Structure–Property Map for Bi _{1– x} La _x VO ₄ Ceramics: Polymorphism, Mixed Conduction, and Dielectric Response

The structural flexibility of BiVO 4 -based ceramics enables diverse electrical functionalities, ranging from oxide ion conductivity to promising dielectric performance. This study investigated the structural evolution, electrical transport, and dielectric behavior of Bi 1– x La x VO 4 (0 ≤ x ≤ 1) ceramics, in which Bi 3 + is progressively substituted by La 3 + . Using variable-temperature X–ray diffraction (VT–XRD) and neutron powder diffraction (NPD), the phase transitions and crystallographic parameters were systematically analyzed. La 3 + substitution induces a structural transformation from monoclinic fergusonite-type BiVO 4 to tetragonal zircon-type BiVO 4 and, ultimately, to a monoclinic monazite-type LaVO 4 phase. Impedance spectroscopy and EMF measurements reveal that electronic conduction dominates across the series, with Bi 0.8 La 0.2 VO 4 exhibiting the highest conductivity of 2 × 10 –3 S cm –1 in air at 700 °C. Dielectric characterization shows that La 3 + incorporation slightly improves thermal stability, with the temperature coefficient of resonant frequency (TCF) in Bi 0.9 La 0.1 VO 4 improving from −280 to −156 ppm/°C. In addition, it features a dielectric constant of 52 and a quality factor of 4360 GHz. These findings provide comprehensive insight into the structure–property relationships in La-substituted BiVO 4 ceramics and demonstrate their potential for applications in electronic and energy-related devices.