High-pressure and high-temperature multianvil synthesis of metastable polymorphs of ${\text{Bi}}_2{\text{O}}_3$: Crystal structure and electronic properties

High-pressure and high-temperature experiments with ${\text{Bi}}_2{\text{O}}_3$ using a 6–8 type multianvil device led to the formation of a metastable polymorph $\left({\text{HP-Bi}}_2{\text{O}}_3\right)$ with noncentrosymmetric trigonal symmetry. This phase relaxes during the course of several months at ambient temperature or more rapidly via annealing, to a second intermediate modification $\left({\text{R-Bi}}_2{\text{O}}_3\right)$. Upon further annealing finally the transformation back to the known ambient phase $\left(\alpha {\text{-Bi}}_2{\text{O}}_3\right)$ takes place. Both crystal structures were solved from high-resolution x-ray and neutron powder-diffraction data. The orientation and stereochemical activity of the ${\text{Bi}}^{3+}$ lone pairs (or inert pairs) is discussed in terms of crystal-chemical considerations and density-functional theory calculations. Whenever suitable, results were verified by experimental determination of the respective properties. The results of the theoretical analyses show that within the structure type of ${\text{HP-Bi}}_2{\text{O}}_3$, bismuth oxide exhibits a pronounced polarization and can be considered as ferroelectric.

Figure 1

High-temperature XRD measurements for ${\text{R-Bi}}_2{\text{O}}_3$. The transformation to $\alpha {\text{-Bi}}_2{\text{O}}_3$ starts at $260°\text{C}$ and is completed at $400°\text{C}$.

Figure 2

Scattered x-ray intensities for the HP (left) and R phases (right) of ${\text{Bi}}_2{\text{O}}_3$ as a function of diffraction angle $2\theta$ for laboratory x ray (top) and neutron (bottom) data. For each are shown the observed pattern (diamonds), the best Rietveld-fit profile (line a), the difference curve between observed and calculated profile (line b), and the reflection markers (vertical bars). The wavelength was $\lambda =1.54059\text{Å}$ for the x ray and $1.5483\text{Å}$ for the neutron data. The higher angle part of the x-ray plots is enlarged by a factor of 5 starting at $58°$ $2\theta$ for the HP phase and by a factor of 3 starting at $45°$ $2\theta$ for the R phase, respectively.

Figure 3

(Color online) Crystal structure of ${\text{HP-Bi}}_2{\text{O}}_3$ viewed along [001] with ${\text{BiO}}_6$ trigonal antiprisms in dark blue and ${\text{BiO}}_5$ square pyramids in light gray.

Figure 4

(Color online) Crystal structure of ${\text{R-Bi}}_2{\text{O}}_3$ viewed along the [010] (left side) and [001] (right side). The ${\text{BiO}}_5$ square pyramids are displayed in red (Bi1), blue (Bi2), green (Bi3), and gray (Bi4) for distinction.

Figure 5

(Color online) Structural variation in ${\text{Bi}}_2{\text{O}}_3$ polymorphs during transformation toward the stable at ambient conditions $\alpha$ phase. The bismuth atoms are divided into the individual A and B layers of the original hexagonal close packing, which are found to evolve different upon phase transformation. A hypothetical, ideal centrosymmetric structure without stereochemically active lone pairs (top) is given for comparison.

Figure 6

Top: volume distortion of the distinct Bi polyhedra within the polymorphs. For comparison also $\alpha {\text{-Bi}}_2{\text{O}}_3$ and the mixed-valent compound ${\text{Ag}}_{25}{\text{Bi}}_3{\text{O}}_{18}$ are depicted. Bottom: projection of the central atom on the barycenter-centroid vector as a normalized measure of stereochemical activity or inertness of the lone pairs.

Figure 7

(Color online) Calibrated DSC measurement for ${\text{HP-Bi}}_2{\text{O}}_3$. A heating rate of $10\text{K}{\text{min}}^{-1}$ was applied.

Figure 8

(Color online) Observed diffuse reflection spectra for ${\text{HP-Bi}}_2{\text{O}}_3$, ${\text{R-Bi}}_2{\text{O}}_3$, and $\alpha {\text{-Bi}}_2{\text{O}}_3$ from 200 to 900 nm (top) and corresponding optical-absorption energies (bottom) calculated from the Kubelka Munk formula.

Figure 9

(Color online) Projected DOSs for HP-, R-, and $\alpha {\text{-Bi}}_2{\text{O}}_3$ phases. The dashed line at 0 eV indicates the position of the Fermi level.

Figure 10

(Color online) Site-specific projected DOS of $\text{Bi}\left(6s\right)$ for HP- and $\alpha {\text{-Bi}}_2{\text{O}}_3$ phases.

Figure 11

(Color online) Domains of the ELF, $\eta =0.96$, in HP-, R-, and $\alpha {\text{-Bi}}_2{\text{O}}_3$ phases. The lone pairs of Bi2 in HP-, Bi1 in R- and $\alpha {\text{-Bi}}_2{\text{O}}_3$ are shown below their respective phases. The Bi1 site in ${\text{HP-Bi}}_2{\text{O}}_3$ and the rest of the Bi sites in R- and $\alpha {\text{-Bi}}_2{\text{O}}_3$ are not shown as they are similar to Bi1 lone pairs.