STUDY OF ELECTRON, PHONON AND CRYSTAL STABILITY VERSUS THERMOELECTRIC PROPERTIES IN Mg2X(X= Si, Sn) COMPOUNDS AND THEIR ALLOYS

We present results of extensive theoretical and experimental investigations of Mg2Siand Mg2Snand their Mg2Si1-xSnxalloys. Electronic and phonon properties of binary compounds were studied by ab initiocalculations. Then, both compounds were synthesized by the solid-state reaction and electrical resistivity and thermopower was measured at high temperature (300–900 K). In both the compounds, the theoretical bandgaps (0.56 eV in Mg2Siand 0.16 eV in Mg2Sn) agree very well with the experimental values (0.6 eV in Mg2Siand 0.17 eV from activation law in Mg2Sn) upon applying the modified Becke–Johnson semilocal exchange potential and including spin–orbit coupling in the calculations. Calculated phonon spectra support crystal stability of both compounds. For Mg2Si, the contributions from Siand Mgare spread over all the spectrum (0–10 THz), whereas in the case of Mg2Sn, a gap opens around 4 THz with Snand Mgcontributions dominating in lower and higher energy range, respectively. The calculated heat capacity at low temperature (0–300 K) fairly agrees with available experimental data. The crystal structure of Mg2Si1-xSnxwith x = 0, 0.25, 0.4, 0.75, 1 was studied by X-ray diffraction measurements. The alloy compositions exist in the ranges 0 < x < 0.4 and 0.6 < x < 1 and the obtained samples are almost single phased. Detailed crystal stability study with temperature revealed that all powder samples started to decompose into MgO, Siand Snat ~ 630 K. For hot pressed bulk materials, the decomposition is much slower than in powder compounds but it still appears. Interestingly, thermoelectric properties measurements performed in Mg2Si1-xSnxshow that both electrical resistivity and thermopower curves are repeatable during temperature cycles up to 770 K. On the other hand, temperature-dependent X-ray powder diffraction suggests that these compounds are not stable. Furthermore, electronic structure calculations of almost 40 impurities (s- and p-block, 3d and 4d transition metal elements) diluted in Mg2Siand Mg2Si0.75Sn0.25were performed by the KKR-CPA method. Based on calculated impurity density of states the character of doping (n or p) is predicted, which, however, strongly depends on the substituted crystallographic site.