Synthesis, Structure, Optical Properties, and Magnetism of the Manganese Chalcogenide Beryllosilicate and Beryllogermanate Sodalites

The compounds Mn₈[BeSiO₄]₆X₂ (X = S, Se, Te) Mn₈[BeGeO₄]₆X₂ (X = S, Se) have been synthesized by solid state reactions of the constituent oxides and MnX at 1000-1100°C in sealed quartz ampules. The materials crystallize with the cubic sodalite unit cell with an ordered Be/Z₄₊ (Z₄₊ = Ge/Si) framework in the space group P43n. Their structures have been refined from powder X-ray and/or powder neutron diffraction data in order to investigate the coordination geometry of the encapsulated Mn₄X unit. The energies of the d-d transitions in the UV/vis spectra are characteristic of high-spin d⁵ tetrahedral manganese. Correlations between structural parameters and IR absorptions are reported for the families of beryllosilicate and beryllogermanate frameworks M₈[BeZO₄]₆X₂ (M = Cd, Mn; Zn, Z = Si, Ge; X = S, Se, Te). Preliminary magnetic measurements on the manganese-containing compounds indicate strong antiferromagnetic interactions, of order 100 K, between Mn₂₊ ions, but the observation of rounded maxima in the susceptibility at 10 K suggests spin-glass freezing rather than long-range magnetic order, indicating a high degree of geometrical frustration between the tetrahedrally coordinated spins of the Mn₁X cluster.