Synthesis, Crystal Structure, and Characterization of Ba(Ti_1/2Mn_1/2)O₃: A High Permittivity 12R-Type Hexagonal Perovskite

Abstract

A 12R-type hexagonal perovskite, Ba(Ti1/2Mn1/2)O3, has been prepared by a mixed oxide route at 1250 °C in air and the crystal structure established by a joint Rietveld refinement of powder X-ray and neutron diffraction data at room temperature [space group R3̄m, a = 5.69135(2) Å, and c = 27.91860(15) Å]. The structure consists of a (hhcc)3 stacking sequence of BaO3 close-packed layers along the c-axis with face sharing M3O12 trimers connected by a vertex sharing octahedron. Partial ordering of the Ti+IV and Mn+IV ions occurs in the B-site substructure; Mn+IV ions occupy the central octahedral site in the trimers, Ti+IV ions occupy the octahedral site in the vertex sharing octahedron, and both ions occupy the outer octahedral site of the trimers. Impedance spectroscopy and microwave dielectric resonance measurements on dense pellets (>97% of the theoretical X-ray density) show the title compound to be a modest dielectric insulator at room temperature with high permittivity, εr ∼ 45 (at both radio and microwave frequencies); a modest unloaded quality factor, Qu ∼ 2026 (at ∼5.75 GHz); and a temperature coefficient of resonant frequency, TCf = −4 ppm K-1.