The structural properties of modulated SrGe6−x (x≈0.45) were investigated by means of single-crystal and powder X-ray diffraction combined with quantum chemical calculations. The framework compound SrGe6−x adopts a defect variant of the EuGa2Ge4-type crystal structure. Samples of the binary compound with nominal compositions 0≤x≤0.5 were synthesized at pressures from 5 to 6 GPa and a temperature of typically 1400 K. The product reveals diffraction peaks of the EuGa2Ge4-type subcell plus additional reflections indicating an ordered superstructure. Detailed crystal structure analysis evidences the incommensurate nature of the superstructure and a modulation of the vacancy ordering in the germanium network. The computations have shown that the non-stoichiometric composition of the framework with its local defect organization affects the calculated charge of the strontium anions. Although the chemical composition is close to a charge-balanced situation, temperature-dependent resistivity measurements showed metal-type conductivity. At ambient pressure SrGe6−x decomposes exothermally and irreversibly at T=680(10) K into SrGe2 and germanium, indicating its metastable nature at ambient pressure.
Dedicated to: Professor Wolfgang Bensch on the occasion of his 65th birthday.
We thank Susann Leipe for high-pressure syntheses, Marcus Schmidt and Susann Scharsach for DTA characterizations as well as Ulrich Burkhardt, Monika Eckert and Sylvia Kostmann for metallographic investigations. Valuable discussions with Frank Wagner and Miroslav Kohout are gratefully acknowledged.
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