths; the resulting structures are 9R + bcc and fcc + bcc, respectively. (C) Experimental paths for 6Li in P-T space to examine the possibility of a reverse fcc → 9R transformation during decompression. Dotted and dot-dashed lines are the transition lines from (A). During decompression, we observed the pure fcc structure deep in what was previously identified as the 9R stability region. (D) Experimental paths for 7Li in P-T space with the same observation of the fcc structure in the 9R stability region. Dotted lines are the transition lines from (B). Points 12 to 14 show the martensitic transition of 7Li during isothermal compression, followed by a transition to fcc. Error bars in all panels denote the estimated experimental uncertainty in the temperature of the sample and are comparable to the symbol size where not shown.

  • Fig. 2 Synchrotron x-ray diffraction patterns of 6Li at different pressures and temperatures.

    Angle-dispersive diffraction measurements were performed using a wavelength of 0.4066 Å. (A) Selected diffraction patterns of 6Li from three different cooling paths (points 1 → 4, 8 → 10, and 11 → 13 in fig. S6). The reflections from bcc (red) and martensitic (blue) phases are labeled by their hkl indices, using the 9R structure for the martensite. Not all 9R peaks are visible because the sample recrystallized to a highly textured quasi–single crystal. (B) Diffraction patterns of 6Li during cooling to the base temperature, 17 K, and isothermal decompression to 0.5 GPa (points 18 → 22 in fig. S6). Only the pure fcc phase (green) was observed (Fig. 2). For clarity, the Compton scattering of the diamonds and reflections from the cryostat window have been removed in both (A) and (B). The data at 4.3 GPa show line broadening consistent with nonhydrostatic strain of up to 1%. This is likely due to a partial escape of the helium pressure-transmitting medium as the pressure on the cell was released.

  • Fig. 3 Phase transitions in lithium as a function of pressure and temperature.

    (A) Experimental observations of bcc, fcc, and martensitic (9R and disordered) polytypes of 7Li upon cooling and warming at zero pressure (16, 18, 21, 29, 37). (B) Calculated thermodynamic phase boundary between bcc and fcc, and metastable bcc-hcp and bcc-9R boundaries. Small symbols indicate the DFT results for 7Li; the corresponding lines are interpolations. The corresponding 6Li lines are a few kelvin lower (fig. S16). White lines indicate the experimental transition lines of 7Li during isobaric cooling from this work and (24). Large symbols show the observed phases upon isothermal pressure changes (Fig. 1D). We can relate the calculated phase boundaries to the temperature-driven transitions at zero pressure [gray bars connecting (A) and (B)]. The equilibrium phase diagram contains only bcc and fcc; the solid black line represents the only phase boundary in this region of P-T space.

  • Fig. 4 Structures of lithium.

    (A) Complex, non-9R stacking sequence in the martensite from the MD simulations, viewed along the [111]bcc body diagonal from the initial bcc crystal. The close-packed layers are shown edge-on, exposing the stacking sequence. Atoms with a local hcp (h) or fcc (k) environment are shown in dark purple and cyan, respectively (43). Other colors mark atoms without close-packed coordination at cell and grain boundaries. There are three separate twins, as well as a region of pure fcc (which is a known finite-size effect) (44). Such fcc regions form only in MD cells with orientations incompatible with a two-twin microstructure. MD simulations in supercells rotated 45° about [100]bcc produced microstructures with only two twins. Simulations starting with bcc nanocrystals produced an ultrafine twinned microstructure that coarsened slowly. Simulations with high cooling rates remained in bcc. (B) Relationship among fcc, hcp, 9R, and bcc. Top: The close-packed structures can be identified by their stacking sequences of hexagonal layers along the z axis. In the ABC notation, the letters refer to different atomic positions in the xy plane. The hk notation removes the arbitrary choice of origin and labels a layer as hexagonal (h) if the layers directly above and below the central layer are of the same type, and as cubic (k) otherwise. The sequence ABACACBCB for the 9R structure translates to hhk hhk hhk. Unit cells are indicated with dark dashed lines. Bottom: Illustration of the martensitic mechanism in which the bcc (110)bcc layers of atoms transform into close-packed (001)hcp layers and shuffle into the appropriate stacking sequence, shown here for the bcc → hcp transformation. The bcc → fcc transition requires a substantial shear strain parallel to (110), three times that required for the transition to 9R.

  • Fig. 5 Simulated diffraction patterns of lithium.

    Comparison of neutron diffraction data (wavelength λ = 1.288 Å) from 7Li at T < 20 K and zero pressure (38) with simulated diffraction patterns from (top to bottom) various candidate phases of lithium, a random sequence of close-packed layers, the stacking sequence of the main twin obtained in the MD simulation, and an unweighted average of all three stacking sequences observed in the simulated martensite shown in Fig. 4A. The neutron diffraction pattern is a combination of bcc and martensite; the positions of the bcc peaks are replicated by the tick marks at the bottom. The Miller indices refer to the 9R structure.

  • Fig. 6 Equations of state for fcc 6Li and 7Li.

    The atomic volume of the fcc phase of 6Li and 7Li at low temperature is plotted as a function of pressure, showing agreement of experiment and theory. The orange and gray lines indicate the third-order Birch-Murnaghan equation of state fits to the data from 7Li and 6Li, respectively, in the region where data were obtained for both isotopes (shaded region). Open symbols show the calculated equation of state for the lithium isotopes in the fcc phase. See (26) for details of all equations of state.

Supplementary Materials

  • Quantum and isotope effects in lithium metal

    Graeme J. Ackland, Mihindra Dunuwille, Miguel Martinez-Canales, Ingo Loa, Rong Zhang, Stanislav Sinogeikin, Weizhao Cai, Shanti Deemyad

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Supplementary Text
    • Materials and Methods
    • Figs. S1 to S17
    • Tables S1 to S6
    • References

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