Synthesis of multi trace element doped Fe_0.6Mn_0.4Ta₂O₆ tantalite reference crystals by Czochralski method

Quality assurance of natural raw materials (e.g. ores) with beam-based microanalytical methods requires a proper set of homogeneous, matrix-matched reference materials (RMs) doped with trace elements relevant to resource technology applications. Natural minerals usually exhibit chemical heterogeneity at μg/g sampling masses and are unsuitable as RMs for in-situ chemical microanalysis. On the other hand, available synthetic RMs (e.g. glasses, pressed pellets) fail to satisfy matrix-match criterion.
A novel strategy has been established to obtaining such RMs through the synthesis of multi trace element doped phases that would be subsequently tested for chemical and structural homogeneity with both microscopic and spectroscopic spatially-resolved microanalytical methods.
A dark brown tantalite crystal from the melt of composition Fe_0.6Mn_0.4Ta₂O₆ has been grown by the conventional Czochralski method (melting point of about 1600°C) with the use of an Ir crucible and a protective atmosphere consisting of 85% Ar, 10% CO₂ and 5% CO. The growth rate of 1 mm/h and rotation rate of 10 rpm were applied. The boule was 17 mm in diameter and 30 mm in length (Fig. 1). Standard mineralogical thin sections were prepared for chemical analysis.
Back-scatter Electrons imaging (BSE) was implemented to check for the presence of impurities and other phases within the crystal. Electron Probe Microanalysis (EPMA) and Particle Induced X-ray Emission (PIXE) were used to determine the composition and homogeneity of the crystal. Compositional maps were prepared for each element for each method. As shown in Fig. 2 both BSE-EPMA and PIXE consistently proved stoichiometric composition of the crystal what is in agreement with stoichiometric composition of the melt. EPMA and PIXE compositional maps showed homogenous lateral distribution of all constituents of the crystal (Fig. 3).
The obtained tantalite phase turned out to be homogenous and stable. A matrix of the same stoichiometry will be used in further experiments – synthesis of tantalite crystals doped with different sets of technological trace elements - Sc, Ti; Y, Zr, W; La, Ce, Nd – each at concentration of 0.01 wt%.
Fig. 2
Mg wt% Ca wt% Fe wt% Mn wt% Ta wt% O wt% Total
EPMA 0.000048 0.00013 5.94 4.29 70.38 18.51 99.13
PIXE b.d. b.d. 6.44 4.08 70.98 18.50 100.00
b.d.= below limit of detection
Fig. 1 Tantalite boule
Fig. 3 PIXE Fe-Kα intensity distribution map