Structural investigations of the bismuth scandate-lead titanate $x{\text{BiScO}}_3\text{−}\left(1-x\right){\text{PbTiO}}_3$ solid solution for $0.10\le x\le 0.40$

A full structural study of the solid solution $x{\text{BiScO}}_3\text{−}\left(1-x\right){\text{PbTiO}}_3$ (BS-PT) has been carried out by neutron powder diffraction using the General Materials Diffractometer (GEM) at ISIS facility in Oxford and complemented by in-house x-ray diffraction studies and high-temperature dielectric measurements. Atomic positions and thermal-displacement parameters have been determined by Rietveld refinement of neutron powder-diffraction data for a total of 14 different compositions of BS-PT, where $0.10\le x\le 0.40$. A gradual structural phase transition from a tetragonal (space group: $P4mm$) phase to a rhombohedral (space group: $R3m$) phase as a function of composition has been observed and the phase boundary is identified as a mixture of a tetragonal and a monoclinic phase with space group $Cm$. However, the refined anisotropic thermal parameters in the region $x\ge 0.39$ are found to be unrealistic for the average crystal structure and instead of a rhombohedral phase, a monoclinic phase $\left(Cm\right)$ is suggested for the compositions $x\ge 0.39$. In addition, temperature-dependent dielectric measurements have been carried out to demonstrate the physical properties of the ceramics including ferroelectric transition temperatures as a function of composition.

Figure 1

(Color online) Room-temperature x-ray diffraction patterns of the $x\text{BS-}\left(1-x\right)\text{PT}$ ceramics as a function of $x$.

Figure 2

(Color online) Evidence of undesired pyrochlore phases for the samples with compositions $x\ge 0.36$. The arrows indicate the peaks which are not from the primary perovskite phase.

Figure 3

(Color online) (a) Variation in the tetragonal axial ratio for BS-PT samples with $x\le 0.35$. Error bars are too small to be visible on the given scale. (b) The Ti displacement as a function of composition in the tetragonal regime of the phase diagram. A third-order polynomial has been fitted in order to visualize the possible nonlinear relationship between them, which provided an apparent maximum at $x\sim 0.16$.

Figure 4

(Color online) The lattice parameters obtained from the refinements for the BS-PT system as a function of composition. The MPB is shown in the region of composition between $x=0.36$ and 0.38. Rhombohedral lattice parameters are shown for compositions $x=0.39$ and 0.40, however these compositions can also be well refined by a single monoclinic model $\left(Cm\right)$.

Figure 5

(Color online) Observed (●), calculated (-), and difference (-) plots for neutron-diffraction patterns of BS-PT samples with $x=0.10$, 0.37 (MPB composition) and 0.40. The data are from the $90°$ bank of the GEM detector.

Figure 6

(Color online) Structural phase transition as a function of temperature for $x=0.10$. A selected range of $2\theta$ shows the transformation of the {210} Bragg Reflection with the temperature.

Figure 7

(Color online) Structural phase transition as a function of temperature for $x=0.39$. The gradual transformation to the high-temperature cubic phase is evident from the transformation of the {210} Bragg peak as a function of temperature.

Figure 8

(Color online) Variation in the peak height and FWHM of the {200} Bragg peak for the composition $x=0.39$ as a function of temperature. The x-ray powder scans were taken under identical experimental conditions.

Figure 9

(Color online) Temperature-dependent dielectric constant of BS-PT ceramics for different compositions at frequencies 50, 100, and 150 kHz.

Figure 10

(Color online) Compositional dependence of the phase transition temperature, $T_m$, obtained from the dielectric plots for BS-PT ceramics.