Zeolite Beta is accepted as a highly faulted intergrowth of two polymorphs, A and B, which are found in a 60:40 ratio. However, a polymorph C was also suggested as a hypothetical structure. Recently, pure silica polymorph C of Beta family has been obtained using 4,4-dimethyl-4-azonia-tricyclo[5.2.2.02,6]undec-8-ene hydroxide as structure directing agent. The formation of all-silica BEC occurs by a phase transformation from a BEA-BEB intergrown material. In order to favour the intermediate product, is necessary to slow the phase transformation.With this study, it has been possible to obtain most selectivity towards the intermediate phase, in such a way that the presented methodology allows obtaining a Beta zeolite with the highest content in polymorph B up to now.SAED patterns from different areas and directions show that most crystallites contain twins of polymorph B or stacking faults. The twinning and stacking faults are observed more frequently near the surface than in the bulk of the crystallites. The structure of polymorph B has been successfully determined by SAED and HRTEM from the areas of unfaulted polymorph B crystals