Developments in Ceramic Materials Research

Coloured ZrSiO4 Ceramic Pigments 263
In fact separate Pr containing phases (either encapsulated into the host lattice or not) were
appreciable in X-ray spectra in the case of the larger Pr amounts and EDX analyses showed,
at the same time, a non homogeneous distribution of the metal.
For V-ZrSiO4 pigments the actual location of vanadium in the zircon structure is still
unclear and has been the subject of many studies, in some cases leading to controversial
interpretations. Several authors suggest that V ions should substitute for Zr in the
dodecahedral lattice positions [18,19]. Exactly the opposite conclusion was reached by Di
Gregorio and de Waal [20,21] that indicated V 4+ preferred the tetrahedral silicon site. A
further possibility some workers have preferred is that both sites can be occupied to a
significant extent [22,23]: Xiayou [22] and Chandley [23] have, in fact, reported very little
difference in the energy of the V 4+ occupying either of the two principal lattice sites. In the
case of V-ZrSiO4 prepared in our laboratory by a sol-gel procedure in the absence of
mineralizers [24] we have observed, also on the grounds of the elaboration of XANES results,
that for vanadium amounts lower than 0.05, the metal is prevailingly substituted to Si in the
tetrahedral positions of the lattice, while for loadings larger than 0.05, additional V is only
localized in the Zr 4+ dodecahedral positions of the lattice.
In the case of iron, Tartaj et al. [4] suggest that the formation of a solid solution between
the iron cations and the tetragonal zirconia lattice occurs before the formation of zircon. This
process generates vacancies which favour the nucleation of zircon. The authors [4] suggest
that after the zircon formation iron (III) segregates with the ensuing formation of hematite and
that hematite is the only responsible of the red colour of the iron zircon pigment. The same
authors have observed by XPS that the surface Fe/ZrSiO4 ratio is comparable to the average
iron content of the sample indicating that hematite is homogeneously distributed in the zircon
matrix. In the case of Fe-ZrSiO4 samples prepared in our laboratory by a sol-gel procedure
[25] we have instead observed, by quantitative elaboration of diffuse reflectance spectra, the
concomitant presence of both interstitial iron and hematite, in partial agreement with what
reported by Berry et al. [7,8].
The presence in the literature of divergent results concerning various aspects of these
systems is, at least in part, due to the manifold features of these materials, and to the close
interplay between different parameters introduced by the preparative steps. In fact the adopted
preparative route plays, in the present authors opinion, a key role in defining the final features
of the pigments. Consistently with these considerations, Monros et al. [5] have recently
shown that, in the case of coral-red zircon pigments, the conditions adopted for the sol-gel
process and particularly the sequence of the iron addition to the reacting mixture lead to the
formation of samples with different phase composition and colour.
In this work Pr-, V-, and Fe-doped ZrSiO4 pigments were obtained by a common sol-gel
path. The reaction, starting from Si and Zr alkoxides, in water-ethanol mixtures, was
employed to obtain the xerogels which were thermally treated in the temperature range 600-
1200°C. The role played by the addition of different mineralizers (LiF, LiCl, NaF, NaCl, KF,
KCl) to the reaction mixture was investigated in the case of V-doped pigments which develop
the desired blue colour only in the presence of an added salt.
X-ray diffraction data were refined by the Rietveld method and UV-Vis diffuse
reflectance spectra were recorded for all samples. XPS analyses were obtained in the case of
V-doped pigments to obtain indications on the localization of both mineralizer and metal
dopant.