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

• Isothermal section of the Ge rich part of the ternary system Ge–Ni–Sr at 700 °C.
• DFT-calculated heat of formation for 3 clathrate-I compounds.
• Experimental and theoretical results explain the formation of the clathrate-I phase.

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.Theoretical 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.