  In their recent publication, Zhao et al. [J. Appl. Phys. 110, 033523 (2011)]
claim to have found a new three-dimensional relationship for
niobium-on-sapphire epitaxy. However, two critical errors were made in the
analysis of x-ray diffraction measurements. The crystal structure of sapphire
(alpha-Al2O3) was erroneously cited as hexagonal close-packed, and
crystallographic orientations of sapphire were misidentified. Correcting these
errors, one finds their claim unjustified.
