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

Abstract In this study, boron-doped apatite-type lanthanum silicate powders (La9.33Si6−xBxO26−0.5x) 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 B³⁺ 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 B³⁺ ions successfully replaced larger Si⁴⁺ 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 B³⁺ doping, confirming that B³⁺ substituted Si⁴⁺ in the form of [BO₄] 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 B³⁺ 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 B³⁺ doping-induced oxygen vacancy formation, which accelerates ion migration, providing a theoretical basis for developing high-performance intermediate-temperature solid oxide fuel cell electrolytes.