Phase development during mixed-oxide processing of a [Na_0.5K_0.5NbO₃]_1−x–[LiTaO₃]_x powder

Abstract

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.

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“…However, the values of peak intensity ratio of all the Sb-modified samples were much lower, ∼0.5-0.8. This change suggests that the LiSbO 3 solid solutions are mainly tetragonal with some co-existing orthorhombic phase, thus giving intensity ratios slightly higher than expected for single-phase tetragonal samples 18 . Second-phase LiSbO 3 was detected in the x = 0.1 samples (Fig.…”

“…The XRD data (Fig. 1) showed little evidence of peak broadening, but a previous study into phase development during conventional powder calcination of NKN-LT 18 indicated that d-spacings were insensitive to small changes in phase composition, and hence no noticeable XRD peak broadening would be observed.…”

“…Samples with 0 x 0.06 showed a perovskite structure, indicating that Li + and Sb 5+ had completely diffused into the NKN-LT lattice to form solid solutions. The intensity ratio of the pair of peaks at 2θ = 45-46.5°in each pattern is used as an indication of the tetragonal/orthorhombic phase content 18 . The lower angle peak in the pair corresponds to the 022 peak of an orthorhombic NKN-LT phase, whereas it is the 002 peak for tetragonal NKN-LT 19,20 .…”

“…The formation of a narrower range of solid solution compositions in x = 0.06 than in x < 0.6 samples could be due to more liquid phase being formed in x = 0.06 samples during sintering. This would increase the rates of mass transport and interdiffusion reactions, and hence increase component reactivity 18 . However, a degree of compositional heterogeneity persists in x = 0.06 samples, as Curie peaks are still relatively broad.…”

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“…Fig. 1 shows the XRD patterns of (1 − x) NKLNTx MnO samples which had been sintered at 1025°C for 2 h. The intensity ratio of the pair of peaks at 2θ = 45-46.5°in each pattern was used as an indication of the tetragonal/orthorhombic phase content 22 . The lower angle peak in the pair corresponds to the (220) peak of an orthorhombic NKN-LT phase, or the (002) peak of the tetragonal phase of NKN-LT 23,24 .…”