Mechanical activation leads to a variety of defects in MoO3. The behav
ior of paramagnetic defects in unmilled MoO3 and in MoO3 mechanically
activated in a planetary mill for 600 min was investigated by ESR spec
troscopy. MoO3 milled for 600 min, and unmilled MoO3 though in a small
er concentration, contain Mo5+ ions; N-2 treatment at elevated tempera
tures increases the Mo5+ concentration in the samples due to oxygen lo
ss. Both MoO3 samples contain octahedrally coordinated Mo5+ species pr
ior to any treatment which interact with a proton in close proximity.
An additional signal is observed in both samples that is characteristi
c of F centers. An inverse Curie-Weiss behavior between 90 and 300 K i
ndicates the pairing of electrons at lower temperatures. Fivefold coor
dinated Mo5+ species in distorted, square-pyramidal symmetry are gener
ated in the bulk of unmilled MoO3 during N-2 flushing at 523 and 623 K
. MoO3 milled for 600 min contains, besides 6-fold coordinated Mo5+ in
C-2 upsilon symmetry, a second Mo5+ species in almost C-4 upsilon sym
metry. The Mo-O distances in the equatorial plane are shortened and th
ese Mo5+ centers are assigned to a precursor structure of crystallogra
phic shear planes. Besides B-fold coordinated Mo5+, tetrahedrally coor
dinated Mo5+ centers are also generated, if milled MoO3 is treated in
N-2 at 623 or 673 K which are assigned to the Mo sites on shear defect
s protruding from the surface. The total intensity of the Mo5+ signals
follows the Curie-Weiss law between 90 and 200 K. Above 200 K a devia
tion from linearity is observed. Hence, a considerable part of the Mo5
+ centers does not contribute to the signal intensity at higher temper
atures. In situ Raman spectroscopy of mechanically activated MoO3 reve
aled the formation of some nonstoichiometric MoO3-x after thermal trea
tment at 673 K in N-2 for 1 h. In addition, diffuse reflectance infrar
ed spectroscopy (DRIFTS) was used to characterize the stability of OH
groups on the surface of MoO3 and in the bulk of the mined sample. The
OH species located in the bulk of MoO3 was found to be stable even at
673 K under flowing N-2.