Synthesis, structure, optical properties, and magnetism of the manganese chalcogenide beryllosilicate and beryllogermanate sodalites

The compounds Mn-8[BeSiO4](6)X-2 (X = S, Se, Te) Mn-8[BeGeO4](6)X-2 (X = S, Se) have been synthesized bq solid state reactions of the constituent oxides and MnX at 1000-1100 degrees C in scaled quartz ampules. The materials crystallize with the cubic sodalite unit cell with an ordered Be/Z(4+) (Z(4+) = 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 d(5) tetrahedral manganese. Correlations between structural parameters and IR absorptions are reported for the families of beryllosilicate and beryllogermanate frameworks M-8[BeZO(4)](6)X-2 (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 Mn4X cluster.