Formation and stability of the clathrate-I structure in the systems Sr–(Ni,Cu,Zn)–Ge based on experimental and DFT studies

The phase relations in the Ge-rich corner of the Sr–Ni–Ge system are investigated at 700 °C by X-ray powder diffraction and electron probe microanalysis. The formation of one ternary compound, SrNiGe3, crystallizing in the BaNiSn3-structure type, is observed. No clathrate-I compound was detected in as cast or annealed state. tical calculations based on density functional theory (DFT) were performed to gain information on the heat of formation for three different clathrate-I compounds, Sr8T6Ge40 (T = Ni, Cu, Zn) and relevant other Ge-rich ternary compounds, namely SrT2Ge2 (T = Cu, Zn) and SrNiGe3. Based on these data and the phase relations at 700 °C energies of formation of the clathrate-I phases were calculated and predictions of the phase equilibria at 0 K are provided.