Tetragonal Lattice Distortion Research Articles

Induced ferroelectricity in strained epitaxial SrTiO3 films on various substrates

The impact of strain on structure and ferroelectric properties of epitaxial SrTiO3 films on various substrate materials—substrates with larger (DyScO3) and smaller (NdGaO3 and CeO2/Al2O3) in-plane lattice constant, respectively—was analyzed. In all cases, (001)-oriented strained epitaxial SrTiO3 was obtained. It is demonstrated that the mismatch of the lattices or, alternatively, the mismatch of the thermal expansion coefficients of films and substrate, imposes biaxial strain on the SrTiO3 films. The strain leads to a small tetragonal distortion of the SrTiO3 lattice and has a large impact on the ferroelectric properties of the films. With decreasing film thickness and at low temperatures the permittivity deviates from the “classical” Curie-Weiss behavior. Furthermore, strain-induced ferroelectricity is observed, which agrees with theoretical predictions. For electric fields parallel to the film, surface-induced ferroelectricity is observed for SrTiO3 that is exposed to in-plane tensile strain, i.e., SrTiO3 on DyScO3 and sapphire. Transition temperatures of To≈210 K and To≈325 K are obtained for SrTiO3 on CeO2/Al2O3 and DyScO3, respectively.

X-ray diffraction analysis of the structure of antisite arsenic point defects in low-temperature-grown GaAs layer

A method for the structure analysis of point defects in a semiconductor layer is developed by combining x-ray diffraction and growth of a superlattice where the concentration of point defects is periodically varied in the growth direction. Intensities of satellite reflections from the superlattice depend predominantly on the atomic structure of point defects, and hence this method can be applicable to the case of a low concentration of point defects. By using this method, the atomic structure of antisite As point defects in GaAs layers grown by molecular-beam epitaxy at low temperatures has been analyzed. Measured intensity ratios of the first-order satellite reflection in the lower angle side to that in the higher angle side for a number of (hkl) reflections are compared with those calculated based on structure models. The analysis has shown that experimental intensity ratios cannot be reproduced by models which include only a uniform tetragonal lattice distortion and local atomic displacements around an antisite As atom. A fairly good agreement between measured and calculated intensity ratios is obtained with a model which account for both gradual change in the tetragonal lattice distortion in the (0 0 1) plane and displacements of neighboring atoms away from the antisite As atom.

Effect of epitaxial strain on the exchange-bias properties of[La0.67Ca0.33MnO3∕La0.33Ca0.67MnO3]15multilayers: Resonant x-ray scattering measurements

Complementary x-ray synchrotron radiation diffraction (XRD) and resonant scattering (RXS) measurements were performed at the $\mathrm{Mn}\phantom{\rule{0.3em}{0ex}}K$-edge between 10 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in order to analyze the effect of epitaxial strain and tetragonal lattice distortions on the exchange bias (EB) mechanism observed in $[{\mathrm{La}}_{2∕3}{\mathrm{Ca}}_{1∕3}{\mathrm{MnO}}_{3}(\mathrm{FM})∕{\mathrm{La}}_{1∕3}{\mathrm{Ca}}_{2∕3}{\mathrm{MnO}}_{3}(\mathrm{AF}){]}_{15}$ multilayers below a blocking temperature, ${T}_{B}$, of $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. XRD measurements showed that the $c∕a$ axial ratio, an indication of the tetragonal lattice distortion in pseudocubic lattice settings, reaches its maximum at the onset of the EB effect and the corresponding structural correlation length varies substantially at the onset of ${T}_{B}$. The in-plane lattice parameter $a$ at ${T}_{B}$ is close to the bulk lattice parameters of the antiferromagnetic (AF) layers, thus indicating that the EB effect is related with the accommodation of strain inside the ferromagnetic (FM) and AF layers. RXS measurements revealed a ferrodistortive (FD) state. The RXS intensity difference signal exhibits a main-edge feature and a postedge feature at $\ensuremath{\sim}6.57\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ that scales linearly with temperature and the $c∕a$ ratio up to $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The disappearance of the postedge feature above $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}(={T}_{B})$ may signify a rearrangement of $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Mn}$ bonding angles due to strain-driven effects at the FM/AF interfaces, inducing disorder in FD octahedral tilt ordering which may pin the local distortions below the ${T}_{B}$.

