Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.

Tuning of band gaps for a two-dimensional piezoelectric phononic crystal with a rectangular lattice

In this paper, the elastic wave propagation in a two-dimensional piezoelectric phononic crystal is studied by considering the mechanic-electric coupling. The generalized eigenvalue equation is obtained by the relation of the mechanic and electric fields as well as the Bloch-Floquet theorem. The band structures of both the in-plane and anti-plane modes are calculated for a rectangular lattice by the planewave expansion method. The effects of the lattice constant ratio and the piezoelectricity with different filling fractions are analyzed. The results show that the largest gap width is not always obtained for a square lattice. In some situations, a rectangular lattice may generate larger gaps. The band gap characteristics are influenced obviously by the piezoelectricity with the larger lattice constant ratios and the filling fractions.

YPO_4 nanocrystals: preparation and size-induced lattice symmetry enhancement

YPO4 and Eu doped YPO4 nanocrystals were prepared via a simple hydrothermal method. The phase purity, rnicrostructure, surface absorbed water and luminescence properties were characterized with X-ray diffraction （XRD）, transmission electron microscopy （TEM）, fluorescence spectrum, infrared spectroscopy, Raman spectroscopy and thermogravimetry analysis（TGA）. Combined with XRD and TEM analysis, it was found that YPO4 nanocrystals crystallized into a single phase of tetragonal （141/amd） zircon structure. YPO4 nanocrystals showed a size reduction with the reaction temperature decreasing. Considering the XRD, IR and Raman results, a lattice expansion and the unit cell symmetry enhancement were observed with the particle size decreasing. A dipole to dipole interaction model was applied to investigate the relationship of the variation of microstrucmre and the particle size. The emission spectrum of Eu doped YPO4 nanocrystals was also presented and the emission bands were clearly ascribed to the f to f transitions of Eu^3＋.

Lattice expansion and smaller CuO_(x)CeO_(2-δ)particles formation by magnesium interaction for low temperature CO oxidation

A series of magnesia doped CuOxCeO_(2-δ)catalysts were prepared by co-precipitation followed by impregnation method and investigated for CO oxidation.The manuscript is devoted to explaining the role of MgO for the formation of active species on the CuOxCeO_(2-δ)surface.The improvement in catalytic activity is ascribed to the formation of various active species due to the interaction of magnesia with CuOxCeO_(2-δ).The catalysts were characterized by PXRD,N_(2)adsorption,H_(2)-TPR,XPS,SEM,EDS and HRTEM techniques.The Mg doped catalyst shows lattice expansion of ceria due to the formation of smaller Ce^(3+)species with oxygen vacancies.The Mg-O bond also takes part in CO activation and oxidation,which results in the increase of CO oxidation.The Mg doped CuOxCeO_(2-δ)catalyst shows improvement in low temperature activity compared with the CuOxCeO_(2-δ).

Exploiting H-induced lattice expansion in β-palladium hydride for enhanced catalytic activities toward oxygen reduction reaction

We have developed an efficient strategy to synthesize an active and durable electrocatalyst of Pd hydride nanocubes(NCs).Instead of the traditional chemical method,the PdH_(0.43)NCs are firstly prepared via a hydrogen diffusion procedure,followed by hydrothermal synthesis of an amorphous CuO layer encapsulating the PdH_(0.43)NCs to prevent the hydrogen atoms from escaping.Obvious lattice expansion is demonstrated playing a pivotal role in the enhancement of their oxygen reduction reaction(ORR)activity and durability.The obtained PdH_(0.43)@CuO NCs catalysts exhibit an ORR mass activity of 0.18 A mg^(−1) at 0.90 V versus reversible hydrogen electrode in an alkaline medium,which is about five times higher than that of commercial Pt/C.Accelerated durability tests show that there is only a 32 mV decay in halfwave potential even after 10,000 potential cycles,indicating the excellent stability of PdH_(0.43)@CuO NCs.Density functional theory(DFT)calculations also indicate that,compared to Pd,PdH_(0.43)has a lower limiting barrier to form OH−during the ORR process.The present study illustrates the importance of lattice expansion caused by hydrogen and offers an available strategy to design highly efficient and durable Pdbased electrocatalysts for alkaline fuel cells.

Thermally enhanced photoluminescence and temperature sensing properties of Sc2W3O12:Eu3+ phosphors

Recently, lanthanide-ion-doped luminescent materials have been extensively used as optical thermometry probes due to their fast responses, non-contact, and high sensitivity properties. Based on different responses of two emissions to temperature, the fluorescence intensity ratio(FIR) technique can be used to estimate the sensitivities for assessing the optical thermometry performances. In this study, we introduce different doping concentrations of Eu^(3+) ions into negative thermal expansion material Sc2W3O12to increase the thermal-enhanced luminescence from 373 K to 548 K, and investigate the temperature sensing properties in detail. All samples can exhibit their good luminescence behaviors thermally enhanced.The emission intensity of Sc2W3O12:6-mol% Eu3+phosphor reaches 147.8% of initial intensity at 473 K. As the Eu3+doping concentration increases, the resistance of the sample to thermal quenching decreases. The FIR technique based on each of the transitions 5D→7F_(1)(592 nm) and 5D→7F_(2)(613 nm) of Eu3+ions demonstrates a maximum relative temperature sensitivity of 3.063% K-1at 298 K for Sc_(2)W_(3)O_(12):6-mol% Eu3+phosphor. The sensitivity of sample decreases with the increase of Eu3+concentration. Benefiting from the thermal-enhanced luminescence performance and good temperature sensing properties, the Sc_(2)W_(3)O_(12):Eu^(3+)phosphors can be used as optical thermometers.

Two-dimensional porous carbon derived from pollen with enlarged interlayer distance enhanced electrocatalytic oxidation of n-valeraldehyde to octane

Electrocatalytic n-valeraldehyde oxidation reaction was an inexpensive and eco-friendly method to control n-valeraldehyde contamination and produce high value-added octane.However,low-cost and readily available electrocatalysts with high current efficiency were urgently needed.Herein,two-dimensional porous carbon derived from pollen with enlarged interlayer distance was built by alkali activation method,applying in electrocatalytic n-valeraldehyde oxidation reaction.The enlarged interlayer distance was verified by X-ray diffraction(XRD)and high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM).Electrocatalytic experiments consequences showed activated biomass derived carbon possessed a higher electrocatalytic activity and octane selectivity than unactivated catalyst.Systematic tests and in situ infrared experiments demonstrated that enlarged interlayer distance was positively correlated with specific surface area of catalysts,large specific surface area provided abundant absorption sites,facilitated the adsorption for n-valeraldehyde,and further promoted the transformation of n-valeraldehyde to octane.This work also provides a new avenue for creating high-performance electrocatalysts in terms of lattice engineering.

Novel hydrogen storage properties of palladium nanocrystals activated by a pentagonal cyclic twinned structure

Researchers appear to have neglected a special form of crystallites, pentagonal cyclic twinning, in which an obvious two-dimensional lattice expansion exists leading to novel physical-chemical properties associated with the changes in geometric and electronic structures. Using the storage and release of hydrogen in Pd nanocrystals as a probe, we have found that icosahedral pentagonal cyclic twinned Pd nanocrystals had distinct hydrogen storage properties, due to the two-dimensional lattice expansions, quite different from those of the octahedral single crystalline counterpart. In addition, the two-dimensional lattice expansion in pentagonal cyclic twinned Pd nanocrystals causes a change in electronic structure, which results in novel catalytic properties involving in situ formation of PdHx pentagonal cyclic twinned nanocrystals.