Induced Lattice Distortion Research Articles

Impact of uniaxial pressure on structural and magnetic phase transitions in electron-doped iron pnictides

We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural ($T_s$) and antiferromagnetic (AF) phase transitions in iron pnictides BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03,0.12$), SrFe$_{1.97}$Ni$_{0.03}$As$_2$, and BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. In antiferromagnetically ordered BaFe$_{2-x}$Ni$_{x}$As$_{2}$ and SrFe$_{1.97}$Ni$_{0.03}$As$_2$ with $T_N$ and $T_s$ ($T_N\leq T_s$), a uniaxial pressure necessary to detwin the sample also increases $T_N$, smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

Extensive domain wall contribution to strain in a (K,Na)NbO3-based lead-free piezoceramics quantified from high energy X-ray diffraction

The origins of high piezoelectric properties in the lead-free (K,Na)NbO3-based tetragonal composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 (KNL-NTS) is investigated by quantifying the intrinsic and extrinsic contributions from high energy X-ray diffraction measurements. The applied methodology, which allows discerning between the intrinsic contribution, related to the field induced lattice distortion, and the extrinsic contributions, related to non-180° domain wall motion, is widely described in this work. The non-180° domain reorientation of the KNL-NTS piezoceramic is quantify from the integrated intensities of the 002 and 200 reflections obtained from line profile, while the shifts in peak position versus the applied electric field is used to obtain the lattice strain contribution. Large non-180° domain wall contribution to the electric field induced macroscopic strain (∼80% of the macroscopic strain) is verified in KNL-NTS.

Space matters: Li+ conduction versus strain effect at FePO4/LiFePO4 interface

FePO4/LiFePO4 (FP/LFP) interfacial strain, giving rise to substantial variation in interfacial energy and lattice volume, is inevitable in the (de)lithiation process of LiFePO4, a prototype of Li ion battery cathodes. Extensive theoretical and experimental research has been focused on the effect of lattice strain energy on FP/LFP interface propagation orientation and cyclic stability of the electrode. However, the essential effect of strain induced lattice distortion on Li+ transport at the FP/LFP interface is typically overlooked. In this report, a coherent interface model is derived to evaluate quantitatively the correlation between FP/LFP lattice distortion and Li+ conduction. The results illustrate that the effect of lattice strain on Li+ conduction depends strongly on FP/LFP interface orientations. Lattice strain induces a 90% decrease of Li+ conductivity in ac-plane oriented (de)lithiation at room temperature. The opposite effect of lattice strain on delithiation and lithiation for ab- and bc-orientations is elucidated. In addition, the effect of lattice strain tends to be more pronounced at a lower working temperature. This study provides an efficient platform to comprehend and manipulate Li+ conduction in the charge and discharge of lithium ion batteries, the large-scale application of which is frequently challenged by limited in-cell ion conduction.

Effects of texture on microstructure, Raman vibration, and ferroelectric properties in 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 ceramics

Structure, polarization, dielectric and piezoelectric properties were studied as functions of electric (E) field and temperature in the textured 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 (BNT–7.5BT) ceramics. The textured BNT–7.5BT ceramics display a high (h00) texture degree up to 0.735 and a coexistence of tetragonal (P4mm+P4bm) phases. The enhanced d33, E-field induced strain, and polarization responses in textured BNT–7.5BT are attributed to the E field induced lattice distortion. High-resolution synchrotron XRD and Raman spectra show a significant phase transition sequence of tetragonal (P4mm+P4bm)→tetragonal (P4bm)→cubic upon heating. The poled specimen displays a long-range-ordered character below depolarization temperature (Td), above which a phase transition takes place from ferroelectric state to relaxor state (TF–R). Compared with Td∼90°C in the randomly oriented BNT–7.5BT ceramic, the textured BNT–7.5BT ceramic shows a higher Td∼140°C. This work suggests that the texture technique can promote structural thermal stability of long-range-ordered ferroelectric state.

