No experimental Fermi surface measurements have been reported or made on low-temperature martensitic lithium

Elatresh et al. (1) (ECAHDB) claim that the low-temperature martensitic structure of lithium “is not the heretofore assigned 9R.” This is based on comparing results of Fermi surface calculations to de Haas–van Alphen (dHvA) data from Springford and coworkers (2, 3). We note that Springford’s group took extensive steps to preserve the body-centered-cubic (bcc) structure and to avoid the temperature-induced martensitic transition. Therefore, the conclusion that their dHvA data are “inconsistent with 9R” is unsurprising, and is in fact intentional.

It appears that unsuccessful attempts were made long ago to perform dHvA experiments on lithium martensite (4). Obtaining dHvA signals from martensitic microstructures is usually impossible: The high defect density produces additional electron scattering that suppresses quantum oscillations (4, 5). The loss of dHvA signal was even used to identify a martensitic transition in rubidium (6). These experimental observations seem to have been overlooked when ECAHDB state that “in the presence of even partial phase transition to 9R there should be quantum-oscillations different from the ones seen [by Springford].”

Springford’s group used micron-sized polycrystalline dispersions of lithium embedded in paraffin wax to retain the bcc structure at liquid-helium temperatures (2, 3). The structure was verified by X-ray diffraction (3). No indication of the martensite was found, and it was concluded that at most 6% martensite phase fraction could have remained undetected (3). The grain size distribution was carefully characterized: The vast majority were below 10 $\mu$m (2, 3), at stark variance with the $\sim$200 $\mathrm{\mu }$m quoted in ref. 1.

ECAHDB’s experimental work involves a completely different sample preparation: Individual lithium chips of significantly larger size ($\sim$100 $\mathrm{\mu }$m) were studied at gigapascal pressures with X-ray diffraction. No attempt was made to suppress the martensitic transition, and the usual bcc/martensite mixture was duly found. No dHvA measurements were performed by ECAHDB; their diffraction experiments are presented as undermining the reliability of Springford’s sample characterization. Since (i) the original sample was shown by X-ray diffraction to be bcc, (ii) the dHvA data are exactly as expected for bcc lithium, and (iii) a martensite is unlikely to give a dHvA signal, to claim that the dHvA data show a signature of something else seems unrealistic.

We conclude that the dHvA measurements by Springford’s group were done on bcc lithium, as originally claimed (3). It might be desirable to obtain dHvA data for the low-temperature phase, especially at pressure, but this is probably impossible (4–6). As no experimental dHvA data are available for the martensite phase of lithium, no conclusions regarding its crystal structure should be drawn from Fermi surface analysis. It is correct that neither the ground state nor the low-temperature structure of lithium is 9R (7), but this cannot be inferred from the work of ECAHDB.