K<sub>2</sub>M<sup>III</sup><sub>2</sub>(M<sup>VI</sup>O<sub>4</sub>)(PO<sub>4</sub>)<sub>2</sub> (M<sup>III</sup> = Fe, Sc; M<sup>VI</sup> = Mo, W), Novel Members of the Lagbeinite-Related Family: Synthesis, Structure, and Magnetic Properties

The possibility of PO<sub>4</sub><sup>3–</sup> for MoO<sub>4</sub><sup>2–</sup> partial substitution in the langbeinite framework has been studied by exploration of the K–Fe­(Sc)–Mo­(W)–P–O systems using the high-temperature solution method. It was shown that 1/3PO<sub>4</sub><sup>3–</sup> for MoO<sub>4</sub><sup>2–</sup> substitution leads to formation of three novel compounds K<sub>2</sub>Fe­(MoO<sub>4</sub>)­(PO<sub>4</sub>)<sub>2</sub>, K<sub>2</sub>Sc­(MoO<sub>4</sub>)­(PO<sub>4</sub>)<sub>2</sub>, and K<sub>2</sub>Sc­(WO<sub>4</sub>)­(PO<sub>4</sub>)<sub>2</sub> with slightly increased lattice parameters and significant distortion of the anion tetrahedra without structure changes. In contrast, the antiferromagnetic structure is modified by substitution in the low-temperature region. The structural peculiarities are discussed in light of bond-valence sums calculations.