Synthesis and Characterization of Single Crystal Zircon-Hafnon Zr(1–x)Hf(x)SiO4 Solid Solutions and the Comparison with the Reaction Products of a TEOS-Based Hydrothermal Route

Two synthesis routes of the zircon–hafnon solid solution series were carried out. The high-temperature route uses the growth of single crystals via a flux mixture that has been cooled down slowly from 1400 °C over 4 weeks. The reaction products were colorless and idiomorphic without byproducts. The hydrothermal tetraethoxysilane (TEOS)-based route represents the low-temperature method at 200 °C for approximately 7 days. The hydrothermal route yielded a white powder and scanning electron microscopy analysis thereof did not reveal any specific idiomorphic properties. However, the synthesis also featured some byproducts besides the zircon–hafnon solid solutions. Thermogravimetric analysis coupled with differential scanning calorimetry, and mass spectroscopy indicated, that hydrothermal reaction products feature the presence of organic residues originating from the starting materials. However, a specific dependency on the hafnium content could not be observed due to the data scatter. Infrared (IR) analysis revealed the presence of Zr/Hf-oxides. The structural characterization demonstrated that properties change constantly with the hafnium amount, however, gradual variations of some properties related to the composition of the solid solution series depend in part on the synthesis route, considering the c/a ratio and IR modes. Furthermore, analyses of the single crystals by Raman spectroscopy and μXRF suggested a nonequilibrated crystal growth based on the starting composition.

In Figures S1-S8 yellow cross symbols indicate the position of the µXRF measurement.The red circle denotes the diameter of the µXRF probe (~ 20 µm)    Estevenon 2020 [23] c(Hf)    Table S12-S22 denote the fitted results for the observed Raman modes of the single crystals (cf. Figure S13) of the zircon-hafnon solid solution series Raman -Linear Mode Fit in dependence on x(Hf) Linear fit results of the observed Raman modes of the zircon-hafnon solid solutions series.Linear µXRF fits based on composition of the solid solutions were also carried out (cf. Figure 8).

Figure S14 :
Figure S14: Comparison of blank KBr matrix (black) and KBr matrix with zircon (red: crushed single crystal)

Table S10 :
R2 values for a, c, c/a, and unit cell volume for Single Crystal XRD, crushed single PXRD, hydrothermal synthesis, and TG based on the linear fit in Figure7

Table S11a :
Crystallite size of crushed single crystals of the zircon-hafnon solid solutions dependence on the Miller indices

Table S11c :
Crystallite size of Zircon-Hafnon solid solutions of hydrothermal synthesis being subjected to TG analysis (1250 °C) dependence on the Miller indices Single crystal zircon-hafnon data solid solution series are provided separately in attached CIF-Files

Table S23 -
S27: IR-Modes of H 2 O and organic residues

Table S23 :
Stretching modes of H 2 O of flux grown grown samples of the solid solution series

Table S27 :
Modes of organic compounds at approx.1395 cm -1 and 1335 cm -1 in the hydrothermal samples of the solid solution series