Origin of discrepancy between electrical and mechanical anomalies in lead-free $(\mathrm{K},\mathrm{Na}){\mathrm{NbO}}_3$-based ceramics

Ferroelectric polymorphic phase coexistence, associated with either the presence of a morphotropic phase boundary or a temperature-driven polymorphic phase transition, is currently acknowledged as the key to high piezoelectric activity and is searched when new perovskite materials are developed, like lead-free alternatives to state-of-the-art $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3$. This requires characterization tools that allow phase coexistence and transitions to be readily identified, among which measurements of the temperature dependences of Young's modulus and mechanical losses by dynamical mechanical analysis stand out as a powerful technique to complement standard electrical characterizations. We report here the application of this technique to $\left({\mathrm{K}}_{1-x}{\mathrm{Na}}_x\right){\mathrm{NbO}}_3$-based materials, which are under extensive investigation as environmentally friendly high sensitivity piezoelectrics. The elastic anomalies associated with the different phase transitions are identified and are shown to be distinctively shifted in relation to the dielectric ones. The origin of this discrepancy is discussed with the help of temperature-dependent Raman spectroscopy and is proposed to be a characteristic of diffuse phase transitions.

Figure 1

X-ray diffraction pattern of a $\left({\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\right){}_{0.97}{\mathrm{Li}}_{0.03}{\mathrm{Nb}}_{0.8}{\mathrm{Ta}}_{0.2}{\mathrm{O}}_3$ ceramic. Miller indexes correspond to those of the orthorhombic perovskite (space group $Bmm2$).

Figure 2

Microstructure of the $\left({\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\right){}_{0.97}{\mathrm{Li}}_{0.03}{\mathrm{Nb}}_{0.8}{\mathrm{Ta}}_{0.2}{\mathrm{O}}_3$ ceramic.

Figure 3

Temperature dependences of (top) Young's modulus and internal friction ${\mathrm{Q}}^{-1}$ and (bottom) dielectric permittivity ${\varepsilon }^{\prime }$ and losses ${\varepsilon }^{\prime \prime }$ for a KNLNT ceramic sample.

Figure 4

Normalized Raman spectra, corrected for the Bose-Einstein factor, at increasing temperatures between $20{}^{\circ }\mathrm{C}$ and $400{}^{\circ }\mathrm{C}$ for a KNLNT ceramic. Gray lines represent the decomposition of the spectrum at $20{}^{\circ }\mathrm{C}$ from the best fit of Eq. (1).

Figure 5

Temperature dependences of (a) wave numbers and (b) FWHM for the $F_{2u}\left(\nu 6\right),F_{2g}\left(\nu 5\right),A_{1g}\left(\nu 1\right)+F_{2g}\left(\nu 5\right),A_{1g}\left(\nu 1\right)$, and $E_g\left(\nu 2\right)$ modes.

Figure 6

Temperature dependence of the integral intensity for the $A_{1g}\left(\nu 1\right)+F_{2g}\left(\nu 5\right)$ mode.

Figure 7

Temperature dependence of the reciprocal dielectric permittivity of a KNLNT ceramic at 1 KHz. The Curie-Weiss regime and the temperature from which the data deviate are indicated by straight lines.