Structural variations and dielectric properties of (Bi1−xLax)

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$. Polycrystalline samples of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of $0\ensuremath{\le}x\ensuremath{\le}0.1$. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate $x$ at room temperature. This structural variation stems from the change in orientation of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. Synchronizing with the disordering of $\mathrm{Si}{\mathrm{O}}_{4}$ chains, ferroelectric phase transition temperature of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ sharply decreases as $x$ increases, and ferroelectricity finally vanishes at around $x=0.03$. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ through propping the ordered structure of one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of ${(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x})}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ with $x\ensuremath{\le}0.01$, where the ordered one-dimensional $\mathrm{Si}{\mathrm{O}}_{4}$ chains remain.