Field-induced percolation of polar nanoregions in relaxor ferroelectrics.

A first-principles-based effective Hamiltonian is used to investigate low-temperature properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics under an increasing dc electric field. This system progressively develops an electric polarization that is highly nonlinear with the dc field. This development leads to a maximum of the static dielectric response at a critical field, E(th), and involves four different field regimes. Each of these regimes is associated with its own behavior of polar nanoregions, such as shrinking, flipping, and elongation of dipoles or change in morphology. The clusters propagating inside the whole sample, with dipoles being parallel to the field direction, begin to form at precisely the E(th) critical field. Such a result, and further analysis we perform, therefore, reveal that field-induced percolation of polar nanoregions is the driving mechanism for the transition from the relaxor to ferroelectric state.

Finite-temperature properties of Ba(Zr,Ti)O3 relaxors from first principles.

A first-principles-based technique is developed to investigate the properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics as a function of temperature. The use of this scheme provides answers to important, unresolved and/or controversial questions such as the following. What do the different critical temperatures usually found in relaxors correspond to? Do polar nanoregions really exist in relaxors? If yes, do they only form inside chemically ordered regions? Is it necessary that antiferroelectricity develop in order for the relaxor behavior to occur? Are random fields and random strains really the mechanisms responsible for relaxor behavior? If not, what are these mechanisms? These ab initio based calculations also lead to deep microscopic insight into relaxors.

Two spectral QSPR models of porphyrin macromolecules for chelating heavy metals and different ligands released from industrial solvents: CH2Cl2, CHCl3 and toluene.

Two simple and reliable correlations are introduced for the prediction of emission and absorption of porphyrins and their derivatives, i.e. metalloporphyrins and ligand coordinated metalloporphyrins. They can be used to sense the extracted precious metals. The proposed models require only simple structural parameters such as the number of carbon, metal and metal-free molecular fragments of desirable porphyrins or their derivatives. Since the proposed models depend on molecular structures of the desired compounds, they can be easily applied for complex molecular structures. Experimental data of 272 porphyrin derivatives were used to derive and test the novel models for the assessment of their emission (Em.) and absorption (Abs.) values in three solvents namely dichloromethane, toluene and chloroform. The values of the coefficients of determination (r 2) for the training set (183 compounds) in dichloromethane and three different test sets, corresponding to the three mentioned solvents, for the emission and absorption correlations were greater than 0.70. The calculated values of the root-mean-square error (RMSE) for the training sets of Em. and Abs. correlations were equal to 7.56 and 4.86 nm, respectively. Further statistical parameters also confirm the high reliability of the new models.

A simple approach for assessment of toxicity of nitroaromatic compounds without using complex descriptors and computer codes.

A simple approach is introduced to assess the toxicity of nitroaromatic compounds in terms of an oral LD50 dose (50% lethal dose) for rats. Most of the presented Quantitative Structure-Activity Relationship (QSAR) models for prediction of in vivo toxicity of nitroaromatics are calculated by quantum computing descriptors which are more difficult to interpret and apply, while the new model requires only the molecular structure of a desirable nitroaromatic compound. The novel model is based on the constitutional descriptors, such as the number of oxygen, sulphur, phosphorous and molecular fragments. Experimental data of 90 nitroaromatics are used to derive and test the new model as the logarithm of LD50 values, i.e. -log (LD50). Although it is based on only simple structural parameters, the reliability of the new model is also higher than the complex QSAR model because the values of the root-mean-square deviation (RMSD) of -log (LD50) for the new and the outputs of the latest QSAR method are 0.342 and 0.377, respectively.

Discovery of novel hydrogen storage materials: an atomic scale computational approach.

Practical hydrogen storage for mobile applications requires materials that exhibit high hydrogen densities, low decomposition temperatures, and fast kinetics for absorption and desorption. Unfortunately, no reversible materials are currently known that possess all of these attributes. Here we present an overview of our recent efforts aimed at developing a first-principles computational approach to the discovery of novel hydrogen storage materials. Such an approach requires several key capabilities to be effective: (i) accurate prediction of decomposition thermodynamics, (ii) prediction of crystal structures for unknown hydrides, and (iii) prediction of preferred decomposition pathways. We present examples that illustrate each of these three capabilities: (i) prediction of hydriding enthalpies and free energies across a wide range of hydride materials, (ii) prediction of low energy crystal structures for complex hydrides (such as Ca(AlH(4))(2) CaAlH(5), and Li(2)NH), and (iii) predicted decomposition pathways for Li(4)BN(3)H(10) and destabilized systems based on combinations of LiBH(4), Ca(BH(4))(2) and metal hydrides. For the destabilized systems, we propose a set of thermodynamic guidelines to help identify thermodynamically viable reactions. These capabilities have led to the prediction of several novel high density hydrogen storage materials and reactions.

First-principles-based effective Hamiltonian simulations of bulks and films made of lead-free Ba(Zr,Ti)O3 relaxor ferroelectrics.

A review of the recent development and application of a first-principles-derived effective Hamiltonian technique to the study of lead-free Ba(Zr,Ti)O3 (BZT) relaxor ferroelectrics is provided. In addition to the computation and analysis of macroscopic properties (such as different types of dielectric responses and electric polarization) and their connections to previous published works, particular emphasis is given to microscopic insights arising from this atomistic technique. These include (i) the numerically-found determination of the physical origin of the relaxor behavior in BZT; and (ii) the prediction of polar nanoregions and the evolution of their morphology as a response to temperature, electric fields and epitaxial misfit strain. Other striking phenomena that were predicted in BZT compounds, such as Fano resonance and field-driven percolation, are also documented and discussed. Finally, a brief perspective of possible remaining computational studies to be conducted in relaxor ferroelectrics, in order to further understand them, is attempted.

Orofacial clefts and risk factors in tehran, iran: a case control study.

BACKGROUND: Non-syndromic cleft lip with or without cleft palate (CL/P) or cleft palate only (CPO) are orofacial clefts with multifactorial etiology. These include environmental factors and heterogeneous genetic background. Therefore, studies on different and homogenous populations can be useful in detecting related factors. The aim of the present study was to evaluate the risk factors in patients with non-syndromic cleft in Tehran, Iran. METHODS: Data from 300 patients and 300 controls were collected between 2005 and 2010. Binary logistic regression analyses were used to calculate relative risk by odds ratio (OR) and %95 confidence interval. RESULTS: Low maternal age (OR=1.06, 95% CI, 1.011-1.113), low socioeconomic status (OR=0.23, 95% CI, 0.007-0.074), maternal systemic disease (OR=0.364; 95% CI, 0.152-0.873) and passive smoking (OR=0.613, 95% CI, 0.430-0.874) increased the risk for CL/P and CPO. There was a significant difference in iron and folic acid use during pregnancy when the case and control groups were compared. CONCLUSION: In assessing for orofacial cleft risk, we should consider lack of folic acid supplementation use, maternal age and systemic diseases and passive smoking as risk factors.

Discovery of incipient ferrotoroidics from atomistic simulations.

An effective Hamiltonian technique is used to investigate the effect of quantum vibrations on properties of stress-free KTaO3 nanodots under open-circuit electrical boundary conditions. We discover that these vibrations suppress the paraelectric-to-ferrotoroidic transition, or, equivalently, wash out the formation of vortex states. Such suppression leads to the saturation of the so-called ferrotoroidic susceptibility at low temperature, and to a peculiar local structure that exhibits short-range, needlelike correlations of the individual toroidal moments.