Diffuse X-ray scattering and reverse Monte Carlo simulation of the short-range order in ytterbium iodine phthalocyanine [YbPc2]I2

Iodine-doped ytterbium phthalocyanine, C64H32N16YbI2, [YbPc2]I2 (YPI), presents an example of a disordered modulated crystal structure of the intergrowth type. The short-range order in YPI has been studied at ambienttemperature 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, \textstyle {{1} \over {2}}, \textstyle {{1} \over {2}}, \textstyle {{1} \over {4}} or \textstyle {{1} \over {2}}, \textstyle {{1} \over {2}}, \textstyle {{3} \over {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 Δz = \textstyle {{1} \over {2}}. The crystal structure of YPI consists of one-dimensional columnar stacks of [{\rm YbPc} _{2} ^{2/3+}] n and of linear iodine chains [{\rm I} _{3} ^{-}] n . The modulation is commensurate since the period c I of iodine chains [{\rm I} _{3} ^{-}] n in the [001] direction is equal to one-half of the period c of the [{\rm YbPc} _{2} ^{2/3+}] n stacks. In the crystal structure, each stack of [{\rm 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].