Crystal structure, microwave dielectric properties and AC conductivity of B-cation deficient hexagonal perovskites La₅M_xTi _4-xO₁₅ (x = 0.5, 1; M = Zn, Mg, Ga, Al)

Three new n = 5 members of the B-cation deficient hexagonal perovskite A_nB_n-1O_3n family, La₅Zn _0.5Ti_3.5O₁₅, La₅GaTi ₃O₁₅ 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, La₅Mg_0.5Ti_3.5O₁₅, 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, 23000-30000 GHz and a negative temperature coefficient of resonant frequency τ_f from -37 ppm/°C to -55 ppm/°C. τ_f 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 τ_f 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.