Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 103
The diffuse reflectance spectrum of Ti(PO3)3 exhibits a strong band centered at 17300
cm -1 which corresponds to the spin allowed transition 2 Eg � 2 T2g in an octahedral symmetry
and can be associated with the ligand-field splitting parameter 10 Dq. A shoulder can be
observed at 14800 cm -1 which is attributed to the Jahn-Teller splitting of the excited 2 Eg state.
The diffuse reflectance spectrum of V(PO3)3 shows two absorptions centered at 24400
and 33600, which are assigned to the spin allowed transitions 3 T1g(F) � 3 T2g(F) and 3 T1g(P),
respectively. Moreover, there is a weak signal about 10200 cm -1 which corresponds to the
spin forbidden transition 3 T1g(F) � 1 Eg, 1 T2g. Using a simple crystal field approximate model
for an octahedral d 2 system the Dq and Rach (B and C) parameters were calculated as Dq=
1610 cm -1 , B= 665 cm -1 and C= 1863 cm -1 .The values obtained are in good agreement with
those given in the literate for this kind of compounds [23-25].
For chromium compounds, Cr(PO3)3 (1) and Cr2(P6O18) (2), two strong absorptions were
observed at 16000, 22500 and 15800, 22000 cm -1 for (1) and (2), respectively. The bands
corresponding to the Mo(PO3)3 (3) were observed at 23000 and 28100 cm -1 . These results are
in good agreement with those observed for the Cr(III) and Mo(III) in octahedral geometry
[23,26-29]. Using a simple crystal field approximate model, the ligand-field splitting
parameter 10 Dq and the Racah parameter B may be calculated from the energies of the two
lowest spin allowed transitions. For (1) we are find Dq= 1600 cm -1 and B= 690 cm -1 and for
(3) Dq= 2300 cm -1 and B= 475 cm -1 . The values obtained for compound (2) are 1580 and 620
cm -1 for the Dq and B parameters, respectively. The B values for (1)-(3) are approximately
70% of those corresponding to the free ions (918 and 610 cm -1 for the chromium and
molybdenum, respectively). This fact implies a significant degree of covalence in the M-O-M
(M= Cr and Mo) bonds.
The weak absorptions observed at 14700, 15400 and 14600, 15300 for (1) and (2),
respectively and 10200 and 10600 for phase (3) may be assigned to the spin forbidden
transitions 4 A2g � 2 Eg 2 T1g considering the ratios Dq/B of 2.46 and 2.55 and 5.03 for the
chromium and molybdenum compounds. The signals of lower energy can be attributed to the
transition 4 A2g � 2 Eg given in the diagram levels of Figure 2. The values of the C parameters
obtained from the forbidden transition are 3390, 3412 and 2180 cm -1 , respectively. Finally,
the weak absorptions observed at 21300, 21800 and 16000 cm -1 for compounds (1), (2) and
(3), respectively, may be assigned to the spin forbidden transition 4 A2g � 2 T2g. In the case of
the molybdenum metaphosphate this band appears at lower frequency than that observed for
the spin-allowed transition 4 A2g � 2 T2g(F) as expected following the energy levels of these
compounds (Figure 2).
The diffuse reflectance data show that both allowed and forbidden transitions have higher
E/B ratios in the molybdenum metaphosphate than in the chromium phases. This fact is due
to the different positions of these metallic ions in the transition series (Table 2). Besides, the
value of the Racah parameter B is higher in the chromium compounds than in the
molybdenum phase, but the percentage of reduction of the B parameter ( ≅ 70%) is indicative
of a similar degree of covalence in the M-O (M= Cr, Mo) bonds, in these compounds.
In the diffuse reflectance spectrum of Fe(PO3)3 very weak characteristic bands belonging
to Fe 3+ are present. These bands correspond to the forbidden transitions of a d 5 high spin
configuration cation from the ground level 6 A1g( 6 S) to the excited 4 T1g( 4 G) , 4 T2g( 4 G) ,
4 A1g( 4 G), 4 Eg( 4 G) and 4 T2g( 4 D) levels. The low intensity of the bands makes difficult the
assignation of their maxima and, so, to calculate both the Dq and Racah (B and C)
parameters.