  Superconductivity intrinsic to the intercalated black phosphorus (BP) with a
transition temperature Tc of 3.8 K, independent of the intercalant, whether an
alkali or an alkaline earth element, has been reported recently by R. Zhang et
al. (2017). However, the reported Tc and the field effect on the
superconducting (SC) transition both bear great similarities to those for the
pure Sn, which is commonly used for BP synthesis under the vapor transport
method. We have therefore decided to determine whether a minute amount of Sn is
present in the starting high purity BP crystals and whether it is the culprit
for the small SC signal detected. Energy-dispersive X-ray spectroscopy results
confirmed the existence of Sn in the starting high purity BP crystals purchased
from the same company as in R. Zhang et al. (2017). We have reproduced the SC
transition at 3.8 K in Li- and Na-intercalated BP crystals that contain minute
amounts of Sn when prepared by the vapor transport method but not in BP
crystals that are free of Sn when prepared by the high-pressure method. We have
therefore concluded that the SC transition reported by R. Zhang et al. (2017)
is associated with the Sn but not intrinsic to the intercalated BP crystals.
