New 10-Layer Hexagonal Perovskites: Relationship between Cation and Vacancy Ordering and Microwave Dielectric Loss

Abstract

The new ceramic materials Ba10Mg0.25Ta7.9O30 and Ba10Co0.25Ta7.9O30 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 Ba8ZnTa6O24, which involves the (hccc)2 stacking sequence, discovered in an earlier study. The microwave dielectric properties of both materials are determined (Ba10Mg0.25Ta7.9O30 Qf = 33 585 at 4.02 GHz, εr = 28, and a temperature coefficient of resonant frequency τf = 30.2 ppm/°C; Ba10Co0.25Ta7.9O30 Qf = 36 693 at 3.78 GHz, εr = 30, τf = 28.6 ppm/°C), and factors controlling dielectric behavior in hexagonal perovskites are identified from the available structural and property data.