NbCl has a hexagonal close packed array of chloride ions. Triangles of niobium occur in octahedral spaces in the chloride array. The compositions with higher chloride have some niobium atoms missing from the structure, creating vacancies and giving rise to nonstoichiometric compounds. NbCl has this pattern of vacancies stretched until the niobium atoms are in pairs rather than triangles. So NbCl can be considered as a solid solution of NbCl and NbCl.
The colour of niobium trichloride varies depending on the niobium:chloride ratio. NbCl is green, while NbCl is brown.