Experimental and theoretical investigation of the structural, chemical, electronic, and high frequency dielectric properties of barium cadmium tantalate–based ceramics

Abstract

Single-phase Ba(Cd1∕3Ta2∕3)O3 powder was produced using conventional solid state reaction methods. Ba(Cd1∕3Ta2∕3)O3 ceramics with 2wt% ZnO as sintering additive sintered at 1550°C exhibited a dielectric constant of ∼32 and loss tangent of 5×10−5 at 2GHz. X-ray diffraction and thermogravimetric measurements were used to characterize the structural and thermodynamic properties of the material. Ab initio electronic structure calculations were used to give insight into the unusual properties of Ba(Cd1∕3Ta2∕3)O3, as well as a similar and more widely used material Ba(Zn1∕3Ta2∕3)O3. While both compounds have a hexagonal Bravais lattice, the P321 space group of Ba(Cd1∕3Ta2∕3)O3 is reduced from P3̱m1 of Ba(Zn1∕3Ta2∕3)O3 as a result of a distortion of oxygen away from the symmetric position between the Ta and Cd ions. Both of the compounds have a conduction band minimum and valence band maximum composed of mostly weakly itinerant Ta5d and Zn3d∕Cd4d levels, respectively. The covalent nature of the directional d-electron bonding in these high-Z oxides plays an important role in producing a more rigid lattice with higher melting points and enhanced phonon energies, and is suggested to play an important role in producing materials with a high dielectric constant and low microwave loss.