First Principles Study of Structural, Electronic, and Mechanical Properties of Lead Selenide and Lead Telluride
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Lead chalcogenides, most notably lead selenide (PbSe) and lead telluride (PbTe), have become an active area of research due to their thermoelectric properties. The high figure of merit (zT) of these materials has brought much attention to them, due to their ability to convert waste heat into electricity. Recent efforts, such as applying pressure or doping, have shown an increase in thermoelectric efficiency. Variation in application and synthesis conditions gives rise to a need for analysis of structural, electronic, and mechanical properties of these materials at different pressures. In addition to the NaCl structure at ambient conditions, lead chalcogenides have a dynamic orthorhombic (Pnma) intermediate pressure phase and a higher pressure, yet stable, CsCl phase. By altering the lattice constant, the application of external pressure is simulated; this has notable effects on total ground-state energy, band gap, and structural phase. Using the Projector Augmented Wave (PAW) Method and the Generalized Gradient Approximation (GGA) in Density Functional Theory (DFT), the phase transition pressures are calculated by finding the differences in enthalpy from total energy calculations. For each phase, elastic constants, bulk modulus, shear modulus, and Young's modulus are calculated, and the NaCl phase is studied with typical dopants, both n-type (Bi, In, and I) and p-type (Na and Tl). Pugh's ratio is employed to give insight on the brittleness of the materials and phases studied. In addition to structural and mechanical properties, the band structure and density of states are analyzed at varying pressures, paying special note to thermoelectric implications. The results presented here will be useful to guide future experiments toward the search for structurally stable thermoelectric materials. Several mechanical properties predicted here are ready to be confirmed by experimental works.