Anisotropic dispersion of the band structure and formation of ring patterns in CBED

An off-axis ring pattern was observed in the (111) zone-axis CBED pattern of an NiO single crystal. The pattern was interpreted as resulting from anisotropic dispersion of the band structure of the two most strongly excited free Bloch waves and strong absorption of the most tightly bound Is Bloch state. Comparison has also been made with the case of MgO which has the same structure as NiO but weaker scattering power. The tightly bound Is Bloch state in MgO was found to play an important role together with a strongly excited free Bloch state and a more conventional zone-axis ring pattern similar to that observed in the (001) zone-axis CBED pattern of silicon was observed.