Powders of the solid lead-free piezoelectric ceramic solution [Na0.5K0.5NbO3]1−x–[LiTaO3]x, x = 0.06, were produced using a mixed-oxide process. Phase analysis indicated the formation of an orthorhombic solid solution at 800 °C, which coexisted with intermediate binary niobate and tantalate phases. A tetragonal main-phase solid solution was formed at ⩾950 °C, along with minor quantities of a tungsten bronze phase. Addition of 3 wt% excess alkali carbonates to the starting powders allowed the orthorhombic solid solution to be retained to 1100 °C and prevented formation of the secondary tungsten bronze phase. Elemental chemical analysis confirmed changes in alkali oxide composition, consistent with volatilization losses, particularly of potassium and lithium oxides. Phase stability near the reported morphotropic phase boundary is shown to be sensitive to alkali oxide content.

A polymorphic phase transition (PPT) is often engineered into lead-free materials to generate high piezoelectric activity at room temperature, limiting their temperature stability. We report [Na0.5K0.5NbO3]0.93–[LiTaO3]0.07 tetragonal ceramics with favorable properties over a broad temperature range due to a high Curie temperature at 447 °C and PPT at −15 °C involving a transition to a monoclinic rather than low temperature orthorhombic phase. Piezoelectric k31 and d31 coefficients varied from 0.19 to 0.14 and −53 to −33 pC/N, respectively, over the range of −15 to 300 °C. Strain-electric field loops provided strains of ∼0.2% and a high-field d33 of 205 pm/V.

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Solid solution formation in the system (1−x) [(Na0.5K0.5NbO3)0.93–(LiTaO3)0.07]−x[BiScO3] has been investigated for compositions, 0≤x≤0.05. The tetragonal c/a ratio decreases with increasing BiScO3 content, with a discontinuity at x=0.02, X‐ray diffraction patterns appear pseudo‐cubic for x>0.02. There is a 10‐fold reduction in grain size between x=0.015 and x=0.02: dielectric peaks become suppressed and broadened at this point. Piezoelectric d33 charge coefficients decrease across the compositional range. Results are discussed in terms of chemical inhomogeneity and grain‐size effects.

A Pechini‐based chemical synthesis route was used to produce powders of Na0.5K0.5NbO3 (NKN). The thermochemistry of the gel was investigated using thermogravimetric analysis‐fourier transform infrared (TGA‐FTIR) evolved gas analysis; in addition, powder FTIR was used to analyze the gel residues after different heat treatments. The final decomposition of the organic components occurred at ∼650°C. However, hydrated–carbonated secondary phase(s) were detected by FTIR in powders that had been heated at 700°C, indicating that the NKN nanopowders are susceptible to a reaction with atmospheric moisture and carbon dioxide. The NKN particle sizes were in the range 50–150 nm after decomposition at 700°C.

A phase-diagram for the Na(0.5)K(0.5)NbO(3)-LiTaO(3) solid solution series (NKN-LT) is presented for compositions ≤ 10 mol% LT, based on the combined results of temperaturevariable X-ray powder diffraction and dielectric measurements. In addition to the reported orthorhombic and tetragonal polymorphs of NKN-LT, a monoclinic phase is revealed. Changes to electrical properties as a function of LT substitution are correlated to phase content. Increasing the LT content from 5 to 7 mol% LT led to improved temperature stability of piezoelectric properties because of the avoidance of the monoclinic-tetragonal polymorphic phase transition during thermal cycling (at >25°C). For 7 mol% LT samples: d(33) = 200 pC/N; T(c) = 440°C; ε(r) = 550 and tan δ = 0.02 (at 20°C). Modification of this composition by solid solution with BiScO(3) led to a decrease in d(33) values. Transmission electron microscopy of a sample of 0.95[0.93 NKN-0.07LT]-0.05BiScO(3) indicated a core-shell grain structure which led to temperature-stable dielectric properties.

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