from ase.build import bulk
from ase import Atom
from ase.io import write

Ni = bulk('Ni', cubic=True) * (2, 2, 2)
H = Atom('H', position=Ni.cell.diagonal()/2)
NiH = Ni + H

write('initial_.data', NiH, format='lammps-data', 
      masses=True, atom_style='atomic')