PARADIM Highlight #101—External User Project (2025)
P. Cardon, S. Adams, S. Smith, and Huiwen Ji (University of Utah)
“Wadsley-Roth”-phased crystal structures have defied accurate crystallographic determination since their discovery in 1965. These structurally challenging materials comprise transition-metal oxides whose oxygen vacancies condensed to give rise to intricate structures and short-range order compounded by second-order Jahn–Teller distortions experienced by the d0 cations. Structural studies have been hampered by the small crystals obtained from annealing, which led to unclear resolutions on the precise space groups of the materials.
Figure 1: (a) The Nb2O5–WO3 binary phase diagram adapted from ref (36) with line compounds Nb12WO33 (6:1) and Nb14W3O44 (7:3) highlighted in red and the 55 wt % WO3 self-flux shown as a dotted red line. (b) Schematic of the laser diode floating zone furnace setup used to grow high-quality crystals of 6:1 and 7:3. Note that the 6:1 growth did not require a flux. (c) Photos of the obtained centimeter-sized crystal boules.
Here, PARADIM’s Bulk Crystal Growth Facility supported the work of Prof. Ji’s group from the University of Utah using the laser diode floating zone technique and a traveling solvent method to obtain centimeter-sized high-quality crystals of niobium tungstate. The characterization revealed additional symmetry than previously assumed. Nb12WO33 crystallizes in I2/m and Nb14W3O44 in the I4/m space group. We also find oxygen-deficient (reduced) crystals to have significant twinning and drastically different electrochemical properties when cycled against a lithium anode from their oxidized counterpart.
In this article, we report the bulk crystal growth of two Wadsley–Roth materials, Nb12WO33 and Nb14W3O44, by using a floating zone technique. The structural characterization of these crystals revealed higher symmetry than previously assumed: Nb12WO33 and Nb14W3O44 adopt the space group of I2/m and I4/m, respectively. Crystals that are fully oxidized and partially reduced due to different synthesis conditions can be separated based on their colors. Using X-ray diffraction, we found that reduced samples have extensive small-angle twinning to accommodate the O deficiencies. These homogeneous single-crystal samples also allow us to measure their intrinsic electrochemical properties. We found that reduced crystals with substantial oxygen deficiencies improve their specific capacities by up to 27%, albeit with faster degradation. In light of the observed twinning, possible hypotheses for the observed contrast in the electrochemistry between the oxidized and reduced crystals were put forward and analyzed based on voltage profile and rate capability tests.
Normally, complex shear plane structures are thought to be inaccessible by zone-based growth techniques. This work demonstrates otherwise – that it is possible to use traveling solvent laser diode floating zone to crystalline complex ordered oxygen vacancy shear structure phases. This enabled not only definitive structural solutions, but also testing of literature predictions of their electrochemical intercalation performance.
Laser diode floating zones plus expertise in design of traveling solvent growth.
The work was initiated by Huiwen Ji (University of Utah) as part of a project on electronic energy materials.
P. Cardon, S. Adams, E. Crites, S. Kushwaha, S. Smith, T. M. McQueen, and H. Ji, "Intrinsic Structures and Electrochemical Properties of Floating Zone Grown Nb12WO33 and Nb14W3O44 Single Crystals," Cryst. Growth Des. 25, 1265–1275 (2025). DOI: 10.1021/acs.cgd.4c01704
This work was supported by the National Science Foundation (NSF) CAREER award (#2145832) and the NSF Graduate Research Fellowship Program (GRFP) award (#2139322). P.C. also acknowledges support from the University of Utah John and Marcia Price College of Engineering (UU COE) Greg McKenna Fellowship award. This research made use of the bulk crystal growth facility of the Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), which is supported by the NSF under Cooperative Agreement No. DMR-2039380. This work also made use of the University of Utah Nanofab EMSAL shared facilities of the Micron Technology Foundation Inc. Microscopy Suite sponsored by the UU COE, Health Sciences Center, Office of the Vice President for Research. TGA was performed at the University of Utah Materials Characterization Lab.
