Layer modulating and electrical properties in acceptor-doped Sr₅Nb₅O₁₇ ceramic with perovskite layer structure

Perovskite layer structured (PLS) oxides exhibit some novel physical properties, such as ultrahigh-temperature piezoelectricity, semiconductivity, and oxygen ionic transport. However, synthesizing 5-layer PLS oxides, such as Sr₅Nb₅O₁₇ ceramics, using traditional solid-state reaction methods is challenging due to their low phase stability. In this study, we propose a new strategy to construct a 5-layer structure from a 4-layer structure. Specifically, Ga-doped Sr₅Nb_4.444Ga_0.556O_16.944 (5-SNGO), an iso-structural material to Sr₅Nb₅O₁₇, was synthesized via a solid-phase reaction. The original design involved co-doping Ga and Mo at the Nb site of the 4-layer parent material Sr₂Nb₂O₇ (4-SNO). However, it was found that only Ga was successfully incorporated into the structure, while Mo remained as SrMoO₄, which was subsequently removed by washing with dilute sulfuric acid. X-ray diffraction (XRD), transmission electron microscopy (TEM), and second-harmonic generation (SHG) analysis confirmed that the synthesized 5-SNGO ceramic exhibits a 5-layer structure with a noncentrosymmetric space group. The material demonstrated a frequency-independent dielectric permittivity of 60 above 1 kHz. Impedance spectroscopy revealed a very high resistivity of 1.95 × 10⁵ ·cm at 900 °C, along with Debye-like dielectric relaxation exhibiting thermal activation behavior. This study presents a novel synthesis approach for constructing 5-layer PLS oxides from a 4-layer structure and provides insights into their structural evolution and electrical properties.