As the first experimentally confirmed ferroelectric metal, LiOsO3 has received extensive research attention recently. Using density-functional calculations, we perform a systematic study on the origin of the metallic ferroelectricity in LiOsO3. We confirm that the ferroelectric transition in this compound is order-disorder like. In addition, we find that the distribution of the conduction electron is extremely anisotropic, which suggests that the electric screening ability is also highly anisotropic. Then, by doing electron screening analysis, we unambiguously demonstrate that the long range ferroelectric order in LiOsO3 results from the incomplete screening of the dipole-dipole interaction along the nearest neighboring Li-Li chain direction. We therefore conclude highly anisotropic screening and local dipole-dipole interactions are two most important keys to form LiOsO3-type metallic ferroelectricity. 72.80.Ga, 61.50.Ah The ferroelectric (FE) instability can be explained by a delicate balance between short-range elastic restoring forces supporting the undistorted paraelectric (PE) structure and long-range Coulomb interactions favoring the FE phase [1]. Itinerant electrons can screen the electric fields and inhibit the electrostatic forces, metallic systems are thus not expected to exhibit ferroelectric like structural distortion. Despite the incompatibility, using a phenomenological theory, Anderson and Blount proposed in 1965 that metals can break inversion symmetry [2]. They found that the FE metal is possible through a continuous structural transition accompanied by the appearance of a polar axis, and the disappearance of an inversion center [2]. In 2004, Cd 2 Re 2 O 7 had been proposed as a rare example of ferroelectric metals [3], however, it was found that although this compound exhibits a second order phase transition to a structure that lacks inversion symmetry, a unique polar axis could not be identified [4], which does not fit the criteria about the FE metal.Very recently, the first convincing success was achieved experimentally in LiOsO 3 [1]. LiOsO 3 remains metallic behavior while it undergoes a second-order phase transition from the high temperature centrosymmetric R3c to a FE-like R3c structure at T s = 140K [1]. Neutron and xray diffraction studies showed that the structural phase transition involves the displacements of Li ions accompanying also a slight shift of O ions [1]. The electronic structure and lattice instability were studied by several groups [6][7][8]. It was found that the local polar distortion in LiOsO 3 is solely due to the instability of the A-site Li ion [6][7][8]. The importance of the Coulomb interaction among 5d electrons and the hybridization between oxygen p orbitals and Os empty e g orbitals has also been emphasized by Giovannetti and Capone[8]. Despite these efforts devoted to understanding the origin of the FE like structural transition in this metallic system, there are still two fundamental issues have not been clearly clarified. The first is the origin of the fer...

Two dimensional materials have many outstanding intrinsic advantages that can be utilized to enhance the photocatalytic efficiency of water splitting. Herein, based on ab initio calculations, we reveal that for monolayer and multilayer rhenium disulphide (ReS2), the band gap and band edge positions are an excellent match with the water splitting energy levels. Moreover, the effective masses of the carriers are relatively light, and the optical absorption coefficients are high under visible illumination. Due to the feature of weak interlayer coupling, these properties are independent of the layer thickness. Our results suggest that ReS2 is a stable and efficient photocatalyst with potential applications in the use of solar energy for water splitting.

No abstract