The appearance of twins and polytypes during high-pressure synthesis has been observed in several systems. It bas been found that even small differences in the p-T conditions or the degree of approach to equilibrium can lead to large differences in the periodicity of the resulting crystal(s) . For example, high-pressure synthesis of boron suboxide B 6 O results in the formation of two polytypes: α-B 6 O with ABCABC stacking of B 12 icosahedra and β-B 6 O with ABAB stacking of B 12 icosahedra ; and twinning of α-B 6 O can produce local β-B 6 O stackings. Thus, the polytypes of all boron-rich chalcogenides B 6 X (X = O, S, Se) are characterized by different stacking of B 12 icosahedra, making them cardinally different from the ʻpolytypesʼ of boron carbide based on varying carbon content and site-specific placement of carbon atoms in the icosahedra .