Nonstoichiometry Role on the Properties of Quantum-Paraelectric Ceramics

Abstract

Among the lead-free perovskite-structure materials, strontium titanate (SrTiO 3-ST) and potassium tantalate (KTaO 3-KT), pure or modified, are of particular importance. They are both quantum paraelectrics with high dielectric permittivity and low losses that can find application in tunable microwave devices due to a dependence of the permittivity on the electric field. Factors as Sr/Ti and K/ Ta ratio in ST and KT ceramics, respectively, can alter the defect chemistry of these materials and affect the microstructure. Therefore, if properly understood, cation stoichiometry variation may be intentionally used to tailor the electrical response of electroceramics. The scientific and technological importance of the stoichiometry variation in ST and KT ceramics is reviewed and compared in this chapter. The differences in crystallographic phase assemblage, grain size, and dielectric properties are described in detail. Although sharing crystal chemical similarities, the effect of the stoichiometry is markedly different. Even if the variation of Sr/Ti and K/Ta ratios did not change the quantum-paraelectric nature of ST and KT, Sr excess impedes the grain growth and decreases the dielectric permittivity in ST ceramics, while K excess promotes the grain growth and increases the dielectric permittivity in KT ceramics.

Dielectric Relaxation, Local Structure and Lattice Dynamics in Mn-Doped Potassium Tantalate Ceramics

“…As reported previously, undoped KT ceramics upon cooling display a continuous increase of the dielectric permittivity to ε ′ ≈ 4000 without frequency dispersion [ 10 , 11 ]. In contrast, for the Mn-doped KT ceramics, the maximum attainable value of ε ′ is only ≈1400 (see Figure 1 a), and the ε ′( T ) dependence reveals a diffuse peak with a frequency-dependent amplitude and position, suggesting a dielectric relaxation.…”

“…were dried to remove moisture, mixed, and milled in alcohol for 5 h in a planetary mill using Teflon containers and zirconia spherical grinding media. The loss of potassium, which is expected to occur during sintering, was compensated by adding 5 wt.% excess of potassium to the initial mixture prior to the milling [ 10 ]. The resulting powders were dried and calcined at 875 °C for 8 h. The calcined powders were milled again for 5 h to reduce the particle size below 5 μm, and uniaxially pressed (100 MPa) into pellets, 10 mm in diameter.…”

“…KT-based compounds have mainly been investigated as single crystals [ 8 , 9 ]. Although ceramics are assumed to possess higher and more controllable dopant contents, being also cheaper to fabricate than single crystals, the scarcity of works on ceramics can be attributed to the relatively low melting temperature of KT, which, combined with the high volatility of potassium, makes it challenging to synthesize dense monophasic ceramics with well-controlled chemical compositions [ 10 ]. Among KT-based compounds, K 1− x Li x TaO 3 solid solutions have been studied intensively, wherein displacements of small Li + ions on K sites generate strong local dipole moments that couple electrostatically to the KT’s polar soft mode [ 8 , 9 ].…”