Pressure-induced long-range magnetic ordering in cobalt oxide

A long-range magnetic ordering transition in cobalt oxide $(\mathrm{CoO})$ was observed around $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature using neutron diffraction. The magnetic structure was type-II antiferromagnetic. High-resolution synchrotron x-ray diffraction showed no evidence of a tetragonal lattice distortion in $\mathrm{CoO}$ in the magnetically ordered phase. This result indicates that the tetragonal lattice distortion occurring with magnetic ordering in the low-temperature structure of $\mathrm{CoO}$ could be associated with cooperative Jahn-Teller effects (orbital ordering), which, however, are decoupled from magnetic ordering under high pressure.

Effect of Ca addition on Grain Size and Crystal Phase of Barium Titanate Nanopowders

The effect of 2.5 at. % Ca addition on the grain size and crystal lattice of hydrothermally produced barium titanate nanopowders was studied by field-emission transmission electron microscopy (FE-TEM). The fabrication of the nanopowders involved the heat treatment of Ba0.975Ca0.025TiO3 and BaTiO3 powders. The grain growth of Ba0.975Ca0.025TiO3 particles is inhibited by the Ca coating. A pore region is observed in both types of particles and the average size of the pores depends on the calcination temperature. In addition, the crystal structure of the nanoparticles was investigated with high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). These methods show that both powders have a crystalline structure with lattice planes typical for a perovskite structure. The lattice shrinkage is explained by Ca substitution at the Ba atomic positions in the unit cell of barium titanate, and the tetragonal lattice distortion is accounted for by Ca substitution at the Ti sites in the perovskite structure. X-ray absorption fine structure (XAFS) analysis shows that the local atomic structures around Ti in Ba0.975Ca0.025TiO3 and BaTiO3 particles are more highly ordered compared with pure single-crystalline BaTiO3.

Transport, magnetic, and structural properties of spinelMnTi2O4and the effect of V doping

We studied the transport, magnetic, and structural properties of spinel ${\mathrm{MnTi}}_{2}{\mathrm{O}}_{4}$ and V-doped samples, ${\mathrm{MnTi}}_{2\ensuremath{-}x}{\mathrm{V}}_{x}{\mathrm{O}}_{4}$, to clarify the relation between charge, spin, and orbital degrees of freedom of $3d$ electrons in this series of compounds. A structural phase transition occurs in ${\mathrm{MnTi}}_{2}{\mathrm{O}}_{4}$ at $180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is presumably dominated by the ordering of ${t}_{2\mathrm{g}}$ orbitals, but it can be easily suppressed by the V doping to the Ti site. Associated with this suppression of the structural anomaly, a ferrimagnetic and electrically conducting state appears in ${\mathrm{MnTi}}_{2\ensuremath{-}x}{\mathrm{V}}_{x}{\mathrm{O}}_{4}$ with finite values of $x$. Fairly large magnetoresistance was observed in this state, whose sign changes from positive to negative with an increase in the V concentration $x$. With approaching the other end of this series, ${\mathrm{MnV}}_{2}{\mathrm{O}}_{4}$, the electrical resistivity increases again and another type of structural phase transition occurs at $53\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Large magnetostriction was observed in ${\mathrm{MnV}}_{2}{\mathrm{O}}_{4}$, the only compound in this series to have both a ferrimagnetic state and tetragonal lattice distortion.

Effect of strain and tetragonal lattice distortions in doped perovskite manganites

A series of high-quality, coherently strained $[{\mathrm{La}}_{2∕3}(\mathrm{Ca}\phantom{\rule{0.3em}{0ex}}\mathrm{or}\phantom{\rule{0.3em}{0ex}}\mathrm{Ba}{)}_{1∕3}{\mathrm{MnO}}_{3}∕{\mathrm{SrTiO}}_{3}]\ifmmode\times\else\texttimes\fi{}N$ superlattices has been prepared on $(100)\phantom{\rule{0.3em}{0ex}}{\mathrm{SrTiO}}_{3}$ and ${\mathrm{NdGaO}}_{3}$ substrates by laser molecular beam epitaxy. The manganite layers are biaxially strained due to lattice mismatch. A quadratic decrease of the metal-to-insulator transition temperature ${T}_{p}$ with increasing biaxial distortion ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$ both for tensile and compressive in-plane strain is found. For $Tg{T}_{p}$, the resistivity versus temperature curves could be well described by the small polaron hopping model with the polaron binding energy increasing with increasing ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$. Furthermore, the magnetoresistance of the manganite films was found to strongly increase with increasing ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$ or decreasing ${T}_{p}$, respectively, following a universal behavior. An anomalous upturn of resistivity in the low-temperature regime $(Tl25\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ was detected, which may be attributed to enhanced Coulomb interaction of the charge carriers resulting from disorder due to the lattice distortion. Our analysis clearly demonstrates the importance of biaxial strain and Jahn-Teller-type lattice distortions for the physics of the doped manganites. It is shown that epitaxial coherency strain can be used to deliberately modify the materials properties.

