A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Four new manganese germanates and silicates, A2MnGeO4 (A = Li, Na) and A2MnSiO4 (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li2MnGeO4 is orthorhombic (Pmn21) layered, isostructural with Li2CdGeO4, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A2MXO4, Pna21 and Pbca AMO2), average bond angles on bridging oxygens (here, Mn–O–X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3m) structure typified by CsAlO2. The sublattices of the magnetic Mn2+ ions in both structure types under study (Pmn21 and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li2MnSiO4 were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and...