In this chapter, properties of novel Si-substituted ferroelectric films are presented. The films are a solid solution between Bi $$_{2}$$ SiO $$_{5}$$ (BSO) and conventional ferroelectric materials such as Bi $$_{4}$$ Ti $$_{3}$$ O $$_{12}$$ , SrBi $$_{2}$$ Ta $$_{2}$$ O $$_{9}$$ , and Pb(Zr,Ti)O $$_{3}$$ , which were formed by a sol-gel spin-coating method. It was found that BSO enhanced crystallization of the ferroelectric materials and finally formed solid solutions with them. As a result, the crystallization temperature of the films decreased by 150--200 C, and the ferroelectric and leakage current characteristics did not degrade even in an ultra-thin film of 13 nm in thickness. It was also found that the ferroelectric and insulating characteristics of the BSO-added films were dramatically improved by annealing in high-pressure oxygen up to 9.9 atm. A three-orders-of-magnitude improvement of the leakage current density was observed in BSO-added BLT films after annealing at 9.9 atm, while a pronounced increase in the saturation polarization was observed in BSO-added SBT and PZT films.