Hexagonal Si-Ge Class of Semiconducting Alloys Prepared by Using Pressure and Temperature.

Multi-anvil and laser-heated diamond anvil methods have been used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500-1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery at ambient pressure of hexagonal Ge-Si solid solutions (P63 /mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be prepared for all Ge-Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge-Si solid solution family, and is of interest for optoelectronic applications.

Effects of temperature and pressure on the optical and vibrational properties of thermoelectric SnSe.

We have conducted a comprehensive investigation of the optical and vibrational properties of the binary semiconductor SnSe as a function of temperature and pressure by means of experimental and ab initio probes. Our high-temperature investigations at ambient pressure have successfully reproduced the progressive enhancement of the free carrier concentration upon approaching the Pnma → Bbmm transition, whereas the pressure-induced Pnma → Bbmm transformation at ambient temperature, accompanied by an electronic semiconductor → semi-metal transition, has been identified for bulk SnSe close to 10 GPa. Modeling of the Raman-active vibrations revealed that three-phonon anharmonic processes dominate the temperature-induced mode frequency evolution. In addition, SnSe was found to exhibit a pressure-induced enhancement of the Born effective charge. Such behavior is quite unique and cannot be rationalized within the proposed effective charge trends of binary materials under pressure.

HP-MoO2: A High-Pressure Polymorph of Molybdenum Dioxide.

High-pressure molybdenum dioxide (HP-MoO2) was synthesized using a multianvil press at 18 GPa and 1073 K, as motivated by previous first-principles calculations. The crystal structure was determined by single-crystal X-ray diffraction. The new polymorph crystallizes isotypically to HP-WO2 in the orthorhombic crystal system in space group Pnma and was found to be diamagnetic. Theoretical investigations using structure optimization at density-functional theory (DFT) level indicate a transition pressure of 5 GPa at 0 K and identify the new compound as slightly metastable at ambient pressure with respect to the thermodynamically stable monoclinic MoO2 (α-MoO2; ΔEm = 2.2 kJ·mol-1).

Dense Si(x)Ge(1-x) (0 < x < 1) materials landscape using extreme conditions and precession electron diffraction.

High-pressure and -temperature experiments on Ge and Si mixtures to 17 GPa and 1500 K allow us to obtain extended Ge-Si solid solutions with cubic (Ia3) and tetragonal (P4(3)2(1)2) crystal symmetries at ambient pressure. The cubic modification can be obtained with up to 77 atom % Ge and the tetragonal modification for Ge concentrations above that. Together with Hume-Rothery criteria, melting point convergence is employed here as a favored attribute for solid solution formation. These compositionally tunable alloys are of growing interest for advanced transport and optoelectronic applications. Furthermore, the work illustrates the significance of employing precession electron diffraction for mapping new materials landscapes resulting from tailored high-pressure and -temperature syntheses.