The thermoelectric figure-of-merit (ZT) for GeTe powder is able to be raised from ∼0.8 to 1.37 at high temperature near ∼500 °C by tuning the Ge vacancy level through a reversible in situ route.

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Polar metals are rare because free carriers in metals screen electrostatic potential and eliminate internal dipoles. Degenerate doped ferroelectrics may create an approximate polar metallic phase.We use first-principles calculations to investigate n-doped LiNbO 3 -type oxides (LiNbO 3 as the prototype) and compare to widely studied perovskite oxides (BaTiO 3 as the prototype). In the rigid-band approximation, substantial polar displacements in n-doped LiNbO 3 persist even at 0.3 e/f.u. ( 10 21 cm −3 ), while polar displacements in n-doped BaTiO 3 quickly get suppressed and completely vanish at 0.1 e/f.u. Furthermore, in n-doped LiNbO 3 , Li-O displacements decay more slowly than Nb-O displacements, while in n-doped BaTiO 3 , Ba-O and Ti-O displacements decay approximately at the same rate. Supercell calculations that use oxygen vacancies as electron donors support the main results from the rigid-band approximation and provide more detailed charge distributions. Substantial cation displacements are observed throughout LiNbO 3−δ (δ = 4.2%), while cation displacements in BaTiO 3−δ (δ = 4.2%) are almost completely suppressed. We find that conduction electrons in LiNbO 3−δ are not as uniformly distributed as in BaTiO 3−δ , implying that the rigid-band approximation should be used with caution in simulating electron doped LiNbO 3type oxides. Our work shows that polar distortions and conduction can coexist in a wide range of electron concentration in n-doped LiNbO 3 , which is a practical approach to generating an approximate polar metallic phase. Combining doped ferroelectrics and doped semiconductors may create new functions for devices.