High-Pressure Melt Curve and Phase Diagram of Lithium

Mungo Frost, Jongjin B. Kim, Emma E. McBride, J. Ryan Peterson, Jesse S. Smith, Peihao Sun, and Siegfried H. Glenzer

Phys. Rev. Lett. 123, 065701 – Published 5 August 2019

Abstract

We investigate the phase diagram of lithium at temperatures of 200 to 400 K, to pressures over 100 GPa using x-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the $h R 1$ and $c I 16$ phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40 and 60 GPa, below which the solid is crystalline. Analysis of the stability fields of the $c I 16$ and $o C 88$ phases suggest the existence of a triple point between these and an undetermined solid phase at 60 GPa between 220 and 255 K.

Received 22 April 2019
Revised 11 June 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.065701

Physics Subject Headings (PhySH)

Crystal melting Pressure effects

Pressure techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Mungo Frost¹, Jongjin B. Kim¹, Emma E. McBride¹, J. Ryan Peterson^1,3, Jesse S. Smith², Peihao Sun^1,3, and Siegfried H. Glenzer¹

¹SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
²High Pressure Collaborative Access Team, X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
³Physics Department, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305, USA

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Issue

Vol. 123, Iss. 6 — 9 August 2019

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Images

Figure 1
Phase diagram of lithium. Filled circles show data points collected in this study, with orange circles indicating crystallinity, evidenced by Bragg diffraction, but with inconclusive indexing. Crosses and open symbols show melting points determined by other studies. Phase boundaries marked with solid lines take into account data from both this and other studies [8, 10, 11, 12, 13, 14, 15, 18, 24, 25]. Gray dashed lines for the $o C 88$ , $o C 40$ , and $o C 24$ phases are from Ref. [8]. The dotted boundary of $c I 16$ near 60 GPa and 230 K indicates the probable location of a triple point, with an uncharacterized phase between $c I 16$ and the melt in blue. On isobaric cooling at low pressure the bcc to fcc transition does not occur, instead the metastable $9 R$ state forms from bcc lithium at lower temperature, as indicated by the striped region and black dotted line [10]. All boundaries below 50 K are extrapolated.
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Figure 2
Pressure vs time for the six runs. Stars indicate where compression was paused to realign the sample, the dagger indicates where the rate of pressure increase on the membrane was dropped during the 320 K run. The much higher compression rate than is customary leads to a much shorter experimental duration and allows data to be collected before the lithium degrades the diamond anvils.
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Figure 3
Top: integrated powder pattern of $h R 1$ lithium at 255 K and 44 GPa. Data (black) with LeBail fit (blue) and residual (below). Tics indicate possible $h R 1$ and tungsten peaks as marked. The unintegrated pattern (inset) is the difference of five frames before and after transformation to the higher pressure phase with negative values suppressed. Middle: a similar plot for $c I 16$ lithium. Diamond and tungsten peaks were masked before integration. Additional inset shows pressure vs volume data for $c I 16$ lithium. Bottom, left, and center: Patterns prepared as above at 53 GPa in the region between $c I 16$ and the melt, red circles indicate lithium diffraction. Right: angles of observed peaks and the possible reflections of various phases. Reflections observed are not compatible with $c I 16$ , while $o C 88$ has a very large number of potential reflections which makes indexing ambiguous. Two candidates from theoretical studies, $C 2$ [4, 28] and $P b c a$ [3, 4] are also shown. $P b c a$ shows the best fit of all structures tried. X-ray diffraction was carried out using 0.4066 Å radiation.
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Figure 4
Observed $d$ spacings of diffraction peaks between 0.9 and 2.3 Å from high pressure lithium. Lines linking points imply that they occur at the same detector location shot to shot and are identified as the same reflection. Pale blue lines indicate the first four diffraction lines of tungsten [27]. Red vertical lines indicate maximum pressure of a run. Data for higher $d$ spacings are presented in the Supplemental Material [29].
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