We have examined the crystallographic and magnetic properties of single crystals of CrI 3 , an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210−220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R3̅ , BiI 3type) and the high-temperature structure is monoclinic (C2/ m, AlCl 3 -type). We find evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI 3 , observing an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Calculations also suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI 3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.

Carefully converged calculations are performed for the band gap of ZnO within many-body perturbation theory (G 0 W 0 approximation). The results obtained using four different well-established plasmon-pole models are compared with those of explicit calculations without such models (the contour-deformation approach). This comparison shows that, surprisingly, plasmon-pole models depending on the f -sum rule gives less precise results. In particular, it confirms that the band gap of ZnO is underestimated in the G 0 W 0 approach as compared to experiment, contrary to the recent claim of Shih et al.

We show that the electric field driven surface instability of viscoelastic films has two distinct regimes: (1) The viscoelastic films behaving like a liquid display long wavelengths governed by applied voltage and surface tension, independent of its elastic storage and viscous loss moduli, and (2) the films behaving like a solid require a threshold voltage for the instability whose wavelength always scales as approximately 4xfilm thickness, independent of its surface tension, applied voltage, loss and storage moduli. Wavelength in a narrow transition zone between these regimes depends on the storage modulus.

Defects and surface reconstructions are thought to be crucial for the long-term stability of high-voltage lithium-manganese-rich cathodes. Unfortunately, many of these defects arise only after electrochemical cycling which occurs under harsh conditions, making it difficult to fully comprehend the role they play in degrading material performance. Recently, it has been observed that defects are present even in the pristine material. This study, therefore, focuses on examining the nature of the disorder observed in pristine Li1.2Ni0.175Mn0.525Co0.1O2 (LNMCO) particles. Using atomic-resolution Z-contrast imaging and electron energy loss spectroscopy measurements, we show that there is indeed a significant amount of antisite defects present in this material, with transition metals substituting on Li metal sites. Furthermore, we find a strong segregation tendency of these types of defects toward open facets (surfaces perpendicular to the layered arrangement of atoms) rather than closed facets (surfaces parallel to the layered arrangement of atoms). First-principles calculations identify antisite defect pairs of Ni swapping with Li ions as the predominant defect in the material. Furthermore, energetically favorable swapping of Ni on the Mn sites was observed to lead to Mn depletion at open facets. Relatively, low Ni migration barriers also support the notion that Ni is the predominant cause of disorder. These insights suggest that certain facets of the LNMCO particles may be more useful for inhibiting surface reconstruction and improving the stability of these materials through careful consideration of the exposed surface.

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We present electronic band structures of transparent oxides calculated using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In(2)O(3), ZnO, CdO and SnO(2) along with the p-type conducting ternary oxides delafossite CuXO(2) (X=Al, Ga, In) and spinel ZnX(2)O(4) (X=Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated.

The GW approximation [1] to many body perturbation theory represents the state of the art technique to calculate the quasiparticle correction to the band gap of solids and has been sucessfully applied to many materials. Although the GW approximation works well with pseudopotentials (PP's) and plane wave basis sets used within Density Functional Theory Local density approximation (DFT-LDA), the II-VI materials are particularly challenging due to the strong p-d hybridization between the cation 'd' and anion 'p' states [2,3]. The DFT-LDA resuslts in p-d overhybridization in the bandstructure which leads to underestimation of the calculated GW band gap. Recently, the self-consistent GW scheme [4] and quasiparticle correction based on the generalized Kohn-Sham schemes [5,6] have been used to improve the GW band gap for II-VI semiconductors and insulators. Although these schemes make closer estimate of the band gap with the GW approximation, there is still need of an elaborate theory and it is a topic of current research. We present the GW bandgap calculated with the ABINIT [7] code, using the PP and plane wave basis set, for II-VI transparent oxides namely ZnO. The quasiparticle corrections are calculated systematically with different number of valence electrons in the cation-pseudopotential. We find that the 20-electron cation pseudopotential yields the best quasiparticle correction. The dependence of self-energy on the exchange interaction between the atomic orbitals is discussed. The postioning of the cation 'd' energy level in the quasiparticle bandstructure is addressed. The correlation between the p-d hybridization and the underestimation of the non-self-consistent GW band gap is made through a comparison of zincblende (ZB) and rocksalt (RS) ZnO. The RS phase includes the inversion symmetry at the Gamma point in the Brillouin zone. The p and d states do not mix at the Gamma point and remain unhy-1

ZnM2O4 (M = Co, Rh, Ir) spinels are considered as a class of potential p-type transparent conducting oxides (TCOs). We report the formation energy of acceptor-like defects using first principles calculations with an advanced hybrid exchange-correlation functional (HSE06) within density functional theory (DFT). Due to the discrepancies between the theoretically obtained band gaps with this hybrid functional and the - scattered - experimental results, we also perform GW calculations to support the validity of the description of these spinels with the HSE06 functional. The considered defects are the cation vacancy and antisite defects, which are supposed to be the leading source of disorder in the spinel structures. We also discuss the band alignments in these spinels. The calculated formation energies indicate that the antisite defects ZnM (Zn replacing M, M = Co, Rh, Ir) and VZn act as shallow acceptors in ZnCo2O4, ZnRh2O4 and ZnIr2O4, which explains the experimentally observed p-type conductivity in those systems. Moreover, our systematic study indicates that the ZnIr antisite defect has the lowest formation energy in the group and it corroborates the highest p-type conductivity reported for ZnIr2O4 among the group of ZnM2O4 spinels. To gain further insight into factors affecting the p-type conductivity, we have also investigated the formation of localized small polarons by calculating the self-trapping energy of the holes.