Structural studies of boracites. IV. Thermal motion in cubic Ni₃B₇O₁₃I at 77K

For pt.III see ibid., vol.9, p.665 (1976). The crystal structure of cubic nickel iodine boracite, Ni₃B₇O₁₃I, has been investigated by neutron diffraction at 77K. Three-dimensional data were collected out to sin theta / lambda =0.73 AA^-1 from a ¹¹B-enriched single-crystal (100) growth sector. The final R-index was 3.4%. To investigate possible anharmonicity in the thermal motion of the metal (Ni) and halogen (I) atoms, third- and fourth-cumulant parameters were considered; only the fourth-cumulant parameters for Ni gave significant improvements in fit. Contrary to previous suggestions, it is not necessary to postulate disorder in the positions of the metal and halogen atoms in order to explain the experimental results.