Structural and ferroelectric transitions in magnetic nickelate PbNiO 3

Density functional calculations have been tremendously useful in understanding the microscopic origin of multiferroicity and in quantifying relevant properties in many multiferroics and magnetoelectrics. Here, we focus on a relatively new and promising compound, PbNiO3. The structural, electronic and magnetic properties of its two polymorphs, i.e. the orthorhombic structure with space group Pnma and the rhombohedral LiNbO3-type structure with space group R3c have been studied by using density functional calculations within DFT + U and hybrid functional schemes. Our data convey an accurate description of the pressure-induced phase transition from the rhombohedral to orthorhombic phase at a predicted critical pressure of 5 GPa in agreement with the measured value of 3 GPa. Both phases show the G-type antiferromagnetic configuration as a magnetic ground state, but differ in the spatial anisotropy associated with nearest-neighbor exchange couplings, which is strongly weakened in the rhombohedral LiNbO3-type phase. The predicted large ferroelectric polarization of the rhombohedral phase (Hao et al 2012 Phys. Rev. B 014116) has been re-explored and analyzed in detail using partial density of states, Born effective charge tensors, charge density difference, electron localization function analysis and distortion mode analysis. The asymmetric bonding between the Pb 6s and O 2p orbitals along the [111]-direction is responsible for the polar cationic displacement, giving rise to a predicted large ferroelectric polarization as high as 3c 100 \u3bcC cm-2. \ua9 2014 IOP Publishing and Deutsche Physikalische Gesellschaft

Tunable ferroelectric polarization and its interplay with spin-orbit coupling in tin iodide perovskites.

Ferroelectricity is a potentially crucial issue in halide perovskites, breakthrough materials in photovoltaic research. Using density functional theory simulations and symmetry analysis, we show that the lead-free perovskite iodide (FA) SnI3, containing the planar formamidinium cation FA, (NH2CHNH2)(+), is ferroelectric. In fact, the perpendicular arrangement of FA planes, leading to a 'weak' polarization, is energetically more stable than parallel arrangements of FA planes, being either antiferroelectric or 'strong' ferroelectric. Moreover, we show that the 'weak' and 'strong' ferroelectric states with the polar axis along different crystallographic directions are energetically competing. Therefore, at least at low temperatures, an electric field could stabilize different states with the polarization rotated by pi/4, resulting in a highly tunable ferroelectricity appealing for multistate logic. Intriguingly, the relatively strong spin-orbit coupling in noncentrosymmetric (FA)SnI3 gives rise to a co-existence of Rashba and Dresselhaus effects and to a spin texture that can be induced, tuned and switched by an electric field controlling the ferroelectric state

Vibrationally resolved NEXAFS at C and N K-edges of pyridine, 2-fluoropyridine and 2,6-difluoropyridine: A combined experimental and theoretical assessment

In the present work, the near edge X-ray absorption spectroscopy (NEXAFS) spectra at both C and N K-edges of pyridine, 2-fluoropyridine, and 2,6-difluoropyridine have been studied both experimentally and theoretically. From an electronic point of view, both transition potential density functional theory and time-dependent density functional theory approaches lead to reliable results provided that suitable basis sets and density functionals are employed. In this connection, the global hybrid B3LYP functional in conjunction with the EPR-III basis set appears particularly suitable after constant scaling of the band positions. For the N K-edge, vertical energies obtained at these levels and broadened by symmetric Gaussian distributions provide spectra in reasonable agreement with the experiment. Vibronic contributions further modulate the band-shapes leading to a better agreement with the experimental results, but are not strictly necessary for semi-quantitative investigations. On the other hand, vibronic contributions are responsible for strong intensity redistribution in the NEXAFS C K-edge spectra, and their inclusion is thus mandatory for a proper description of experiments. In this connection, the simple vertical gradient model is particularly appealing in view of its sufficient reliability and low computational cost. For more quantitative results, the more refined vertical Hessian approach can be employed, and its effectiveness has been improved thanks to a new least-squares fitting approach

