William D. Pyrz

Atomic-level imaging of Mo-V-O complex oxide phase intergrowth, grain boundaries, and defects using HAADF-STEM

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo5O14-type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase equilibria of complex Mo-V-O materials. This study exemplifies the utility of STEM for the characterization of local structure at crystalline phase boundaries.

Using Aberration-corrected STEM Imaging to Explore Chemical and Structural Variations in the M1 Phase of the MoVNbTeO Oxidation Catalyst

We report results of aberration-corrected STEM imaging of the orthorhombic M1 phase in the MoVNbTeO selective oxidation catalyst prepared using two different solution techniques. Atomic coordinates and cation site occupancies for both sample preparations are compared with a previously reported Rietveld-based model for this structure. The high angle annular dark-field (HAADF) images from the two preparations varied significantly only in the occupancy of the heptagonal channels. The M1 sample prepared under ambient conditions exhibited partial occupancy, whereas the hydrothermally prepared sample had heptagonal channels that were primarily vacant. Compared to the Rietveld model, both the atomic positions and the occupancies were found to be consistent, with the exception of the Mo4 and the two Te sites. The Mo4 site shows a lower HAADF intensity than expected, suggesting that appreciable V content may be present, whereas both Te sites had low Te occupancy as compared with the refined model, likely due to e-...