Effects of Vacancies on Properties of Relaxor Ferroelectrics: a First-Principles Study

A first-principles-based model is developed to investigate the influence of lead vacancies on the properties of relaxor ferroelectric Pb(Sc1/2Nb1/2)O3 (PSN). Lead vacancies generate large, inhomogeneous, electric fields that reduce barriers between energy minima for different polarization directions. This naturally explains why relaxors with significant lead vacancy concentrations have broadened dielectric peaks at lower temperatures, and why lead vacancies smear properties in the neighborhood of the ferroelectric transition in PSN. We also reconsider the conventional wisdom that lead vacancies reduce the magnitude of dielectric response.Comment: 11 pages, 1 figur

Spin-phonon coupling effects in transition-metal perovskites:a DFT+UU and hybrid-functional study

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$, are investigated using density-functional methods. The calculations are carried out both with a hybrid-functional (HSE) approach and with a DFT+$U$ approach using a $U$ that has been fitted to HSE calculations. The phonon frequency shifts obtained in going from the FM to the AFM spin configuration agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the $A$MnO$_3$ materials class with $A$=Ca, Sr, and Ba, this frequency shift decreases as the A cation radius increases for the $\Gamma$ phonons, while it increases for R-point phonons. In La$M$O$_3$ with $M$=Cr, Fe, and Cr/Fe, the phonon frequencies at $\Gamma$ decrease as the spin order changes from AFM to FM for LaCrO$_3$ and LaFeO$_3$, but they increase for the double perovskite La$_2$(CrFe)O$_6$. We discuss these results and the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.Comment: 13 pages, 7 figures, 9 table

Effects of atomic short-range order on the properties of perovskite alloys in their morphotropic phase boundary

The effects of atomic short-range order on the properties of Pb(Zr_{1-x}Ti_x)O_3 alloy in its morphotropic phase boundary (MPB) are predicted by combining first-principles-based methods and annealing techniques. Clustering is found to lead to a compositional expansion of this boundary, while the association of unlike atoms yields a contraction of this region. Atomic short-range order can thus drastically affect properties of perovskite alloys in their MPB, by inducing phase transitions. Microscopic mechanisms responsible for these effects are revealed and discussed.Comment: 4 pages, with 2 postscript figures embedded. Uses REVTEX4 and graphicx macro

Comparación de la clasificación visual de la madera en condiciones húmedas y secas de acuerdo con la norma española UNE 56544 y la alemana DIN 4074-1

Visual strength grading is widely used to estimate mechanical properties of structural timber. National visual grades are allocated to strength classes according to European standard EN1912. The recent discussion about the proper function of visual strength grading standards and the assignment of strength classes shows the need for further research in this field. Spanish-sourced radiata, Scots, Salzmann and maritime pine timber samples were visually graded in wet and dry condition according to the Spanish UNE56544 (2011) and German DIN4074-1 (2012) standards. Rejection was far higher dry due to distortion (warping) parameters. However, this rejection could be significantly mitigated by adopting a higher twist limit (2 mm / 25 mm width). UNE 56544 is more suitable for visually grading these species because it was specifically designed for them and a Spanish source. However, both standards underestimated the Scots pine rejection pieces which mechanical properties fulfill the MEG and S10 grades.La clasificación visual es ampliamente utilizada para estimar las propiedades mecánicas de la madera. Las calidades visuales son asignadas a clases resistentes de acuerdo a la norma europea EN1912. La reciente discusión sobre el apropiado funcionamiento de las normas de clasificación visual y de la asignación de clases resistentes hace necesario seguir investigando sobre el tema. Madera de pino radiata, silvestre, laricio y pinaster fue clasificada visualmente en condiciones húmedas y secas de acuerdo a la norma española UNE56544 y la alemana DIN4074-1. Muchas más piezas fueron rechazadas en seco debido a las deformaciones. Sin embargo, este rechazo se ve significativamente reducido adoptando un nuevo límite de alabeo (2 mm / 25 mm). UNE56544 es más adecuada para la clasificación visual de estas especies pues fue diseñada para ellas y procedencia española. Sin embargo, ambas normas infravaloran el lote rechazado de pino silvestre, cuyas propiedades mecánicas cumplen MEG y S10

