Lattice dynamics in intermetallic Mg2Ge and Mg2Si

Phase relations and thermoelasticity of magnesium silicide at high pressure and temperature

Within the exploration of sustainable and functional materials, narrow bandgap magnesium silicide semiconductors have gained growing interest. Intriguingly, squeezing silicides to extreme pressures and exposing them to non-ambient temperatures proves fruitful to study the structural behavior, tune the electronic structure, or discover novel phases. Herein, structural changes and thermoelastic characteristics of magnesium silicides were probed with synchrotron x-ray diffraction techniques using the laser-heated diamond anvil cell and large volume press at high pressure and temperature and temperature-dependent synchrotron powder diffraction. Probing the ambient phase of Mg₂Si (anti-CaF₂-type Mg₂Si, space group: Fm3¯m) at static pressures of giga-Pascals possibly unveiled the transformation to metastable orthorhombic anti-PbCl₂-type Mg₂Si (Pnma). Interestingly, heating under pressures introduced the decomposition of Mg₂Si to hexagonal Mg₉Si₅ (P6₃) and minor Mg. Using equations of state (EoS), which relate pressure to volume, the bulk moduli of anti-CaF₂-type Mg₂Si, anti-PbCl₂-type Mg₂Si, and Mg₉Si₅ were determined to be B₀ = 47(2) GPa, B₀ ≈ 72(5) GPa, and B₀ = 58(3) GPa, respectively. Employing a high-temperature EoS to the P-V-T data of anti-CaF₂-type Mg₂Si provided its thermoelastic parameters: B_T0 = 46(3) GPa, B'_T0 = 6.1(8), and (∂B_T0/∂T)_P = -0.013(4) GPa K^-1. At atmospheric pressure, anti-CaF₂-type Mg₂Si kept stable at T = 133-723 K, whereas Mg₉Si₅ transformed to anti-CaF₂-type Mg₂Si and Si above T ≥ 530 K. This temperature stability may indicate the potential of Mg₉Si₅ as a mid-temperature thermoelectric material, as suggested from previous first-principles calculations. Within this realm, thermal models were applied, yielding thermal expansion coefficients of both silicides together with estimations of their Grüneisen parameter and Debye temperature.

Effect of the Fabrication Technique on the Thermoelectric Performance of Mg-Based Compounds-A Case Study of n-Type Mg2Ge

High performance, low cost, and low toxicity have been the main characteristics associated with magnesium-based thermoelectric materials. Nevertheless, the high volatility of magnesium creates challenges in the synthesis of these materials. In this work, n-type Mg₂Ge is synthesized using a solid-state technique, fully characterized, and compared with Mg₂Ge fabricated through different processes. We have found that Bi is an ineffective dopant in Mg₂Ge and precipitates into Mg₂Bi₃. Regardless of the technique used, the loss of Mg by evaporation and formation of precipitates in Bi-doped samples resulted in a low charge carrier concentration and, consequently, a low power factor. The precipitates significantly reduced the lattice thermal conductivity, however, leading to a figure-of-merit, zT, of 0.4 at 725 K, improving the previously reported figure-of-merit, zT, of 0.2 for Sb-doped Mg₂Ge. This work highlights the impact of the fabrication technique on the thermoelectric performance of Mg-based compounds.

Thermoelectric performance of n-type Mg2Ge

Magnesium-based thermoelectric materials (Mg₂X, X = Si, Ge, Sn) have received considerable attention due to their availability, low toxicity, and reasonably good thermoelectric performance. The synthesis of these materials with high purity is challenging, however, due to the reactive nature and high vapour pressure of magnesium. In the current study, high purity single phase n-type Mg₂Ge has been fabricated through a one-step reaction of MgH₂ and elemental Ge, using spark plasma sintering (SPS) to reduce the formation of magnesium oxides due to the liberation of hydrogen. We have found that Bi has a very limited solubility in Mg₂Ge and results in the precipitation of Mg₂Bi₃. Bismuth doping increases the electrical conductivity of Mg₂Ge up to its solubility limit, beyond which the variation is minimal. The main improvement in the thermoelectric performance is originated from the significant phonon scattering achieved by the Mg₂Bi₃ precipitates located mainly at grain boundaries. This reduces the lattice thermal conductivity by ~50% and increases the maximum zT for n-type Mg₂Ge to 0.32, compared to previously reported maximum value of 0.2 for Sb-doped Mg₂Ge.