  We report a structural study of the Weyl semimetals TaAs, TaP, NbP, and NbAs,
utilizing diffraction techniques (single crystal x-ray diffraction and energy
dispersive spectroscopy) and imaging techniques (transmission electron
microscopy/scanning transmission electron microscopy). We observe defects of
various degrees, leading to non-stoichiometric single crystals of all four
semimetals. While TaP displays a large pnictide deficiency with composition
TaP$_{0.83(3)}$, and stacking faults accompanied by anti-site disorder and site
vacancies, TaAs displays transition metal deficiency with composition
Ta$_{0.92(2)}$As and a high density of stacking faults. NbP also displays
pnictide deficiency, yielding composition NbP$_{0.95(2)}$, and lastly, NbAs
display very little deviation from a 1:1 composition, NbAs$_{1.00(3)}$, and is
therefore recommended to serve as the model compound for these semimetals.