Ab-initio design of perovskite alloys with predetermined properties: The case of Pb(Sc_{0.5} Nb_{0.5})O_{3}

A first-principles derived approach is combined with the inverse Monte Carlo technique to determine the atomic orderings leading to prefixed properties in Pb(Sc_{0.5}Nb_{0.5})O_{3} perovskite alloy. We find that some arrangements between Sc and Nb atoms result in drastic changes with respect to the disordered material, including ground states of new symmetries, large enhancement of electromechanical responses, and considerable shift of the Curie temperature. We discuss the microscopic mechanisms responsible for these unusual effects.Comment: 5 pages with 2 postscript figures embedde

Nondestructive testing used on timber in spain: A literature review

Nondestructive testing (NDT) includes several highly efficient techniques for the estimation of the physical and mechanical properties of structural timber. Apart from visual grading, scientific research using Nondestructive testing on timber has been used in Spain since the 1990s. Nondestructive testing can be used for two different purposes: timber grading and the assessment of existing timber structures. The most common devices used in Spain are portable ones based on ultrasound, stress waves, vibration and probing techniques. Many statistical linear models for estimating the mechanical properties of new sawn timber and timber from existing structures have been proposed. Furthermore, several factors that affect Nondestructive testing measurements have been studied (moisture content, temperature, specimen dimensions, sensors position-grain angle…) and adjustment factors have also been proposed. Species have been characterized for visual grading standards from the 1980s to date. The large number of research works using different species, devices and procedures shows the need of homogenization and standardization of Nondestructive testing use. This paper presents a review of research works using Nondestructive testing on timber in Spain, in order to add to knowledge, elucidate the concepts to unify Nondestructive testing used and promote research group collaboration in the near future

First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys

We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x (BS-PT) alloys recently proposed by Eitel et al. as promising materials for piezoelectric actuator applications. We show that (i) BS-PT displays very large structural distortions and polarizations at the morphotropic phase boundary (MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the ferroelectric and piezoelectric properties of BS-PT are dominated by the onset of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as far as the computed values of the piezoelectric coefficient d_15 are concerned. While our results are generally consistent with experiment, they also suggest that certain intrinsic properties of BS-PT may be even better than has been indicated by experiments to date. We also discuss results for PZT that demonstrate the prominent role played by Pb displacements in its piezoelectric properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm

Optimized local modes for lattice dynamical applications

We present a new scheme for the construction of highly localized lattice Wannier functions. The approach is based on a heuristic criterion for localization and takes the symmetry constraints into account from the start. We compare the local modes thus obtained with those generated by other schemes and find that they also provide a better description of the relevant vibrational subspace.Comment: 6 pages, ReVTeX, plus four postscript files for figure

Understanding the Unique Structural and Electronic Properties of SrFeO2

We report a first-principles study of SrFeO$_2$, an infinite-layer oxide with Fe atoms in a perfect square-planar coordination down to essentially 0 K. Our results reveal this striking behavior relies on the double occupation of the 3$d_{z^2}$ orbitals of high-spin Fe$^{2+}$. Such an electronic state results from the hybridization of iron's 3$d_{z^2}$ and 4$s$ orbitals, which enables a large reduction of the intra-atomic exchange splitting of the $z^2$ electrons. The generality of the phenomenon is discussed.Comment: 4 pages with 3 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Multi-phonon scattering and Ti-induced hydrogen dynamics in sodium alanate

We use ab initio methods and neutron inelastic scattering (NIS) to study the structure, energetics, and dynamics of pure and Ti-doped sodium alanate (NaAlH_4), focusing on the possibility of substitutional Ti doping. The NIS spectrum is found to exhibit surprisingly strong and sharp two-phonon features. The calculations reveal that substitutional Ti doping is energetically possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al--H bond breaking. Our results hint at new ways of improving the hydrogen dynamics and storage capacity of the alanates.Comment: 5 pages, with 4 postscript figures embedded. Uses REVTEX4 and graphicx macro