Relationship between microwave and lattice vibration properties in Ba(Zn_1/3Nb_2/3)O₃-based microwave dielectric ceramics

The dielectric properties of (1 - x)Ba(Zn1/3Nb 2/3)O3-xBa (Ga1/2Ta1/2)O3 (BZN-xBGT) microwave (MW) ceramics, with x between 0 and 0.2, and those of 0.9Ba(Zn0.6Co0.4)1/3Nb2/3O 3-0.1Ba(Ga0.5Ta0.5)O3 (BZCN-BGT) were studied at MW, terahertz (THz) and infrared (IR) frequencies at temperatures from 10 to 300 K. At room temperature, the temperature coefficient of resonance frequency (τf) near 3 GHz decreases from 28 ppm K-1 in undoped BZN to 2 ppm K-1 in BZN-0.2BGT and reduces to zero in BZCN-BGT. The addition of BGT to BZN depresses the dielectric Q value, but incorporation of Co improves the Q values, yielding Q ∼ 30 000 at 3 GHZ in BZCN-BGT. The relative permittivity (ε′) exhibits only limited variation with composition (ε′ values in the range 34.4-36.0). IR and THz spectra as well as the low-temperature MW dielectric measurements revealed a weak dielectric relaxation below phonon frequencies, possibly arising from charges caused by inhomogeneous distribution of the B-site ions with differing valences. The IR reflectivity spectrum of BZN-0.2BGT is significantly different (smeared) compared with other compositions, which may be caused by disorder on the B sites and by an amorphous phase at the grain boundaries.