Crystal structure and microwave dielectric properties of a new A6B5O18-type cation-deficient perovskite Ba3La3Ti4NbO18

A new microwave dielectric ceramic Ba3La3Ti4NbO18 has been synthesized and characterized. Through Rietveld refinement of X-ray powder diffraction data, Ba3La3Ti4NbO18 is identified as the A6B5O18 type cation-deficient perovskites with space group R3¯m and lattice constants a = b = 5.6198(1) Å, and c = 41.6402(4) Å, V = 1138.90(1) Å3 and Z = 3. The crystal structure can be described as consisting of identical perovskite-like blocks, five corner-sharing (Ba, La)O6 octahedra thick, separated by layers of vacant octahedral. The (Ba, La)O bond lengths range from 2.526(5) to 3.262(7) Å and the (Ti, Nb)O bond lengths range from 1.784(5) to 2.227(6) Å. Further, the effects of ionic polarizability and crystal structure on the microwave dielectric properties of A6B5O18 ceramics in the BaO-La2O3-TiO2-Nb2O5 system are discussed.