Incommensurate modulations of Bi-III and Sb-II

Single-crystal diffraction data from the incommensurate host-guest composite structure of Bi-III-stable between 2.8 and 7.7 GPa-are found to contain very weak satellite reflections arising from structural modulations attributed to interactions between the host and guest structures. The principal effects of the modulation are a quasipairing of the guest atoms along their chains and some reduction in the closest approach of the host and guest atoms. It is suggested that these modulations play a key role in the stability of the structure. A similar modulated structure fits powder-diffraction data from Sb-II.