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Bismuth compounds are considered the most suitable alternatives to lead-based systems for ferroelectric and piezoelectric applications. Nevertheless little has been done to systematically study the relationships that link the two main classes of ferroelectric bismuth compounds: perovskite and bismuth layer compounds (Aurivillius compounds). The knowledge of these relationships could be a key to improve the performances of the bismuth compounds and could help to better tailor their properties for specific applications. The objective of this project was twofold: 1. To understand the structural and electrical relationships between perovskite and Aurivillius compounds within the Na^Bi^TiO^-Bi^Ti^O^-BaTiCb (NBT-BIT-BT) system; 2. To explore the possibility of obtaining a new perovskite of formula Bi2MgTiOô. A study of 83 different compositions allowed us to sketch the possible phase diagram of the NBT-BIT-BT system. XRD structural characterisation highlighted the importance of low angle analysis in the study of layered compounds. The presence of monophasic regions in the phase diagram stressed the flexibility of this class of compounds that are able to withstand high percentage of vacancies before collapsing. It was shown that, after taking into account some electronic considerations, a simple geometrical model can be use as "rule of thumb" to predict the stability of Aurivillius compounds. New unreported compounds were found in the system and a mechanism for the formation of the Aurivillius compounds is proposed. The dielectric and piezoelectric properties were linked to the fundamental layered structure of the Aurivillius phases showing that all these properties depend on the number of perovskite blocks present. The higher the number of perovskite blocks, higher the values of resistivity and relative permittivity.