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We report the heat capacities of MTa2O6 (M = Co,Ni,Fe,Mg). Antiferromagnetic ordering is observed at 6.67 K (Co), 10.55 K (Ni) and 8.1 K (Fe). Short range correlations above Tc contribute substantially to the heat capacity of the magnetic compounds. The magnetic part in the heat capacity of the Co and Ni compound can be described by a very anisotropic square planar S = 1/2 Ising model. Magnetic ordering in FeTa2O6 takes place in a S = 1 triplet ground state. The short range order contributions are compared to the heat capatities of S = 1 afm chains

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The study of the reactions between Se/Te mixtures and SbF5 or AsF5, resp., and between Se4(AsF6)2 and Te4(AsF6)2 results in the preparation of the compounds (I)‐(V) which contain the title cations (reactions in SO2, except for (IV) where a SO2/SO2ClF mixture is used as solvent, ratios of the starting materials and reaction times are given in a table, yields not given).