It was found that the data for polycrystalline samples, at 690 to 906C, were described by:

D (cm2/s) = 6.2 x 10-1 exp[-45.20(kcal/mol)/RT]

M.E.Yanitskaya, A.A.Zhukhovitskii, S.Z.Bokshtein: Doklady Akademii Nauk SSSR, 1957, 112, 720