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Recent advances in perovskite ferroelectrics have fostered a host of exciting sensors and actuators. Defect engineering provides critical control of the performance of ferroelectric materials, especially lead-free ones. However, it remains a challenge to quantitatively study the concentration of defects due to the complexity of measurement techniques. Here, a feasible approach to analyzing the A-site defect and electron in alkali metal niobate is demonstrated. The theoretical relationships among defect concentration, conductivity, and oxygen partial pressure can be established based on the defect chemistry equilibria. The type and concentration of defects are reflected through the conductivity variation with oxygen partial pressure. As a result, the variation of defect concentration gives rise to defect-driven interfacial polarization, which further leads to distinct properties of the ceramics. e.g., abnormal dielectric behavior. Furthermore, this study also suggests a strategy to manipulate defects and charges in perovskite oxides for performance optimization., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

In this work, the (Na 0.5 Bi 0.5 )(Mo 1-x W x )O 4 (x = 0.0, 0.5 and 1.0) ceramics were prepared via solid state reaction method. All the samples can be well densified at sintering temperature about ~720 °C. Dense and homogeneous microstructure with grain size lying between 2~8 μm can be observed from scanning electron microscopy (SEM). Microwave dielectric permittivity of the (Na 0.5 Bi 0.5 )(Mo 0.5 W 0.5 )O 4 ceramic was found to be temperature-independent in a wide range between 25~120 °C with a temperature coefficient of frequency (TCF) ~-6 ppm/°C, a permittivity ~28.9, and Qf values 12,000~14,000 GHz. Crystal structure was refined using Rietveld method and lattice parameters are a = b = 5.281 (5) Å and c = 11.550 (6) Å with a space group I 4 1 /a (88). The (Na 0.5 Bi 0.5 )(Mo 1-x W x )O 4 ceramics might be good candidate for low temperature co-fired ceramics (LTCC) technology.

With growing concern over world environmental problems and increasing legislative restriction on using lead and lead-containing materials, a feasible replacement for lead-based piezoceramics is desperately needed. Herein, we report a large piezoelectric strain (d 33 *) of 470 pm/V and a high Curie temperature (T c ) of 243 °C in (Na 0.5 K 0.5 )NbO 3 -(Bi 0.5 Li 0.5 )TiO 3 -BaZrO 3 lead-free ceramics by doping MnO 2 . Moreover, excellent temperature stability is also observed from room temperature to 170 °C (430 pm/V at 100 °C and 370 pm/V at 170 °C). Thermally stimulated depolarization currents (TSDC) analysis reveals the reduced defects and improved ferroelectricity in MnO 2 -doped piezoceramics from a macroscopic view. Local poling experiments and local switching spectroscopy piezoresponse force microscopy (SS-PFM) demonstrates the enhanced ferroelectricity and domain mobility from a microscopic view. Distinct grain growth and improvement in phase angle may also account for the enhancement of piezoelectric properties.

In the present work, the microwave dielectric ceramic (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was prepared by using the solid-state reaction method. (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was found to crystallize in the scheelite structure, in which Ag(+) and Bi(3+) occupy the A site randomly with 8-coordination while Mo(6+) and W(6+) occupy the B site with 4-coordination, at a sintering temperature above 500 °C, with lattice parameters a = b = 5.29469(2) Å and c = 11.62114(0) Å, space group I4(1)/a (No. 88), and acceptable Rp = 9.38, Rwp = 11.2, and Rexp = 5.86. High-performance microwave dielectric properties, with permittivity ∼26.3, Qf value ∼10,000 GHz, and temperature coefficient ∼+20 ppm/°C, were obtained in the sample sintered at 580 °C. Its chemical compatibility with aluminum at its sintering temperature was revealed and confirmed by both X-ray and energy dispersive spectrometer analysis. This ceramic could be a good candidate for ultralow-temperature cofired ceramics.