1:2 Cation order in A(Li1/3(Nb,Ta)2/3)O3 microwave perovskites

Abstract

In a family of perovskites with the general stoichiometry $({\mathrm{A}}_{\text{2/3}}^{\text{2+}}{\mathrm{La}}_{\text{1/3}}\mathrm{)(}{\mathrm{B}}_{\text{1/3}}^{\text{1+}}{\mathrm{B}}_{\text{2/3}}^{\text{5+}}){\mathrm{O}}_3,$ three 1:2 ordered perovskites were prepared with $\mathrm{A}=\mathrm{Sr},$ ${\mathrm{B}}^{\text{1+}}=\mathrm{Li},$ ${\mathrm{B}}^{\text{5+}}=\mathrm{Nb},$ Ta and with $\mathrm{A}=\mathrm{Ca},$ ${\mathrm{B}}^{\text{1+}}=\mathrm{Li},$ ${\mathrm{B}}^{\text{5+}}=\mathrm{Nb}.$ These compounds can be sintered to high density at temperatures $\text{⩽1350\hspace{0.05em}°}\mathrm{C},$ have dielectric constants in the range 25–30, microwave quality factors as high as 26 000, and negative temperature coefficients of resonant frequency. A 1:2 polymorph was not observed for $({\mathrm{Ca}}_{\text{2/3}}{\mathrm{La}}_{\text{1/3}}\mathrm{)(}{\mathrm{Li}}_{\text{1/3}}{\mathrm{Ta}}_{\text{2/3}}){\mathrm{O}}_3$ which adopted a 1:1 ordered structure. The larger charge difference between the ions involved in the B-site ordering in these systems compared to other 1:2 ordered microwave perovskites such as $\mathrm{Ba}({\mathrm{Zn}}_{\text{1/3}}{\mathrm{Nb}}_{\text{2/3}}){\mathrm{O}}_3,$ is reflected by a higher stability of the order which is stable up to their decomposition/melting temperature.