Preparation of a Silicon-Site Boron-Doped Apatite-Type Lanthanum Silicate Solid Electrolyte and Study of the Conductivity Enhancement Mechanism

In this study, boron-doped apatite-type lanthanum silicate powders (La _9.33 Si _{6− x} B _x O _{26−0.5 x} ) were successfully synthesized at 600°C via the urea‒nitrate combustion method. High-conductivity solid electrolytes were obtained by sintering at 1500°C, and the effects of B ³⁺ doping at silicon sites on the material structure were investigated. Optimization of the sintering temperature revealed that samples sintered at 1500°C exhibited a shrinkage rate, with a uniform grain size and no pores. The smaller B ³⁺ ions successfully replaced larger Si ⁴⁺ ions, leading to lattice contraction and a reduction in the unit cell volume from 587.83 ų to 584.36 ų. IR analysis revealed that the infrared vibration peaks of Si-O bonds shifted to higher wavenumbers due to B ³⁺ doping, confirming that B ³⁺ substituted Si ⁴⁺ in the form of [BO ₄ ] tetrahedra. XPS results demonstrated an increased oxygen vacancy concentration and enhanced covalency of Si–O bonds via charge compensation effects. Electrochemical impedance tests indicated that when the B ³⁺ doping amount reached x  = 0.4, the material achieved the highest ionic conductivity of 1.14×10 ^− 3 S/cm at 600°C. This study reveals the conductivity enhancement mechanism through B ³⁺ doping-induced oxygen vacancy formation, which accelerates ion migration, providing a theoretical basis for developing high-performance intermediate-temperature solid oxide fuel cell electrolytes.