Characterization of Ba4.5Re9Ti18O54 (Re = La, Nd) microwave dielectric ceramics

Ba4.5Re9Ti18O54 (Re = La, Nd) ceramics were prepared via a solid state mixed oxide route. X-ray diffraction (XRD) analysis revealed the formation of the major Ba4.5Re9Ti18O54 phase along with rutile (TiO2) as a secondary phase. Rietveld structure refinement of the recorded XRD data confirmed that the crystal structure of Ba4.5Nd9Ti18O54 (BNT) was orthorhombic (Pnma) with unit cell parameters a = 22.3412 , b = 7.6824 and c = 12.1952 . Ba4.5La9Ti18O54 (BLT) exhibited high relative permittivity (95.6), low quality factor (2,102 GHz) and a high temperature coefficient of resonance frequency (+352 ppm/A degrees 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/A degrees 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.