Multi-temperature structure of thermoelectric Mg2Si and Mg2Sn

A multi-temperature structural study of Mg2Si and Mg2Sn was carried out from 100 to 700 K using synchrotron X-ray powder diffraction. The temperature dependence of the lattice parameters can be expressed as a = 6.3272 (4) + 6.5 (2) × 10−5 T + 4.0 (3) × 10−8 T 2 Å and a = 6.7323 (7) + 8.5 (4) × 10−5 T + 3.8 (5) × 10−8 T 2 Å for Mg2Si and Mg2Sn, respectively. The atomic displacement parameters (ADPs) are reported and analysed using a Debye model for the averaged U iso giving Debye temperatures of 425 (2) K for Mg2Si and 243 (2) K for Mg2Sn. The ADPs are considerably smaller for Mg2Si than for Mg2Sn reflecting the weaker chemical bonding in the Mg2Sn structure. Following the heating, an annealing effect is observed on the lattice parameters and peak widths in both structures, presumably due to changes in the crystal defects, but the lattice thermal expansion is almost unchanged by the annealing. This work provides accurate structural parameters which are of importance for studies of Mg2Si, Mg2Sn and their solid solutions.