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 degrees C using liquid sodium as a growth medium. Black Ca2GeN2 is isostructural with the previously reported strontium analogue. It is tetragonal P4(2)/mbc (no. 135) with a = 11.2004(8) Angstrom, c = 5.0482(6) Angstrom, and Z = 8. It contains GeN24- 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 GeN48- tetrahedra and is monoclinic P2(1)/c (no. 14) with a = 9.2823(8) Angstrom, b = 6.0429(5) Angstrom, c = 11.1612(9) Angstrom, beta = 116.498(6)degrees, and Z = 4. Clear, colorless Ca5Ge2N6, also with fully oxidized germanium, contains infinite chains, (1)(infinity)[GeN2N2/25-], 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.