Crystal structure, microwave dielectric properties and AC conductivity of B-cation deficient hexagonal perovskites La5MxTi4–xO15 (x = 0.5, 1; M = Zn, Mg, Ga, Al)

Three new n = 5 members of the B-cation deficient hexagonal perovskite AnBn−1O3n family, La5Zn0.5Ti3.5O15, La5GaTi3O15 and La5AlTi3O15, have been synthesised and their crystal structures determined. Their microwave dielectric properties and AC conductivities, as well as those of a known material, La5Mg0.5Ti3.5O15, were characterized. These four analogues adopt a 10H structure with an AO3 stacking sequence of (hhccc)2 with empty octahedral sites between two hexagonal layers. Zn, Mg, Ga and Al atoms prefer the octahedral sites between two cubic layers to those between cubic and hexagonal layers. The Mg phase exhibits more extensive B-cation order than the Zn, Ga and Al phases. These three new materials exhibit relatively high dielectric permittivity ε, 28–37, modest Q × f values, 23 000–30 000 GHz and a negative temperature coefficient of resonant frequency τf from −37 ppm/°C to −55 ppm/°C. τf of these four members and other n = 5 La-based and Ba-based members of B-cation hexagonal deficient perovskites are compared and the correlations between their τf and tolerance factors are discussed. Based on the comparison of these four materials with BaLa4Ti4O15, correlations between the defects, conductivity and microwave dielectric loss are also addressed.