Characterization of Ba_4.5Re₉Ti₁₈O₅₄ (Re = La, Nd) microwave dielectric ceramics

Ba_4.5Re₉Ti₁₈O₅₄ (Re = La, Nd) ceramics were prepared via a solid state mixed oxide route. X-ray diffraction (XRD) analysis revealed the formation of the major Ba_4.5Re₉Ti₁₈O₅₄ phase along with rutile (TiO₂) as a secondary phase. Rietveld structure refinement of the recorded XRD data confirmed that the crystal structure of Ba_4.5Nd₉Ti₁₈O₅₄ (BNT) was orthorhombic (Pnma) with unit cell parameters a = 22.3412 Å, b = 7.6824 Å and c = 12.1952 Å. Ba_4.5La₉Ti₁₈O₅₄ (BLT) exhibited high relative permittivity (95.6), low quality factor (2,102 GHz) and a high temperature coefficient of resonance frequency (+352 ppm/°C). The substitution of Nd for La caused a decrease in both the relatively permittivity and temperature coefficient of resonance frequency to 84.2 and 167 ppm/°C respectively and an increase in quality factor to 8,007 GHz. Raman spectroscopic analysis revealed that lattice defects may be responsible for the observed decrease in quality factor of BLT ceramics in comparison to BNT. The Raman shifts at 533.5 and 613.6 cm⁻¹, related to Ti–O bond stretching, decreased for BNT ceramics, which may be a possible reason for the observed decrease in relative permittivity.