Tailoring the electronic and optical properties of anatase TiO2 by (S, Nb) co-doping from a DFT plus U calculation

Abstract The geometrical structure, defect formation, electronic and optical properties of S-doped, Nb-doped, and (S, Nb)-codoped anatase TiO 2 were successfully calculated by the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory with plus U method. Firstly, the geometrical structure demonstrates that the (S, Nb) co-doping can effectively induce lattice distortion and reduce the recombination of electron–hole pairs. Note that the (S, Nb)-codoped system further reduces the band gap compared with pure and mono-doped TiO 2 due to the mixture of S 2p and Nb 4d states appears in the gap, which results in an obvious red-shift in the optical absorption spectra and improves the photocatalytic activity. Moreover, the (S, Nb) co-doping should be grown under Ti-rich conditions while S or Nb mono-doping is expected to be easier under O-rich conditions. The above results would be beneficial to further developing for titanium dioxide photocatalyst.

Unstable spin-ice order in the stuffed metallic pyrochlorePr2+xIr2−xO7−δ

Specific heat, elastic neutron scattering, and muon spin rotation ($\mu$SR) experiments have been carried out on a well-characterized sample of "stuffed" (Pr-rich) Pr$_{2+x}$Ir$_{2-x}$O$_{7-\delta}$. Elastic neutron scattering shows the onset of long-range spin-ice "2-in/2-out" magnetic order at $T_M = 0.93$ K, with an ordered moment of 1.7(1)$\mu_\mathrm{B}$/Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 \AA\ and 0.7 ns, respectively. $\mu$SR experiments yield an upper bound 2.6(7) mT on the local field $B_\mathrm{loc}^{4f}$ at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7$\mu_B$ moments (120--270 mT, depending on muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr$^{3+}$ ions by the $\mu^+$-induced lattice distortion. For this to be the only effect, however, $\sim$160 Pr moments out to a distance of $\sim$14 \AA\ must be suppressed. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat, invokes slow correlated Pr-moment fluctuations in the ordered state that average $B_\mathrm{loc}^{4f}$ on the $\mu$SR time scale (${\sim}10^{-7}$ s), but are static on the time scale of the elastic neutron scattering experiments (${\sim}10^{-9}$ s). In this picture the dynamic muon relaxation suggests a Pr$^{3+}$ $4f$ correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.

Electronic structure of Gd/N co-doped anatase TiO2 by first-principles calculations

The lattice parameters, energy band structures, density of states, and optical absorption spectra of Gd-doped, N-doped, and Gd/N co-doped anatase TiO2 were calculated by the first-principles based on the density functional theory. The calculated results indicate that the three kinds of doping all induce lattice distortion for TiO2, but the structures still keep unchanged. Gd doping introduces an empty energy band in the forbidden band of TiO2, and N doping introduces an impurity energy level above the maximum valence band. Gd/N co-doping forms an impurity energy level and a full-filled energy band in the forbidden band of TiO2, where the impurity energy level lies above the full-filled energy band. The experimental results for the photocatalytic activity enhancement of TiO2 by the Gd/N co-doping can be explained perfectly from the calculation results.

Increased uniaxial perpendicular anisotropy in tetragonally distorted FeCo-Ti-N films

Tetragonally distorted FeCo-Ti-N films were prepared on a Rh(001) underlayer. Uniaxial perpendicular anisotropy was observed for FeCo-Ti-N films with a thickness of 23.5 nm, and the c/a lattice distortion reached 1.08. The anisotropy constant (Ku) was estimated to be a minimum of 0.57 MJ·m−3 (5.7 × 106 ergs·cm−3). These results show that the combination of Ti and N additive elements could be effective for inducing lattice distortion in FeCo-based alloys on a Rh under layer, and that the lattice distortion increases Ku.

Modulated multiferroic phases and electric polarization in Mn1−xRuxWO4+δ

In this work, we investigate the multiferroic phase stability against 4d Ru substitution of 3d Mn in Mn1−xRuxWO4+δ. The Ru substitution results in both Ru3+ and Ru4+ occupations at the Mn sites, while oxygen interstitials in the lattice are expected owing to the charge neutrality requirement. It is suggested that the local electric dipoles as induced by the charge valence variations of Ru cancel with each others, making no contribution to the ferroelectric polarization. The Ru-substitution induced lattice distortion and chemical disorder may be responsible for the enhanced stability of the ferroelectric AF2 phase over the non-ferroelectric AF1 phase, thus leading to the observed polarization enhancement.

