We have used the high intensity x-ray beam on beamline I15 to examine the crystal structures of the alkali metals Na and K to above 100 GPa (1 million atmospheres). In Na, we have utilised the low melting temperature near 100 GPa to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using single-crystal diffraction. We confirm that at this pressure sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. In K, our ab initio electronic structure calculations revealed that an orthorhombic structure (oP8), also found in sodium, had a lower enthalpy than other candidate crystal structures recently reported to be stable in K at high pressure. Subsequent powder-diffraction studies to above 100 GPa on I15 confirmed oP8 to be the stable phase of potassium from 54(2) to 90(2) GPa.
Figure 1: One quadrant of a composite diffraction image showing representative data from cI16-Na at 108 GPa. Eight observed reflections are marked and indexed. One part of the image has been enhanced to reveal the (112) reflection. Figure taken from McMahon et al, Proc.Nat. Acad. Sci. 104, 17297 (2007)
At ambient conditions, the valence electrons in the alkali metals have only very weak interactions with the ion cores. These metals are therefore regarded as “simple”, with electronic properties that are well described by the nearly-free electron (NFE) model. At high pressures, however, the NFE model is no longer valid, and the behaviour of such metals under these conditions is of fundamental interest.
The alkali metals all adopt the body-centred cubic (bcc) crystal structure at ambient conditions, but compression leads to the formation of a wide variety of lower-symmetry and often very complex crystal structures . In Na, this is accompanied by a dramatic reduction in the melting temperature, which falls from 1000 K at 30 GPa to ~300 K at 118 GPa . We have used this unique behaviour to grow a single-crystal of the first of the complex structures of Na, with the aim of determining its full crystal structure.
The wealth of complex behaviour observed in the alkali metals at high pressures has prompted a large number of computational studies. A recent such study of Li, K, Rb, and Cs, using an evolutionary algorithm to search for the lowest-energy structures to very high pressures, predicted that the phase transition sequence in K above 25 GPa would mirror that known in Rb and Cs, with transitions first to the tetragonal tI4 structure and then to the orthorhombic oC16 structure . In an extension of our diffraction studies of Na, we found that an orthorhombic structure with eight atoms per unit cell (oP8-Na) is the stable phase between 117 and 125 GPa at room temperature . Although this structure was not reported as a low-energy candidate in the computational study of K, our own calculations suggested that this phase would be seen on compression prior to the tI4 and oC16 phases. To investigate this, we used powder-diffraction techniques on I15 to investigate K to above 100 GPa.
The Na data were collected on I15 using single-crystal techniques and an incident wavelength of 0.3444 Å, while the powder study of K used a wavelength of 0.4403 Å. The beam size in each case was approximately 50 microns.
The Na data were straightforwardly indexed as coming from a body-centred cubic unit cell with a lattice parameter of 5.461(1) Å. The systematic absences were consistent with the cI16 structure reported previously , in which atoms lie on the 16c sites (x, x, x) of spacegroup I-43d. The final single-crystal refinement of 15 unique reflections converged to an R factor of 3.3% and a goodness of fit of 1.37, and the previously unknown value of x was determined accurately as 0.044(1).
The structure refinement of Na on I15 resulted in the first publication from Diamond, and was the first single-crystal structure refinement ever carried out above 100 GPa. Using the same diffraction techniques, which were developed as a Long Term Project at the ESRF, we have subsequently pushed single-crystal studies of Na to 165 GPa at the ESRF, and have revealed a wealth of extremely complex structures near the melting minimum at 118 GPa .
Figure 2: The crystal structure of oP8-K at 58 GPa, as viewed down the crystallographic b axis. Light grey atoms are at y=1/4, and dark grey atoms are at y=3/4.
In K, only the known phases (bcc, fcc and host-guest composite) were observed up to 54(2) GPa, where new peaks began to appear in the diffraction patterns. The transition was complete at 56 GPa. While the new phase was clearly not from the tI4 structure predicted by Ma et al. , its diffraction pattern was very similar to that expected from the oP8 structure predicted in our calculations. A subsequent Rietveld refinement using the calculated oP8 structure gave an excellent fit. Further compression showed the oP8 phase of potassium to be stable to 90(2) GPa, where it transforms to the tI4 phase, with a further transition to the oC16 phase at 96(3) GPa.
Our results on K showed that it differs from the high-pressure behaviour of Rb and Cs in sharing the oP8 phase with Na, even though K has the possibility of s-d hybridisation of electron states, as found in Rb and Cs, rather than the proposed p-d hybrid character of Na .
In conclusion, I15 has been used to push structural studies of Na and K to above 100 GPa. In Na at 108 GPa, we confirm that sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. In K, we find the orthorhombic oP8 structure to be the stable phase from 54 to 90 GPa, where it transforms to the tetragonal tI4 structure, which in turn transforms to the orthorhombic oC16 structure at 96 GPa.
 M.I. McMahon and R.J. Nelmes, Chem. Soc. Rev. 35, 943 (2006).
 E. Gregoryanz et al., Phys. Rev. Lett. 94, 185502 (2005).
 Y. Ma, A. R. Oganov, and Y. Xie, Phys. Rev. B, 78, 014102 (2008).
 E. Gregoryanz et al., Science, 320, 1054 (2008).
 K. Syassen, in «High Pressure Phenomena», Proceedings of the International School of Physics, (IOS Press, Amsterdam), 266–268 (2002).
Principal Publications and Authors
M. I. McMahon, E. Gregoryanz, L. F. Lundegaard, I. Loa,
C. Guillaume, R. J. Nelmes, A. K. Kleppe, M. Amboage,
H. Wilhelm, and A. P. Jephcoat, Structure of sodium above 100 GPa by single-crystal x-ray diffraction, Proc. Nat. Acad. Sci. 104, 17297–17299 (2007).
L. F. Lundegaard, M. Marqués, G. Stinton, G. J. Ackland,
R. J. Nelmes, and M. I. McMahon, Observation of the oP8 crystal structure in potassium at high pressure, Physical Review B, 80, 020101(R) (2009).
Engineering and Physical Sciences Research Council, UK.
Research carried out at Diamond on I15 and the ESRF on ID09 and ID27.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
Copyright © 2018 Diamond Light Source
Diamond Light Source Ltd
Harwell Science & Innovation Campus