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.