Blogging a crystal structure a day in 2014


Contributed by

Chris Ling

Conducting heat with a very complicated structure

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software

It shows a view along the <110000> direction, drawn over the equivalent of 8 × 8 × 8 fluorite-type subcells. The fractional occupancy cutoff for drawing atoms is 1/2. Bi atoms are yellow, Nb atoms and polyhedra are blue, oxygen atoms are red.

What is it?

The high-temperature form of bismuth oxide, δ-Bi2O3, is the best solid-state oxide-ionic conductor known. Pure δ-Bi2O3 has never been seriously considered for practical applications (such as fuel-cell membranes) due to its limited thermal stability, but chemically stabilised variants that preserve its average structure to room temperature, along with a significant part of its conductivity, have genuine potential. The most elegant and effective of these are the Type-II phases, which show the highest conductivities across the widest ranges of chemical and thermal stability, but remain poorly understood because none of their (3+3)-dimensional "hypercubic" structures has even been determined.

A large (cm-scale) crystal of Type-II Bi2O3–Nb2O5 was grown by refluxing bismuth oxide in a floating-zone furnace. Neutron diffraction data was collected from this crystal on the instrument D19 at the Institut Laue-Langevin (France) and solve what is by far the most complex and detailed structure of its type. The result reveals ordered strings of corner-connected NbO6 octahedra separating continuous disordered oxygen-deficient channels, explaining its substantially higher conductivity compared to other stabilised δ-Bi2O3 phases. This is the first-ever full (unconstrained) refinement of a 3+3-dimensional incommensurately modulated structure.

Where did the structure come from?

A crystallographic information file can be obtained as part of the Supporting Information associated with: Ling, C.D., Schmid, S, Blanchard, P.E.R., Petricek, V., McIntyre, G.J., Sharma, N., Maljuk, A., Yaremchenko, A.A., Kharton, V.V., Gutmann, M., Withers, R.L., J. Am. Chem. Soc. 2013, 135, 6477–6484.