New Calcium Germanium Nitrides:  Ca2GeN2, Ca4GeN4, and Ca5Ge2N6

We report three new calcium germanium nitrides synthesized as crystals from the elements in sealed niobium tubes at 760 °C using liquid sodium as a growth medium. Black Ca2GeN2 is isostructural with the previously reported strontium analogue. It is tetragonal P42/mbc (no. 135) with a = 11.2004(8) Å, c = 5.0482(6) Å, and Z = 8. It contains GeN2 4- units which have 18 valence electrons, and consequently are bent, like the isoelectronic molecule SO2. In contrast, clear, orange Ca4GeN4 with fully oxidized germanium contains isolated GeN4 8- tetrahedra and is monoclinic P21/c (no. 14) with a = 9.2823(8) Å, b = 6.0429(5) Å, c = 11.1612(9) Å, β = 116.498(6)°, and Z = 4. Clear, colorless Ca5Ge2N6, also with fully oxidized germanium, contains infinite chains, 1 ∞[GeN2N2/2 5-], of corner-sharing tetrahedra similar to those found in pyroxenes. However, the precise structure of this latter phase has not yet been determined because of twinning problems.