more water molecules in the first coordination sphere of Cr(III) by a weaker ligand,
e.g., the lattice WO 3 oxygen.
Returning to ESR investigation, the signal d in Fig. 5.21, g = 3.5, is attributable
to Cr(III) centers in a very distorted crystal field. In annealed samples, a new series
of signals become evident, corresponding to a low-spin Cr species, whose g values
all indicate a d
1 electronic configuration in axial or rhombic field. The magnetic
parameter values are g ‖ (A) = 1.967, g † (A) = 1.886, g 1 (B) = 1.949, g 2 (B) = 1.912,
g 3 = 1.823, and both species are attributable to isolated Cr(V) centers whose
symmetry is strongly distorted by the presence of [Cr = O]
3+ groups (chromyl).
3400
3000
3000
3200
0
2000 4000
gauss
gauss
3400
3000
3000
3200
gauss
6000 8000
0
2000 4000
gauss
6000 8000
0
60
30
0
0
40
80
EPR indons. / a.u
EPR indons. / a.u
EPR indons. / a.u
EPR indons. / a.u
700
1400
300
150
0
d
d
d
d
δ
δ
β
β
γ
γ
c
c
c
c
b
b
b
b
a
a
a
a
g 1 (A) = 1.967
g 1 (A) =
g 1 (B) =
g 2 (B) =
g 3 (B) =
g 3 (A) = 1.886
(a)
(b)
Fig. 5.22 ESR spectra of the same samples as in Fig. 5.21, while a annealed at 400 °C and
b annealed at 700 °C [18]
116
5 The Symmetry Properties Describe the Electronic Structure …