Developments in Ceramic Materials Research

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S. Ardizzone, C. L. Bianchi, G. Cappelletti et al.
Results in Figure 10 confirm that the blue colour is solely due to V 4+ species. The role of
the mineralizer can be considered to be that of stabilizing the tetravalent state of vanadium,
assisting the charge balance in the zircon lattice.
From the diffuse reflectance spectra L*, a* and b* parameters have been calculated. The
obtained data are reported in Figure 11. While the sample V0.1 without the mineralizer is
green (b* small and positive), in the presence of all the mineralizers, except for KF and KCl,
the b* parameter is large and negative. The lowest values of b*, corresponding to best
development of the blue colour, are obtained with NaF and LiF as the mineralizers. The lower
L* value of NaF indicates that the colour is more intense. The large and positive b* values for
samples added with potassium salts indicate that the samples show an intense (large L*)
yellow colour.
CONCLUSION
Pr-, V-, and Fe-doped zircon pigments were prepared by combining a sol-gel reaction
with calcinations steps in the range 600-1200°C. Investigations of the structural features of
the calcined products show that all the three metals promote the formation of the zircon lattice
at lower temperatures with respect to the un-promoted samples. The three metals show
different promoting effects on the formation of the ZrSiO4 structure at all temperatures. The
promotion sequence, V>Fe>Pr, follows, inversely, the sequence of the ionic radius of the
metal ions. In order to interpret this effect the localization of the metals in the zircon lattice
must be discussed. In the case of V-doped pigments the addition of the guest metal was
shown to provoke an expansion of the zircon lattice. This expansion is referred to the
substitution of a smaller ion with a bigger ion. Considering the present three guest metal ions
it can be observed that the size of V 4+ is the closest one to that of Si in the tetrahedral sites
and that the size of the ions increase progressively in passing from V to Pr. The present
results allow therefore to draw two different conclusions with respect to this point. First, the
sequence is to be related primarily to the matching between the size of the guest metal and
that of the species in the lattice; second since the size of Zr in the lattice is much larger than
that of Si, the above reported sequence apparently shows that for all the three metals the
promoting effect is related to the occupation by the guest ion of the tetrahedral Si 4+ positions.
The colour properties of the three groups of pigments differ not only intrinsically due to
the different colours but also with respect to the colour dependence on the metal content of
the samples. The colour of the V-doped samples is invariably green, and in no case assumes
the desired blue shade. Apparently there is no direct relation, at least in the case of V-ZrSiO4,
between the formation of the zircon structure and the development of the blue colour. To
promote the blue colour mineralizers (monovalent salts) must be added to the mixture.
The presence of alkaline metals (Li, Na, K) both as fluorides and as chlorides provokes
important variations in the structural and optical features of the pigments. The promotion of
the zircon structure follows the order Li>Na>K. In the case of the presence of Li salts zircon
forms, to significant extents, even at 600°C. However the most intense blue colour of the
pigment is not obtained in the presence of Li salts but by addition of NaF. The low intensity
of the blue colour of the Li-promoted pigments is interpreted, on the grounds of XPS