Doping control of Cu in pH-tuned hydrothermal growth of ZnO nanowires

Un-doped and Cu-doped ZnO nanowires were grown on glass substrates by hydrothermal method. To investigate the effect of pH values on dopant concentration and distribution in the as-grown ZnO nanowires, we carried out hydrothermal process in conjunction with controlling pH values ranging from 5.5 to 7.0. The results show that the disorder and chemical impurity induced lattice distortion are clearly affected by pH values. In Cu 2p3/2 core level, Cu element has a mixed valence state (Cu+, Cu2+). Cu2+ state rather than Cu+ was greatly affected by pH value of solution, resulting in controlled dopant concentration. As such, the dopant concentration is highest at 5.5 of pH, which was confirmed by X-ray diffractometry and micro-Raman spectroscopy. In addition, energy dispersive X-ray spectroscopic elemental mapping indicates the uniform distribution of Cu in ZnO nanowires.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Enhanced hydrogenation and reduced lattice distortion in size selected Pd-Ag and Pd-Cu alloy nanoparticles

Important correlation between valence band spectra and hydrogenation properties in Pd alloy nanoparticles is established by studying the properties of size selected and monocrystalline Pd, Ag, Cu, Pd-Ag, and Pd-Cu nanoparticles. The X-ray photoelectron spectroscopy and elastic recoil detection analysis show that size induced Pd4d centroid shift is related to enhanced hydrogenation with H/Pd ratio of 0.57 and 0.49 in Pd-Ag and Pd-Cu nanoparticles in comparison to reported bulk values of 0.2 and 0.1, respectively. Pd-alloy nanoparticles show lower hydrogen induced lattice distortion. The reduced distortion and higher hydrogen reactivity of Pd-alloy nanoparticles is important for numerous hydrogen related applications.

Molecular dynamics study of strengthening by nanometric void and Cr alloying in Fe

Ferritic steels are the main candidates for the structural components of future fusion reactors. Because of the complexity of their structure, most of the simulation works are focused on the base phase of these materials such as alpha-Fe or Fe–Cr alloy. In this work molecular dynamics (MD) simulation is used to investigate the influence of chromium on the plasticity of bcc Fe–Cr alloy. Recent interatomic potentials for the Fe–Fe interactions are compared, namely Chiesa 2009 and Malerba 2010, with a comparison to Ackland 1997, Mendelev 2003 and Ackland 2004, widely used in this field. For the Fe–Cr system the potentials of Ackland 2004 for Fe–Fe and of Olsson 2005 for Fe–Cr and Cr–Cr interactions are used. Firstly, the recent Fe–Fe interatomic potentials are assessed by studying the interaction of the edge dislocation with a 2nm void at 10K. Secondly, the impact of Cr in Fe on the mobility of an edge dislocation for contents between 0 and 100at.% is studied. It appears that its glide stress increases with Cr content up to about 60at.%, and then decreases. The high flow stress observed at 60at.% Cr does not relate to Cr intrinsic mechanical properties, but to the induced lattice distortion.

Electric field driven variation in magnetoresistance of Co/Cu/Fe/BaTiO3 heterostructure

We demonstrate control of the magnetoresistance of Co/Cu/Fe giant magnetoresistance structures on BaTiO3(001) by electric field. The magnetoresistance clearly increases with electric field strength in the three different ferroelectric phases of BaTiO3, while the electric field dependence is more significant in the orthorhombic and rhombohedral phases with hysteretic behaviors, associated with the different ferroelectric domain reversal processes of BaTiO3. The results clearly show that electrically induced lattice distortion at the Fe/BaTiO3 interface can provide an alternative means to manipulating the electronic structure of the Fe layer by electric field.

Diffusion dynamics of the relaxed excited state in SrTiO3

AbstractThe generation and diffusion dynamics of the optically induced lattice distortion in the relaxed excited state of SrTiO3 are studied by a transient birefringence experiment. The observed signal of the lattice distortion has a component which decays in the millisecond region. Its amplitude shows a nonlinear dependence on the pump power that indicates the relaxed excited state is generated through a multiphoton absorption process.We observed the lattice distortion induced by a separated pump beam from the probe beam. The signal of the lattice distortion appears later as the separation between the pump and probe beams becomes larger. Experimental results suggest that the optically induced lattice distortion diffuses spatially and its direction is along the crystal axes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magneto-Absorption in the α Phase of Solid Oxygen at Megagauss Magnetic Fields

Optical absorption spectra of solid oxygen α phase and liquid oxygen were measured in high magnetic fields over 100 T using a single turn coil method. The absorption originates from the bimolecular transition was observed. The spectra strongly reflect the local magnetic interaction between two oxygen molecules. The shape of the absorption spectrum of the α oxygen changed dramatically with increasing magnetic fields, even though the absorption intensity of liquid oxygen simply declined. This difference cannot be explained by only the local magnetic interaction, and we speculate that the field induced lattice distortion plays an important role in the solid oxygen α phase.

