Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3–xLiNbO3 {[Lix(Na0.5K0.5)1−x]NbO3} (x=0.04–0.20) have been synthesized by an ordinary sintering technique. The materials with perovskite structure is orthorhombic phase at x⩽0.05 and becomes tetragonal phase at x⩾0.07, a phase K3Li2Nb5O15 with tetragonal tungsten bronze structure begins to appear at x=0.08 and becomes dominant with increasing the content of LiNbO3. A morphotropic phase boundary between orthorhombic and tetragonal phases is found in the composition range 0.05<x<0.07. Analogous to Pb(Zr,Ti)O3, the piezoelectric and electromechanical properties are enhanced for compositions near the morphotropic phase boundary. Piezoelectric constant d33 values reach 200–235pC∕N. Electromechanical coefficients of the planar mode and the thickness mode reach 38%–44% and 44%–48%, respectively. The Curie temperatures (TC) of [Lix(Na0.5K0.5)1−x]NbO3 (x=0.04–0.20) are in the range of 452–510°C, at least 100°C higher than that of conventional Pb(Zr,Ti)O3. Our results show that [Lix(Na0.5K0.5)1−x]NbO3 is a good lead-free high-temperature piezoelectric ceramic.

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A Raman scattering study of (Na0.5K0.5)NbO3 (NKN)-LiNbO3 (LN) lead-free piezoceramics has been carried out on nominal compositions of (1-x)NKN-xLN (0 ≤x ≤0.70). The Raman spectra demonstrated a variety of changes with x, mainly classified as lattice translations involving motions of the alkaline cations and internal modes of NbO6 octahedra. At 0.05 ≤x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO6 octahedra occurred preferably, which is consistent with the evidence for a morphotropic phase boundary (MPB). Furthermore, an abrupt shift was observed in the peak position of the symmetric stretching mode v 1 to a higher frequency, resulting from the distortion of O-Nb-O angles caused by incorporating small Li ions into the perovskite units. This is considered to be a prologue for the structural transformation in the NKN-LN solid solution from orthorhombic to tetragonal symmetry. At compositions in which x increases above the MPB, the translational mode of the Li+ cation emerged clearly, and the overall scattering pattern gradually changed to complex patterns caused by the formation of a tungsten-bronze K3Li2Nb5O15 (KLN) secondary phase and an increase in the number of distorted LiNbO3-type crystal units.

Li7La3Zr2O12 (LLZO) is a promising electrolyte material for all-solid-state battery due to its high ionic conductivity and good stability with metallic lithium. In this article, we studied the effect of crucibles on the ionic conductivity and air stability by synthesizing 0.25Al doped LLZO pellets in Pt crucibles and alumina crucibles, respectively. The results show that the composition and microstructure of the pellets play important roles influencing the ionic conductivity, relative density, and air stability. Specifically, the 0.25Al-LLZO pellets sintered in Pt crucibles exhibit a high relative density (∼96%) and high ionic conductivity (4.48 × 10(-4) S cm(-1)). The ionic conductivity maintains 3.6 × 10(-4) S cm(-1) after 3-month air exposure. In contrast, the ionic conductivity of the pellets from alumina crucibles is about 1.81 × 10(-4) S cm(-1) and drops to 2.39 × 10(-5) S cm(-1) 3 months later. The large grains and the reduced grain boundaries in the pellets sintered in Pt crucibles are favorable to obtain high ionic conductivity and good air stability. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy results suggest that the formation of Li2CO3 on the pellet surface is probably another main reason, which is also closely related to the relative density and the amount of grain boundary within the pellets. This work stresses the importance of synthesis parameters, crucibles included, to obtain the LLZO electrolyte with high ionic conductivity and good air stability.

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Li 7 La 3 Zr 2 O 12 (LLZO) has been reported to react in humid air to form Li 2 CO 3 on the surface, which decreases ionic conductivity. To study the reaction mechanism, 0.5-mol Ta-doped LLZO (0.5Ta-LLZO) pellets are exposed in dry (humiditỹ 5%) and humid air (humidity~80%) for 6 weeks, respectively. After exposure in humid air, the formation of Li 2 CO 3 on the pellet surface is confirmed experimentally and the room-temperature ionic conductivity is found to drop from 6.45910 À4 S cm À1 to 3.61910 À4 S cm À1 . Whereas for the 0.5Ta-LLZO samples exposed in dry air, the amount of formed Li 2 CO 3 is much less and the ionic conductivity barely decreases. To further clarify the reaction mechanism of 0.5Ta-LLZO pellets with moisture, we decouple the reactions between 0.5Ta-LLZO with water and CO 2 by immersing 0.5Ta-LLZO pellets in deionized water for 1 week and then exposing them to ambient air for another week. After immersion in deionized water, Li + /H + exchange occurs and LiOH H 2 O forms on the surface, which is a necessary intermediate step for the Li 2 CO 3 formation. Based on these observations, a reaction model is proposed and discussed. K E Y W O R D Sgarnets, impedance spectroscopy, ion exchange, lithium oxide, Raman spectroscopy

Piezoelectric constant and temperature-dependent dielectric constant measurements have been performed on ⟨110⟩-oriented (1 − x)Pb (Mg1/3 Nb2/3)O3–xPbTiO3 crystals with different compositions under different poling fields. The width of the morphotropic phase boundary region (0.30 < x < 0.35) is determined on the basis of two abnormal regions of the dielectric and piezoelectric properties. An irreversible rhombohedral–monoclinic MA–monoclinic MC–tetragonal phase transition sequence was observed directly from the dielectric constant versus temperature results for ⟨001⟩-poled rhombohedral crystals with compositions near the rhombohedral–monoclinic phase boundary. The structure of the morphotropic phase is shown to be monoclinic with space group Pm.

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