•We predict a single atomic layer thin silicene allotrope which is similar to T-graphene and bct-carbon.•There exist linear energy dispersion relations near the Fermi surface and the cross points form a loop.•It behaves as a nodal line semimetal.Materials with Dirac point are so amazing since the charge carriers are massless and have an effective speed of light. However, among the predicted two-dimensional silicon allotropes with Dirac point, no one has been directly proved by experiment. This fact motivates us to search for other two-dimensional silicon allotropes. As a result, another stable single atomic layer thin silicon allotrope is found with the help of CALYPSO code in this work. This silicene allotrope is composed of eight-membered rings linked by Si–Si bonds with buckling formation. The electronic calculation reveals that it behaves as a nodal line semimetal with the linear energy dispersion relation near the Fermi surface. Notably, the ab initio molecular dynamics simulations display that the original atomic configuration can be remained even at an extremely high temperature of 1000 K. Additionally, hydrogenation could induce a semimetal-semiconductor transition in this silicene allotrope. We hope this work can expand the family of single atomic layer thin silicon allotropes with special applications.

Materials with competitive spin interactions can show multiple quantum magnetic states under external manipulation, which is important for spin-related research and applications. Although the magnetism in graphene nanoribbons has been intensively studied, robust spin interactions have not been reported. Here, we explored graphene nanoribbons modified with Ti and V atomic chains that show competitive spin interactions and robust spiral magnetism. On the basis of first-principles calculations, we systematically investigate the possibility of inducing robust and competitive magnetic ordering in graphene nanoribbons through transition-metal (TM) atomic chain decoration (denoted as TM-ZGNR, TM = Ti–Co). On the basis of the Heisenberg XY model including nearest-neighbor (NN) and next-nearest-neighbor (NNN) magnetic interactions, Ti-ZGNR and V-ZGNR are predicted to have spiral magnetic order due to spin frustration. By Monte Carlo simulations, the Néel temperatures for Ti-ZGNR and V-ZGNR are estimated to be 45 and 100 K, respectively. Our studies will benefit further spin-control research on graphene nanoribbons.

•Several CaC structures were found by CALYPSO code.•The stable CaC with both thermodynamically and dynamically stabilities shows P4/mmm space group.•The stable CaC exhibits metallic nature, instead of half-metallic character.Because of no experimental report on the half-metallic CaC theoretically found by Gao et al. in 2007, it is necessary to explore the nature of this fact, and the reason is revealed via density-functional theory study in this work. By a particle swarm structural search firstly, several CaC allotropes are found, including the half-metallic one with F-43m space group. Among the found compounds, six allotropes have the negative formation energies with Ccma, P4/mmm, Immm, Cm (8), Pmm2, and Cmmm space groups while the half-metallic one has positive formation energy of 2.67 eV. Furthermore, through the study on the dynamical properties of these six thermally stable CaC allotropes, only the one with P4/mmm symmetry exhibits dynamical stability. Based on the above fact, CaC with P4/mmm symmetry, instead of the F-43m one, perhaps can be experimentally synthesized due to both the thermodynamic and dynamically stabilities.

•The half-metallic nature of Sr2FeMoO6 persists in the whole range of strain applied.•A magnetic transition from ferrimagnetic to ferromagnetic phase is found in the critical strain.•Spin crossovers of Fe and Mo ions are found in the critical strains.Using the density-functional theory, the effect of biaxial mechanical strain on the electronic and magnetic properties of double perovskite oxide Sr2FeMoO6 has been studied. Our calculations reveal that Sr2FeMoO6 exhibits a magnetic transition from ferrimagnetic to ferromagnetic phase in the strain region of 7–8%, accompanied by a spin crossover of Mo ion from intermediate-spin to low-spin state. For the ferrimagnetic Sr2FeMoO6, the electronic configuration of Fe ion shows a spin transition from high-spin to intermediate-spin state around the strain of −7% to −6%, causing a steep decrease of the magnetic moment from ~3.9 to ~2.3 μB. Additionally, the half-metallic nature of Sr2FeMoO6 persists in the whole range of strain applied in this work. The results may expand the technology application of Sr2FeMoO6 in the spintronic field.

•The effect of oxygen vacancy on the half-metallic character of perovskite oxide Sr2FeMoO6 has been investigated theoretically.•Sr2FeMoO6−δ possesses half-metallic nature in a large range of δ.•The energy gap decreases with increasing the content of oxygen vacancy.The effect of oxygen vacancy on the half-metallic characteristic of perovskite oxide Sr2FeMoO6 has been investigated using density-functional calculations. The results reveal that the compounds behave as half-metal in the whole range of oxygen vacancies discussed in this work, and the energy gap in the spin-up channel decreases with increasing the content of oxygen vacancy. For all compounds, the electronic configurations of Fe and Mo ions are in the high-spin and intermediate-spin states, respectively. With increasing the content of oxygen vacancy, the magnetic moment of Fe ion decreases and that of Mo ion increases. The above fact hints that Sr2FeMoO6−δ possesses half-metallic nature in a large range of δ, which may open a promising way to experimentally synthesize the practical half-metal with a few ineluctable oxygen vacancies.

After substitution of carbon by nitrogen, the electronic structures of the porous graphene have been studied by the density functional theory calculations. The N substitutional site without hydrogen passivation leads to a tunable energy gap of the two-dimensional porous polymer, depending on the number of N atoms in a unit cell. Moreover, the increasing number of N in an aromatic ring enhances the binding energies between hydrogen molecules and pre-adsorbed Li atoms from 1H2 to 3H2. Thus, porous polymer materials through controllable synthesis techniques will improve their potential applications in photosplitting of water as well as hydrogen storage.

After substitution of carbon by nitrogen, the electronic structures of the porous graphene have been studied by the density functional theory calculations. The N substitutional site without hydrogen passivation leads to a tunable energy gap of the two-dimensional porous polymer, depending on the number of N atoms in a unit cell. Moreover, the increasing number of N in an aromatic ring enhances the binding energies between hydrogen molecules and pre-adsorbed Li atoms from 1H2 to 3H2. Thus, porous polymer materials through controllable synthesis techniques will improve their potential applications in photosplitting of water as well as hydrogen storage.