  The effect of annealing on the superconducting properties of niobium single
crystals cut from the same master boule was studied by local and global
magnetic measurements, as well as scanning tunneling microscopy (STM). The
formation of large hydride precipitates was observed in unannealed samples. The
variation in structural and magnetic properties was studied after annealing
under high vacuum at 800 C, 1400 C, and near the melting point of niobium (2477
C) for a few seconds. The initial samples had a high hydrogen content.
Polarized optics and magneto-optical studies show that the formation of large
niobium hydride precipitates is suppressed already by 800 C annealing. However,
the overall superconducting properties in the annealed samples did not improve
after annealing, and in fact, worsened. The superconducting transition
temperature decreased, the upper critical field increased, and the pinning
strength increased. Parallel studies were conducted using STM, where the sample
was annealed initially at 400 C, measured, annealed again at 1700 C, and
measured again. These studies revealed a ``dirty'' superconducting gap with a
significant spatial variation of tunneling conductance after annealing at 400
C. The clean gap was recovered after annealing at 1700 C. It is likely that
these results are due to oxygen redistribution near the surface, which is
always covered by oxide layers in as-grown crystals. Overall, the results
indicate that vacuum annealing at least up to 1400 C, while expected to remove
a large amount of hydrogen, introduces additional nanosized defects, perhaps
hydride precipitates, that act as efficient pair-breaking and pinning centers.
