Diffuse X-ray scattering and reverse Monte Carlo simulation of the short-range order in ytterbium iodine...

Iodine-doped ytterbium phthalocyanine, C(64)H(32)N(16)YbI(2), [YbPc(2)]I(2) (YPI), presents an example of a disordered modulated crystal structure of the intergrowth type. The short-range order in YPI has been studied at ambient temperature by the diffuse X-ray scattering technique. Both Yb and I atoms are disordered. In a unit cell of the average structure, the Yb atoms occupy two positions, 1/2, 1/2, 1/4 or 1/2, 1/2, 3/4, with occupancy 0.5. In the neighbouring unit cells in the (001) plane there is, however, a strong tendency towards an alternating distribution of ytterbium in positions shifted by Delta z = 1/2. The crystal structure of YPI consists of one-dimensional columnar stacks of [YbPc(2)(2/3+)](n) and of linear iodine chains [I(3)(-)](n). The modulation is commensurate since the period c(I) of iodine chains [I(3)(-)](n) in the [001] direction is equal to one-half of the period c of the [YbPc(2)(2/3+)](n) stacks. In the crystal structure, each stack of [YbPc(2)(2/3+)](n) may occur in two settings shifted by c/2 and each iodine chain in three settings shifted along [001] by one-third of the period c(I). The short-range order of iodine chains distributed over three settings results in a complicated pattern. Iodine chains of the same setting tend to form ribbons perpendicular to <100> and running along [001].