Ethanol-mediated stress promotes autophagic survival and aggressiveness of colon cancer cells via activation of Nrf2/HO-1 pathway

Epidemiological studies suggest that chronic alcohol consumption is a lifestyle risk factor strongly associated with colorectal cancer development and progression. The aim of the present study was to examine the effect of ethanol (EtOH) on survival and progression of three different colon cancer cell lines (HCT116, HT29, and Caco-2). Our data showed that EtOH induces oxidative and endoplasmic reticulum (ER) stress, as demonstrated by reactive oxygen species (ROS) and ER stress markers Grp78, ATF6, PERK and, CHOP increase. Moreover, EtOH triggers an autophagic response which is accompanied by the upregulation of beclin, LC3-II, ATG7, and p62 proteins. The addition of the antioxidant N-acetylcysteine significantly prevents autophagy, suggesting that autophagy is triggered by oxidative stress as a prosurvival response. EtOH treatment also upregulates the antioxidant enzymes SOD, catalase, and heme oxygenase (HO-1) and promotes the nuclear translocation of both Nrf2 and HO-1. Interestingly, EtOH also upregulates the levels of matrix metalloproteases (MMP2 and MMP9) and VEGF. Nrf2 silencing or preventing HO-1 nuclear translocation by the protease inhibitor E64d abrogates the EtOH-induced increase in the antioxidant enzyme levels as well as the migration markers. Taken together, our results suggest that EtOH mediates both the activation of Nrf2 and HO-1 to sustain colon cancer cell survival, thus leading to the acquisition of a more aggressive phenotype

Heme oxygenase-1 posttranslational modifications in the brain of subjects with Alzheimer disease and mild cognitive impairment. Free Radic.

a b s t r a c t Alzheimer disease (AD) is a neurodegenerative disorder characterized by progressive cognitive impairment and neuropathology. Oxidative and nitrosative stress plays a principal role in the pathogenesis of AD. The induction of the heme oxygenase-1/biliverdin reductase-A (HO-1/BVR-A) system in the brain represents one of the earliest mechanisms activated by cells to counteract the noxious effects of increased reactive oxygen species and reactive nitrogen species. Although initially proposed as a neuroprotective system in AD brain, the HO-1/BVR-A pathophysiological features are under debate. We previously reported alterations in BVR activity along with decreased phosphorylation and increased oxidative/nitrosative posttranslational modifications in the brain of subjects with AD and those with mild cognitive impairment (MCI). Furthermore, other groups proposed the observed increase in HO-1 in AD brain as a possible neurotoxic mechanism. Here we provide new insights about HO-1 in the brain of subjects with AD and MCI, the latter condition being the transitional phase between normal aging and early AD. HO-1 protein levels were significantly increased in the hippocampus of AD subjects, whereas HO-2 protein levels were significantly decreased in both AD and MCI hippocampi. In addition, significant increases in Ser-residue phosphorylation together with increased oxidative posttranslational modifications were found in the hippocampus of AD subjects. Interestingly, despite the lack of oxidative stressinduced AD neuropathology in cerebellum, HO-1 demonstrated increased Ser-residue phosphorylation and oxidative posttranslational modifications in this brain area, suggesting HO-1 as a target of oxidative damage even in the cerebellum. The significance of these findings is profound and opens new avenues into the comprehension of the role of HO-1 in the pathogenesis of AD. & 2012 Elsevier Inc. All rights reserved. Introduction Increased oxidative and nitrosative stress represents one of the main mechanisms involved in the pathogenesis of neurodegenerative disorders such as Alzheimer disease (AD), which exhibits a large impairment of neuronal structure and molecular pathways due to oxidative stress-induced posttranslational modifications on both proteins and lipids AD is an age-related neurodegenerative disorder characterized histopathologically by the presence of senile plaques, neurofibrillary tangles (NFTs), and synapse loss in selected brain region