New 10-layer hexagonal perovskites: Relationship between cation and vacancy ordering and microwave dielectric loss

The new ceramic materials Ba₁₀Mg_0.25Ta _7.9O₃₀ and Ba₁₀Co_0.25Ta _7.9O₃₀ have been synthesized in isolation and their dielectric and crystallographic properties characterized fully. The crystal structure determination, using a combination of X-ray and neutron powder diffraction and electron microscopy, reveals a 10-layer perovskite structure that involves the (hcccc)₂ stacking sequence with a pair of face-sharing octahedral sites separating three layers of corner-sharing octahedra. The structure is discussed in comparison with the related eight-layer Ba₈ZnTa₆C₂₄, which involves the (hccc) ₂ stacking sequence, discovered in an earlier study. The microwave dielectric properties of both materials are determined (Ba₁₀Mg _0.25Ta_7.9O₃₀ Qf = 33 585 at 4.02 GHz, ε_r = 28, and a temperature coefficient of resonant frequency τ_f = 30.2 ppm/°C; Ba₁₀Co_0.25Ta _7.9O₃₀ Qf = 36 693 at 3.78 GHz, Tau;_r = 30, τ_f = 28.6 ppm/°C), and factors controlling dielectric behavior in hexagonal perovskites are identified from the available structural and property data.