Structural variations and dielectric properties of (Bi1-xLax)(2)SiO5 (0 <= x <= 0.1): Polycrystallines synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric Bi2SiO5. Polycrystalline samples of (Bi1-xLax)(2)SiO5 are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of 0 <= x <= 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 SiO4 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 SiO4 tetrahedra. Synchronizing with the disordering of SiO4 chains, ferroelectric phase transition temperature of (Bi1-xLax)(2)SiO5 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 Bi2SiO5 through propping the ordered structure of one-dimensional SiO4 chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of (Bi1-xLax)(2)SiO5 with x <= 0.01, where the ordered one-dimensional SiO4 chains remain.