Novel preparation and single crystal analysis of layered metal-
Faculty Mentor: Branton J. Campbell, Physics and Astronomy
Layered transition-metal-doped bismuth vanadates, commonly known as BIMEVOX oxides,
have very attractive oxygen-ion conductivity properties at moderate temperatures and show
promise in applications as electrode materials and membranes for oxygen separation. Of great
interest is the relationship between the atomic structure of BIMEVOX materials and their useful
physical properties. We are investigating this relationship in order to better understand which
structural features produce the best ionic conductors. This research requires the preparation of
pure powder and single-crystal samples. Here, we present a novel sol-gel synthesis procedure
that reduces preparation time, reduces impurities and yields superior single-crystal samples.
Amongst solid-state ionic conductors, the layered bismuth vanadates (Fig. 1) have the highest
known oxygen conductivities. The bridging oxygen sites (red/gray) that connect the corner-
sharing VO6 octahedra (blue) are actually 25% vacant. With shuffle-puzzle-like motion, oxygen
atoms and vacancies flow in opposite directions within the vanadium-oxide sheets under the
influence of an applied voltage.
The traditional BIMEVOX synthesis methods (Fig. 2) use high temperature annealing of the
different components. This method relies on the mechanical and diffusive mixing of the
individual atomic species. Clumps of material must be physically broken down with pressure and
heat before a chemical reaction occurs. Traditional high-temperature syntheses typically involve
slow annealings at 750°C, leading to Bi evaporation and the subsequent formation of impurity
phases. Our sol-gel method, however, completely dissolves the precursors in an aqueous
solution which allows and provides atomic-scale homogeneity at the outset (Fig. 2). The desired
chemical reactions then proceed quickly to completion at much lower temperatures (570° C) and
without mechanical grinding.
Single-crystal BIMEVOX samples (Fig. 4) are grown from pressed powder pellets (Fig. 3). The
pellets are heated to the melting point of BIMEVOX (around 850°C) and cooled slowly to
encourage crystal formation. Crystal sizes range from 1-500 microns in size.
The broad features in the diffuse-scattering pattern from BINiVOX encode information about
nanoscale oxygen vacancy defect structures in the material that may ultimately determine its
macroscopic oxygen-ion conductivity. We plan to further investigate these structures using
state-of-the art x-ray scattering instruments here at BYU and at national synchrotron x-ray user
facilities. Our new x-ray diffractometer is equipped with a rotating anode source, focusing x-ray
optics, a 16-megapixel x-ray camera, two kappa-geometry goniometers, and both high and low-
temperature capabilities. It has been optimized specifically for the demands of these difficult
The novel sol-gel synthesis of layered transition-metal-doped bismuth vanadates presented here
offers several advantages over traditional synthesis procedures. Through the sol-gel method,
greater atomic homogeneity is achieved, along with lower reaction temperatures and shorter
annealing times. These advantages result in higher-purity BIMEVOX powder products. We
plan on synthesizing a wide range of Fe and Ni doped BIMEVOX powder products ranging from
x=0.0 to x=0.6. With this wide range of compositions, single crystals will be grown and
analyzed using x-ray diffraction and diffuse scattering techniques. Our single-crystal and
powder samples will be critical to the characterization of defect structure-property relationships
in the BIMEVOX family of solid-state oxygen-ion conductors.
Figure 1 – Diagram of layered transition-metal-doped bismuth vanadates. The red/gray spheres
represent the bridging oxygen sites, and the blue octahedra represent the corner-sharing VO6
Figure 2 – Diagram of traditional BIMEVOX synthesis
methods versus novel sol-gel synthesis.
Figure 3 –This pellet was formed from BIMEVOX powder heated to 850°C
Figure 4 – This single crystal was obtained by slowly cooling a BIMIVOX powder pellet