Synthesis and investigation of langbeinite-related strontium- and iron-containing phosphates


Abstract: The partial substitution of potassium atoms by strontium atoms in initial matrix K$_{3}$Fe$_{2}$(PO$_{4})_{3}$ with formation of complex phosphates K$_{3-2x}$Sr$_{x}$Fe$_{2}$(PO$_{4})_{3}$ ($x =$ 0.75, 1.0, and 1.25) was determined using a solid-state reaction method. According to powder X-ray diffraction (XRD) data, the prepared phosphates with $x =$ 1.0 and 1.25 crystallize in a cubic system (space group ${P2}_{1}$3) and belong to langbeinite family compounds, while for the sample with $x =$ 0.75 a mixture of langbeinite-type phase and a minor amount of K$_{3}$Fe$_{2}$(PO$_{4})_{3}$ was obtained. The decreasing trend of lattice parameter $a$ with increase of Sr amount in the phosphate confirmed the incorporation of strontium in the langbeinite-related structure. The thermal analysis results indicate that the obtained langbeinite-related compounds are stable upon heating and their melting points are higher than 1000 $^{\circ}$C. It should be noted that the determined iron-containing phosphates can be obtained at temperature lower than $^{\circ}$C, which creates a perspective for using them as matrixes for immobilization of radioactive strontium.

Keywords: Langbeinite, X-ray diffraction, phosphate, strontium, thermogravimetry/differential thermal analysis

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