Developments in Ceramic Materials Research

a
c
Synthesis, Spectroscopic and Magnetic Studies… 101
b
Figure 1. Crystal structure of (a) M(PO 3) 3 (M= Ti, V, Cr, Mo and Fe) and (b) Cr 2(P 6O 18).
4.2. IR Spectroscopy
Different kinds of phosphates with a definite structural type (orto, pyro, chaing o ring
structure) can be identified by IR spectroscopy [20,21]. Metaphosphate phases are classified
in two large families of chain o ring structures. For these condensed phosphates an
approximation may be needed due to the existence of two types of P-O bonds: for an
“external” oxygen, the P-Oext distance is shorter (and the stretching frequency accordingly
higher) than for an “internal” (or bridging) oxygen whitin the P-O-P chaing o ring. Moreover,
in short chain polyphosphates a further splitting takes place owing the existence of PO2 and
PO3 groups in the oxoanions. The common features that exhibit all of metaphosphates are the
existence in the PO2 groups of P-Oext distances shorter than those observed in the PO3 and
PO4 groups of diphosphate and phosphate compounds, respectively [21,22].
In Table 1 the most important bands of the metaphosphates studied in this work are
given, together with their empirical assignments. In all cases can be observed the most
characterized feature of a metaphosphate (whatever its chain or ring structure) the existence
of one or several and strong IR bands at frequencies higher than 1200 cm -1 .
4.3. UV-VIS Spectroscopy
The most important bands obtained for the M(PO3)3 (M III = Ti, V, Cr, Mo and Fe)
metaphosphates from the absorption electronic spectra, collected by the diffuse reflectance
method, are listed in Table 2.