  A series of MD and DFT simulations were performed to investigate hydrogen
self-clustering and retention in tungsten. Using a newly develop machine
learned interatomic potential, spontaneous formation of hydrogen platelets was
observed after implanting low-energy hydrogen into tungsten at high fluxes and
temperatures. The platelets formed along low miller index orientations and
neighboring tetrahedral and octahedral sites and could grow to over 50 atoms in
size. High temperatures above 600 K and high hydrogen concentrations were
needed to observe significant platelet formation. A critical platelet size of
six hydrogen atoms was needed for long term stability. Platelets smaller than
this were found to be thermally unstable within a few nanoseconds. To verify
these observations, characteristic platelets from the MD simulations were
simulated using large-scale DFT. DFT corroborated the MD results in that large
platelets were also found to be dynamically stable for five or more hydrogen
atoms. The LDOS from the DFT simulated platelets indicated that hydrogen atoms,
particularly at the periphery of the platelet, were found to be at least as
stable as hydrogen atoms in bulk tungsten. In addition, electrons were found to
be localized around hydrogen atoms in the platelet itself and that hydrogen
atoms up to 4.2 Angstrom away within the platelet were found to be bonded
suggesting that the hydrogen atoms are interacting across longer distances than
previously suggested. These results reveal a self-clustering mechanisms for
hydrogen within tungsten in the absence of radiation induced or microstructural
defects that could be a precursor to blistering and potentially explain the
experimentally observed high hydrogen retention particularly in the near
surface region.
