Developments in Ceramic Materials Research

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S. Ardizzone, C. L. Bianchi, G. Cappelletti et al.
is the major component. The phase compositionof Fe-ZrSiO4 (Figure 2d) is intermediate
between V- and Pr-doped samples, the presence of zircon being around 5%. In this case the
presence of hematite, either present as an incapsulated phase or segregated can be
appreciated.
Figure 2. Powder X-ray diffraction lines of a) non doped, b) Pr, c) V, and d) Fe doped pigments at the
same metal/Zr content (0.02 molar ratio) calcined at 1000°C. T and M: tetragonal polymorph and
monoclinic zirconia (ZrO 2), respectively; Z: zircon (ZrSiO 4); H: hematite. Inset: the relative phase
enrichment, by Rietveld refinement.
The role played by the metal on the promotion of the crystal growth and formation of the
zircon lattice is apparent. This effect has been observed previously in the literature in the case
of all the present metal dopants. In the literature the final formation of ZrSiO4 is considered to
occur first, by crystallization of a phase having the t-ZrO2 structure followed by a phase
transformation from t-ZrO2 to m-ZrO2 and, almost simultaneously, by the reaction between
m-ZrO2 and an amorphous silica rich phase, leading to zircon [31,32].
The direct cross-comparison between the effects produced by the three metals in samples
obtained by the same preparative route is, instead, not commonly reported in the literature.
From results in Figure 2, vanadium appears to be the best promoter of the structure and the
promoting sequence is: V>Fe>Pr. The same sequence is maintained also at a higher
calcination temperature (1200°C). Figure 3 reports the amount of zircon phase in the samples,
obtained by quantitative elaboration of X-ray diffraction data, as a function of the metal
loading. The comparison between the three series of pigments is striking.