  We present extensive spectroscopic observations for one of the closest type
Ia supernovae (SNe Ia), SN 2014J discovered in M82, ranging from 10.4 days
before to 473.2 days after B-band maximum light. The diffuse interstellar band
(DIB) features detected in a high-resolution spectrum allow an estimate of
line-of-sight extinction as Av=1.9+/-0.6 mag. Spectroscopically, SN 2014J can
be put into the high-velocity (HV) subgroup in Wang's classification with a
velocity of Si~II 6355 at maximum light as about 12200 km/s, but has a low
velocity gradient (LVG, following Benetti's classification) as 41+/-2 km/s/day,
which is inconsistent with the trend that HV SNe Ia generally have larger
velocity gradients. We find that the HV SNe Ia with LVGs tend to have
relatively stronger Si III (at ~4400 Angstrom) absorptions in early spectra,
larger ratios of S II 5468 to S II 5640, and weaker Si II 5972 absorptions
compared to their counterparts with similar velocities but high velocity
gradients. This shows that the HV+LVG subgroup of SNe Ia may have intrinsically
higher photospheric temperature, which indicates that their progenitors may
experience more complete burning in the explosions relative to the typical HV
SNe Ia.
