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

• Published in: The Journal of chemical physics Vol. 154; no. 14; p. 144701
• Main Authors: Gaida, Nico Alexander, Niwa, Ken, Sasaki, Takuya, Hasegawa, Masashi
• Format: Journal Article
• Language: English
• Published: United States 14.04.2021
• ISSN: 1089-7690
• DOI: 10.1063/5.0044648

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 Mg Si (anti-CaF -type Mg Si, space group: Fm3¯m) at static pressures of giga-Pascals possibly unveiled the transformation to metastable orthorhombic anti-PbCl -type Mg Si (Pnma). Interestingly, heating under pressures introduced the decomposition of Mg Si to hexagonal Mg Si (P6 ) and minor Mg. Using equations of state (EoS), which relate pressure to volume, the bulk moduli of anti-CaF -type Mg Si, anti-PbCl -type Mg Si, and Mg Si were determined to be B = 47(2) GPa, B ≈ 72(5) GPa, and B = 58(3) GPa, respectively. Employing a high-temperature EoS to the P-V-T data of anti-CaF -type Mg Si provided its thermoelastic parameters: B = 46(3) GPa, B' = 6.1(8), and (∂B /∂T) = -0.013(4) GPa K . At atmospheric pressure, anti-CaF -type Mg Si kept stable at T = 133-723 K, whereas Mg Si transformed to anti-CaF -type Mg Si and Si above T ≥ 530 K. This temperature stability may indicate the potential of Mg Si 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.