ABSTRACT
Most of the state-of-the-art thermoelectric materials are inorganic semiconductors. Owing to the directional covalent bonding, they usually show limited plasticity at room temperature1,2, for example, with a tensile strain of less than five per cent. Here we discover that single-crystalline Mg3Bi2 shows a room-temperature tensile strain of up to 100 per cent when the tension is applied along the (0001) plane (that is, the ab plane). Such a value is at least one order of magnitude higher than that of traditional thermoelectric materials and outperforms many metals that crystallize in a similar structure. Experimentally, slip bands and dislocations are identified in the deformed Mg3Bi2, indicating the gliding of dislocations as the microscopic mechanism of plastic deformation. Analysis of chemical bonding reveals multiple planes with low slipping barrier energy, suggesting the existence of several slip systems in Mg3Bi2. In addition, continuous dynamic bonding during the slipping process prevents the cleavage of the atomic plane, thus sustaining a large plastic deformation. Importantly, the tellurium-doped single-crystalline Mg3Bi2 shows a power factor of about 55 microwatts per centimetre per kelvin squared and a figure of merit of about 0.65 at room temperature along the ab plane, which outperforms the existing ductile thermoelectric materials3,4.
ABSTRACT
The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B-X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly -80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.
ABSTRACT
Motivated by the challenges in the harnessing of energy and the continuing trend of miniaturizing devices, an exhaustive evaluation of the electronic, mechanical and piezoelectric properties of surface modified penta-graphene (PG), including fluorinated PG (F-PG-F), hydrofluorinated PG (H-PG-F) and hydrogenated PG (H-PG-H), was carried out via a first-principles approach based on density functional theory. We first predicted the H-PG-F system and calculated its phonon dispersion and magnetic properties. All three systems were found to exhibit an e31 piezoelectric effect, and the e31 (96.88 pC m-1) effect of H-PG-F was found to be much greater than that of the other two systems. So, it could be concluded that hydrofluorination can significantly enhance the piezoelectric properties of PG. The binding energy and formation energy of the H-PG-F system were found to be the lowest among the three surface modified PG systems, showing that the H-PG-F system is the most energetically favorable state. The e31 piezoelectricity can be potentially engineered into a PG monolayer by surface modification, providing an avenue for monolithic integration of electronic and electromechanical devices with a PG monolayer for use in mechanical stress-sensors, nano-sized actuators and energy harvesting systems. The H-PG-F system stands out in terms of its combination of a larger piezoelectric coefficient (e31 = 96.88 pC m-1), negative Poisson's ratio and low formation energy (-3.37 eV) and is recommended for experimental exploration.
