code: def _split_compound_string_(compound_string):
      
  
      formula = compound_string.replace(']', '').split('[')[0]
      phase = compound_string.replace(']', '').split('[')[1]
  
      return formula, phase
docs: Split a compound's combined formula and phase into separate strings for
      the formula and phase.
  
      :param compound_string: Formula and phase of a chemical compound, e.g.
        'SiO2[S1]'.
  
      :returns: Formula of chemical compound.
      :returns: Phase of chemical compound.