Atomically engineered interfaces yield extraordinary electrostriction.

Electrostriction is a property of dielectric materials whereby an applied electric field induces a mechanical deformation proportional to the square of that field. The magnitude of the effect is usually minuscule (<10-19 m2 V-2 for simple oxides). However, symmetry-breaking phenomena at the interfaces can offer an efficient strategy for the design of new properties1,2. Here we report an engineered electrostrictive effect via the epitaxial deposition of alternating layers of Gd2O3-doped CeO2 and Er2O3-stabilized δ-Bi2O3 with atomically controlled interfaces on NdGaO3 substrates. The value of the electrostriction coefficient achieved is 2.38 × 10-14 m2 V-2, exceeding the best known relaxor ferroelectrics by three orders of magnitude. Our theoretical calculations indicate that this greatly enhanced electrostriction arises from coherent strain imparted by interfacial lattice discontinuity. These artificial heterostructures open a new avenue for the design and manipulation of electrostrictive materials and devices for nano/micro actuation and cutting-edge sensors.

Electromechanically active pair dynamics in a Gd-doped ceria single crystal.

Oxygen-defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cation and oxygen vacancy (VO) pairs (i.e. Ce-VO), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-VO pairs in a grain boundary-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV cm-1. The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at the Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on density of states and orbitals at the valence band edge. Our research indicates that electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise by one order of magnitude, i.e., from 5 × 10-4 at room temperature to 5 × 10-3 at -193 °C, due to an increase in the population of the electrically active pairs. At a constant VO concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not.

Atomic-scale insights into electro-steric substitutional chemistry of cerium oxide.

Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and electronic conducting materials. Previous atomistic analysis has widely covered the effects of substitution on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations has rarely been explored. Here, we investigate soluble monovalent (Li+, Na+, K+, Rb+), divalent (Fe2+, Co2+, Mn2+, Mg2+, Ni2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+), trivalent (Al3+, Fe3+, Sc3+, In3+, Lu3+, Yb3+, Y3+, Er3+, Gd3+, Eu3+, Nd3+, Pr3+, La3+) and tetravalent (Si4+, Ge4+, Ti4+, Sn4+, Hf4+, Zr4+) cation substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on the diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cation-substituted systems follows the order Gd3+ > Ca2+ > Na+. With the same charge, a larger size mismatch with the Ce4+ cation yields a lower oxygen ionic diffusivity, i.e., Na+ > K+, Ca2+ > Ni2+, Gd3+ > Al3+. Based on these trends, we identify species that could tune the oxygen ionic diffusivity: we estimate that the optimum oxygen vacancy concentration for achieving fast oxygen ionic transport is ≈2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values as the temperature is increased. We find that co-substitutions can enhance the impact of the single substitutions beyond that expected by their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrate the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that govern the oxygen diffusion coefficient and migration energy would enable design criteria to be defined for tuning the ionic properties of the material, e.g., by co-substitutions.

Quantification of chemical elements in blood of patients affected by multiple sclerosis.

Although some studies suggested a link between exposure to trace elements and development of multiple sclerosis (MS), clear information on their role in the aetiology of MS is still lacking. In this study the concentrations of Al, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, Li, Mg, Mn, Mo, Ni, Pb, Sb, Si, Sn, Sr, Tl, V, W, Zn and Zr were determined in the blood of 60 patients with MS and 60 controls. Quantifications were performed by inductively coupled plasma (ICP) atomic emission spectrometry and sector field ICP mass spectrometry. When the two groups were compared, an increased level of Co, Cu and Ni and a decrement of Be, Fe, Hg, Mg, Mo, Pb and Zn in blood of patients were observed. In addition, the discriminant analysis pointed out that Cu, Be, Hg, Co and Mo were able to discriminate between MS patients and controls (92.5% of cases correctly classified).

Concentration of elements in serum of patients affected by multiple sclerosis with first demyelinating episode: a six-month longitudinal follow-up study.

Twenty-six chemical elements and oxidative status were determined in serum of 12 patients with first demyelinating episode and brain magnetic resonance imaging compatible with the disease at different time points. Quantifications of Al, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, Li, Mg, Mn, Mo, Ni, Pb, Sb, Si, Sn, Sr, V, Tl, W, Zn and Zr, as well as of serum oxidative status and antioxidant capacity were carried out. The results were compared with values obtained from healthy subjects living in the same geographic area. Concentration variability, expressed as coefficient of variation (CV), was evaluated over a six months longitudinal follow-up. The CV was higher for Li and Pb, while showed minimal variation for Ca, Cu, Mg and Zn--elements strictly body regulated. Significant difference (p < or = 0.05) in mean concentrations of Ba, Ca, Cd, Cr, Li, Mn, Mo, Ni, Sb, Si, Sn and Zr between patients at time 0 and controls was also found.