Assessing structural bonding aspects of multiband superconductors through impurity‐induced local lattice distortions: A case study on MgB2

AbstractTo examine how a change in the host cation affects the structural and bonding properties of the local ionic environment, we have performed a series of ab initio simulations of substitutional impurities such as zinc, copper, and zirconium ions incorporated into the magnesium sublattice of MgB2. It was shown that at a low doping level, the induced lattice distortions and additional forces are noticeable small. For the given set of substituents, it was also determined how the impurity ion modifies the initial charge distribution around the host cation position and therefore, affects both the overall picture of the electronic states involved in the chemical bonding and the intrinsic charge‐transfer channels. In the context of the cation properties, by analyzing specific changes in local charge structures caused by the presence of the impurity, the existence of two different trends in the Mg‐site substitution effect was found and discussed. © 2012 Wiley Periodicals, Inc.

Germanium strips for channeling: Study of the crystal quality after slicing and chemical etching

Abstract Aiming at the fabrication of germanium strips with lateral faces provided of excellent crystal quality for swift proton beams channeling, two different cutting techniques (diamond saw, DS, and wire electrical discharge machining, WEDM) were adopted to obtain germanium strips from Ga doped wafers (100 mm diameter, 500 μm thickness). After cutting, the damaged layer was removed by using two different chemical etching compositions, whose etch rate, anisotropy ratio and effects on morphology were investigated. The etch pits density of the as received material was evaluated through selective chemical etching and inspection with optical microscope, whereas the surface morphology of the as cut surface and after chemical treatment was investigated by scanning electron microscopy (SEM). The physical nature, amount and in-depth distribution of the cut induced lattice distortions were analyzed by low energy channeling Rutherford Backscattering Spectrometry (c-RBS), Raman spectroscopy and high resolution X-ray diffraction (HRXRD). The same set of techniques was also applied to monitor the structure evolution with increasing etching time. In the case of WEDM cut samples, though a remarkable recovery of the pristine crystal quality was achieved through removal of the sub-surface damage by prolonged wet etching as pointed out by c-RBS and HRXRD, the trend in crystal quality enhancement versus etching time is not as regular and reproducible as evidenced by the same techniques for DS samples.

Strain control of orbital polarization and correlated metal-insulator transition in La2CoMnO6 from first principles

Transition metal oxides show rich physical properties which are often influenced by external perturbation. Here, we report by first-principles calculations that strain enables switching of orbital occupancy of Co2+ in La2CoMnO6 and hence modifies electronic structure of La2CoMnO6. We find a metal-insulator transition in La2CoMnO6, which originates from the interplay between the strain induced lattice distortion and electron correlations. Such transition is understood upon the orbital polarization of Co2+, which takes on the dxz/yz character under compression, yet switches to the dx2-y2 character under tension. The orbital switching presented should hold promise for many technological applications.

Nd-Doping Induced Lattice Distortion in TiO2 Nanoparticles

Nd-doping effects are investigated in TiO2 nanoparticles with various annealing temperatures T from 70°C to 1100°C by means of x-ray diffraction (XRD) and Raman scattering spectroscopy. XRD results indicate that the sample shows a rutile phase at 1100°C, which changes to anatase phase at 900°C. With decreasing T down to 300°C, a significant lattice shrink is found, that is, the lattice parameter c is significantly suppressed while the a value shows a gradual decrease. With further decrease of T, the c-value shows an unexpected increase while the α-value keeps a gradual decrease. Thus, a lattice distortion takes place with changing the annealing temperature. In Raman investigation, all the Raman modes for the anatase phase show hardening behaviors with decreasing T in the range 900–300°C, and then the Eg and A1g modes show softening behaviors below 300°C, suggesting the variation of the lattice distortion. The variation of the lattice distortion at different annealing temperatures is ascribed to different depositions of Nd ions on the surface of TiO2 nanoparticles.