Phase structural transition and microwave dielectric properties in isovalently substituted La1−xLnxTiNbO6 (Ln=Ce, Sm) ceramics

Abstract

Aeschynite-type La1−xLnxTiNbO6 (Ln=Ce, Sm, x=0–1) ceramics were prepared via a conventional solid state method. Analysis of X-ray diffraction, Raman, infrared reflectivity spectra and the microstructures revealed a series of composition-induced phase evolution in sequence: monoclinic→coexistence of monoclinic and orthorhombic→orthorhombic structure, i.e. M→M+O→O. The critical compositions of distinguishing the dominant M or O phase were x=0.15 in La1−xCexTiNbO6 and x=0.10 in La1−xSmxTiNbO6 ceramics, exactly corresponding to the average ionic radius of rare earth ions (IR) ~1.027Å. The crystal structure and microwave dielectric properties of RETiNbO6 (RE=rare earth) ceramics strongly depended on IR. Near-zero τf was achieved in the Ce-sample with x=0.153 (εr=28.9, Q×f=17,275GHz@6.54GHz) as well as in the Sm-sample with x=0.098 (εr=28.2, Q×f=19,186GHz@6.78GHz). Eventually, RETiNbO6 would form O euxenite (-τf), O aeschynite (+τf), and M aeschynite (-τf), as IR<0.945Å, 0.945Å <IR<1.027Å, and 1.027Å<IR<1.032Å, respectively. The infrared reflectivity study also confirmed that the structural phonon oscillations in the infrared region dominated the dielectric performance in the microwave region for this system.