Structural variations and dielectric properties of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ ($0\le x\le 0.1$): Polycrystallines synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses

This paper focuses on effects of isovalent La substitution on the crystal structure and dielectric properties of ferroelectric $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$. Polycrystalline samples of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ are synthesized by crystallization of Bi-Si-O and Bi-La-Si-O glasses with a composition range of $0\le x\le 0.1$. The crystal structure changes from monoclinic to tetragonal with increasing La-substitution rate $x$ at room temperature. This structural variation stems from the change in orientation of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra that form one-dimensional chains when they are in the ordered configuration, thus suggesting that lone-pair electrons play an important role in sustaining one-dimensional chains of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra. Synchronizing with the disordering of $\mathrm{Si}{\mathrm{O}}_4$ chains, ferroelectric phase transition temperature of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ sharply decreases as $x$ increases, and ferroelectricity finally vanishes at around $x=0.03$. The present results demonstrate that lone-pair electrons of Bi play an important role in the ferroelectricity of $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$ through propping the ordered structure of one-dimensional $\mathrm{Si}{\mathrm{O}}_4$ chains with stereochemical activity. Furthermore, an additional phase transition has been first discovered in the low-temperature region of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ with $x\le 0.01$, where the ordered one-dimensional $\mathrm{Si}{\mathrm{O}}_4$ chains remain.

Figure 1

A comparison of perovskite-type, aurivillius-type, and brownmillerlite-type crystal structures with that of $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$. The crystal structure of $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$ can be understood by successive replacement of one in every two octahedral layers by fluorite-type $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_2$ layers and parallel-aligned one-dimensional tetrahedral chains, which are involved in the aurivillius-type and brownmillerlite-type crystal structures, respectively. (See text for details.)

Figure 2

The crystallization process of Bi-Si-O glass, as observed by in situ x-ray-diffraction measurements on heating from 300 to 700 °C. Bottom panels indicate calculated diffraction patterns for $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$ and $\mathrm{B}{\mathrm{i}}_2{\mathrm{O}}_3$.

Figure 3

(a) Powder-diffraction patterns of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ observed at room temperature, where baselines are systematically offset. The bottom panel indicates a calculated pattern for pure $\mathrm{B}{\mathrm{i}}_2\mathrm{Si}{\mathrm{O}}_5$. (b) A magnified graph of the diffraction patterns for a range from 32.0 to 34.5 °.

Figure 4

Schematic for (a) the ordered and (b) the disordered $\mathrm{Si}{\mathrm{O}}_4$ one-dimensional chains. The broken square indicates possible orientations of the $\mathrm{Si}{\mathrm{O}}_4$ tetrahedron.

Figure 5

Raman-scattering spectra of ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ ($x=0.00$, 0.01, 0.03, and 0.05). The arrow and the circle in the figure denote the peaks that characterize the spectra for $x=0.00$ and 0.05.

Figure 6

Temperature dependences of (a) dielectric permittivity and (b) tanδ for ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ ($x=0.00$, 0.01, 0.03, 0.05, 0.07, and 0.10) ceramics observed at 100 kHz over the temperature range from 2 to 760 K. The data plots $x=0.01$, 0.03, 0.05, 0.07, and 0.10 are, respectively, shifted for the sake of visibility.

Figure 7

Temperature dependence of dielectric permittivity and tanδ for ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ observed with several test frequencies. The inset in each panel presents temperature variation of tanδ from 0 to 250 K with magnified scale.

Figure 8

Temperature dependences of lattice parameters for ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{L}{\mathrm{a}}_x\right)}_2\mathrm{Si}{\mathrm{O}}_5$ with $x=0.00$ and 0.10. For $x=0.00$ with the monoclinic symmetry, the lattice parameters $a$, $b$, $c$, and β are plotted in the panels (a), (b), (c), and (d), respectively. The lattice volume $V$ and the in-plane anisotropy parameter $b$/$c$ are, respectively, presented in panels (e) and (f). For $x=0.10$ with tetragonal symmetry, on the other hand, the lattice parameters $a$ and $c$ and the volume $V$ are plotted in panels (g), (h), and (i), respectively.