Phase composition, degree of cation ordering, and dielectric properties of complex perovskites with general formula Ba(B' 1 3 B" 2 / 3 )O 3 , where B' = Mg, Zn, and Ni and B" = Nb and Ta, were analyzed. It was found that allthe studied complex perovskites attained high degrees of 1:2 cation ordering at temperatures specific to each composition. A high temperature order-disorder phase transition in Ba(Zn 1 / 3 Nb 2 / 3 )O 3 occurred below 1380 °C. Ba(Ni 1 / 3 Nb 2 / 3 )O 3 (BNN) and Ba(Mg 1 / 3 Nb 2 / 3 )O 3 (BMN) pervoskites remained 100% ordered at temperatures as high as 1500 and 1620 °C, respectively. It was found that in BMN and BNN extrinsic factors, such as the second phase (i.e., Ba 3 Nb 5 O 1 5 ) and point defects, dominated the dielectric loss at microwave frequencies. Ba(Mg 1 / 3 Ta 2 / 3 )O 3 (BMT) remained single phase up to 1630 °C. Above this temperature, the Ba 3 Ta 5 O 1 5 second phase was detected. A decrease in the 1:2 cation ordering and increase of dielectric loss in BMT occurred at sintering temperatures above 1590 °C. It was also revealed by electron paramagnetic resonance that all samples studied contained a substantial amount of paramagnetic point defects. These defects contributed to extrinsic dielectric loss at microwave frequencies, thus degrading the Q factor.

Synthesis and dielectric properties of barium tantalates and niobates with complex perovskite structure

La(Mg0.5Ti(0.5−x)Snx)O3 perovskite ceramics with composition (x=0.0−0.5) are prepared by the solid state reaction method. The ceramics are characterized by x-ray diffraction, far-infrared reflectance, Raman spectroscopy, and microwave dielectric properties. The symmetry of ceramics is monoclinic with P21/n space group. Intrinsic dielectric constant and loss are estimated by fitting reflectance to the four-parameter semiquantum model. Transverse optic phonon mode strengths and average phonon damping are calculated. The modes corresponding to B-site ordering are identified in Raman spectra and the A1g mode of La(MgTi)0.5O3 is analyzed by assuming two merging modes. The variation of long-range order is correlated with full width half maximum of the A1g mode. Microwave measurements are carried out in the frequency range of 8–10 GHz. The dielectric constant (e′) is found to gradually decrease from 28.4 to 19.7 with an increase in tin concentration, whereas the temperature coefficient of resonant frequency (τf)...

Far-infrared, Raman spectroscopy, and microwave dielectric properties of La(Mg0.5Ti(0.5−x)Snx)O3 ceramics

Complex perovskite microwave dielectric ceramics (MWDCs) of A ( B 1 / 3 ′ B 2 / 3 ′ ′ ) O 3 -type (A = Ba, Sr,…; B′ = Mg, Zn,…; B″ = Nb, Ta), which exhibit excellent dielectric properties, have currently been widely used in microwave and millimetre wave devices. Vibrational spectra, including both Raman and far-infrared (FIR) spectra, are powerful tools to investigate the atomic thermal vibrational properties of MWDCs and reveal the intrinsic origin of dielectric properties. In this review, lattice dynamics and phonon characteristics of the A ( B 1 / 3 ′ B 2 / 3 ′ ′ ) O 3 -type MWDCs are summarised and presented in detail to introduce remarkable progress in this field and make a guide for the design of novel advanced MWDCs. The atomic sites and the corresponding modes in Raman and FIR spectra are identified and illuminated. The effects of the processing conditions and the ordered superstructures in the nanoscale region on vibrational modes are summarised systemically. Intrinsic properties can be extrapolated from the fitting results of FIR spectroscopy, which were also discussed based on the Kramers–Kronig relations, Lorentz three-parameter classical model and four-parameter semi-quantum model. The correlations between vibrational modes (phonons), crystal structures, and dielectric properties are created, which can help to build the mathematical models so as to understand the structure–property relationship of MWDCs better.

Lattice dynamics and phonon characteristics of complex perovskite microwave ceramics

The effects of CuO and TiO2 additives on the microstructure and microwave dielectric properties of Al2O3 ceramics were investigated. Al2O3 ceramics with CuO and TiO2 additions can be well sintered to achieve 93∼98% theoretical densities below 1,360 °C due to Ti4Cu2O liquid phase sintering effect. The Qf values decreased with increasing CuO and TiO2 content, due to the formation of the second phase Ti4Cu2O. However, the varying behaviors of the dielectric constant (ɛr) and temperature coefficients (τf) were associated with phase constitutions, as a result of the change of CuO and TiO2content. The τf can be shifted close to 0 ppm/°C by controlling the content of CuO and TiO2. The specimens with 0.5 wt.% CuO and 7 wt.% TiO2 sintered at 1,360 °C for 4 h showed ɛr of 11.8, Qf value of 30,000 GHz, and τf of −7 ppm/°C.

Sintering and dielectric properties of Al2O3 ceramics doped by TiO2 and CuO

This paper reports on the structural and electrical properties of LiPO3 glass over a wide range of frequencies (10 Hz - 2 MHz) and temperatures (290 K -627 K). The temperature dependence of a.c. conductivity is studied at different frequencies. The d.c. conductivity is due to the hopping of lithium ions. A theoretical model and mathematical fit of conductivity measurements are used for characterization of the ionic hopping motion connected with the mobility of conducting Li+ ions. The acoustic attenuation spectroscopy can be useful technique for the study of relaxation processes in ion conducting glasses too. Raman experiments showed the rings and chains configuration at the medium range.

V. Sivasubramanian

Papers

Far-IR reflectance study on B-site disordered Ba (Zn1/3Ta2/3)O3 dielectric resonator