Preponderant influence of disordered P4bm phase on the piezoelectricity of critical compositions of ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$-based ferroelectrics

We investigated the phase boundaries of three ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$-(NBT) based piezoelectric systems NBT-${\mathrm{BaTiO}}_3$ (NBT-BT), NBT-${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3$ (NBT-KBT) and NBT-${\mathrm{SrTiO}}_3$ (NBT-ST), all exhibiting similar piezoelectric responses ($d_{33}\sim 180pC/N$). We found that, apart from a qualitative difference (NBT-KBT/NBT-BT exhibits R3c-P4mm interferroelectric instability and NBT-ST does not), the critical compositions of all the three systems exhibit onset of a disordered P4bm phase. Our study suggests that the preponderant effect of this distortion overrides the interferroelectric instability, limiting the piezoelectric response of NBT-BT/NBT-KBT like that in NBT-ST.

Figure 1

(a) Composition dependence of weak-field piezoelectric coefficient (${\mathrm{d}}_{33}$) of (1-x)NBT-(x)ST, (1-y)NBT-(y)BT and (1-z)NBT-(z)KBT. Electric field dependence of (b) polarization, (c) bipolar strain, and (d) unipolar strain of $x=0.20,y=0.06$, and $z=0.20$.

Figure 2

X-ray powder-diffraction profiles of pseudocubic ${\left\{111\right\}}_{\mathrm{pc}}$, and ${\left\{200\right\}}_{\mathrm{pc}}$ of the critical compositions of NBT-xST(x = 0.20), NBT-yBT (y = 0.06) and NBT-zKBT (z = 0.20) in (a) unpoled and (b) poled specimens.

Figure 3

(a) Comparison of the ${}^5{\mathrm{D}}_0{\to }^7{\mathrm{F}}_0$ (570-585 nm) Stark profiles in the PL spectra of Eu-doped unpoled specimens of NBT-xST (x = 0.20), NBT-yBT (y = 0.06) and NBT-zKBT (z = 0.20). In contrast to the PL spectrum of x = 0.20 which possesses one stark line in the band, y = 0.06 and z = 0.20 exhibit two Stark lines. (b–d) show the ${}^7{\mathrm{F}}_0$ Stark profiles of the critical compositions of unpoled and poled specimens of the three systems. (e) The Stark profiles of KBT (tetragonal, P4mm) denoted by $T$ along with profiles of poled and unpoled z = 0.20. $R$ denotes the Stark profile of the R3c phase.

Figure 4

(a) Evolution of the superlattice peaks corresponding to in-phase tilt $0.5{\left\{310\right\}}_{\mathrm{pc}}$ and antiphase tilt $0.5{\left\{311\right\}}_{\mathrm{pc}}$ in the neutron powder-diffraction (NPD) patterns of unpoled NBT-xST. (b) The NPD pattern of x = 0.24 fitted R3c + P4bm structure model at 30 °C. The arrows indicate misfit regions around the superlattice peaks. (c) Comparison of unpoled and poled NPD patterns of NBT, x = 0.20, y = 0.06, and z = 0.20.

Figure 5

(a) Evolution of NPD patterns of poled NBT-0.20ST sample with temperature. (b) Variation of the intensity of $0.5{\left\{331\right\}}_{\mathrm{pc}}$ and $0.5{\left\{551\right\}}_{\mathrm{pc}}$ superlattice reflections with temperature. (c) Variation of the intensity of ${\left\{310\right\}}_{\mathrm{pc}}$ and ${\left\{334\right\}}_{\mathrm{pc}}$ superlattice reflections with temperature. (d) Temperature dependence of the depoling current of x = 0.20. The sharp peak at ${\mathrm{T}}_{\mathrm{d}}\sim 85{}^{\circ }\mathrm{C}$ in (d) characterizes the depolarization temperature. (e) Variation of ${\mathrm{d}}_{33}$ as a function of annealing temperature of poled x = 0.20, y = 0.06 and z = 0.20. The depolarization temperatures (as obtained from the peak in the temperature dependent thermal depoling current measurements) are shown with dashed vertical lines in (e).

Figure 6

Phase diagrams of unpoled (a) NBT-(x)ST, (b) NBT-(y)BT, and (c) NBT-(Z)KBT. ${\mathrm{T}}_{\mathrm{d}}\to$ depolarization temperature; ${\mathrm{T}}_2\to R3c-P4bm$ phase transition temperature.