ESR study of theEu2+g-value in the metallic phase of cubic hexaborideCa1−xEuxB6(0.15≲x⩽1.00)

The angular, magnetic field, and temperature dependence of the ${\mathrm{Eu}}^{2+}(4{f}^{7},S=7∕2)$ $g$-value in ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}{\mathrm{B}}_{6}(0.15\ensuremath{\lesssim}x\ensuremath{\leqslant}1.00)$ is measured by means of electron spin resonance at two microwave frequencies, 9.4 and $34.4\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$. The $g$-value is found to be anisotropic and magnetic-field-dependent. The reduction with field of the positive $g$-shift is interpreted in terms of the exchange interaction between the localized ${\mathrm{Eu}}^{2+}4{f}^{7}$ electrons with the conduction ${\mathrm{Eu}}^{2+}$ $5d$-like electrons and B $2p$-like holes. The angular, temperature, and $x$ dependence of the anisotropy of the $g$-value can be attributed to demagnetization effects due to the platelet-like shape of the samples. High-resolution synchrotron x-ray diffraction experiments show the absence of distortions larger than $\ensuremath{\delta}a∕a\ensuremath{\sim}{10}^{\ensuremath{-}4}$. However, a mechanism for the anisotropy based on a weak tetragonal lattice distortion arising from the crystal surface cannot be excluded.

Comparison of Bi 3Fe 5O 12 film giant Faraday rotators grown on (111) and (001) Gd 3Ga 5O 12 single crystals

Bismuth iron garnet (Bi 3Fe 5O 12, BIG) epitaxial thin films were grown on single crystal (Gd 3Ga 5O 12, GGG) (111) and (001) substrates by rf-magnetron sputtering technique. Processing parameters have been optimized to obtain high deposition rate (2.74 μm/h) and the surface rms roughness less than 10 nm. X-ray diffraction reveals films epitaxial quality: exclusive (111) or (001) orientation with narrow rocking curves and strong in-plane texture. Films possess low optical loss and magneto-optical Faraday rotation (FR) as high as 5 deg/μm at 677 nm wavelength. Comparative analysis of films grown on (111) and (001) substrates clearly shows significant superiority of BIG/GGG(001) film. For this film, the coercive field ∼100 Oe appears to be 2.5 times lower while the optical transmission to be 10% higher than that for BIG/GGG(111) film. Enhanced magneto-optical performance of BIG/GGG(001) films relies upon better accommodation of the film-to-substrate mismatch strain through the tetragonal BIG lattice distortions compared to the rhombohedral one in BIG/GGG(111) films.

Magnetocrystalline anisotropy inL10 FePt and exchangecoupling in FePt/Fe3Pt nanocomposites

The magnetic and structural properties of Fe–Pt nanocomposites and relatedidealized structures have been investigated by a combination of experimental andtheoretical techniques. The dependence of magnetocrystalline anisotropy (MCA) ofL10 FePt on the ratio of the tetragonal lattice parameters,c/a, has been calculated with a relativistic version of the full potential local orbitalmethod, assuming complete chemical order and fixed unit-cell volume. It hasbeen found that the well known tetragonal lattice distortion in this phase has arelatively small influence on the MCA (compared to the influence of chemicalordering) and even reduces the MCA. The calculated in-plane anisotropy is negligible.The structure, magnetic properties and magnetization reversal processes ofFe100−xPtx (x = 40,45, and 50) powders produced by mechanical milling and subsequent annealing have beeninvestigated. Structural studies reveal that upon annealing of the as-milled powders consisting of fineFe/FePt(A1)/Pt lamellae, chemicallyhighly ordered L10 FePt and, in the case of the Fe-rich compositions,L12 Fe3Pt areformed. The nanometre scale multilayer structure preserved after annealing gives rise to large effectsof exchange interactions between the crystallites of the phases. With decreasing Pt concentrationx, the remanence enhancement increases, due to the increase of theFe3Pt fraction, whereas the coercivity and the switching fields for irreversible magnetizationreversal are reduced.

Shape and Internal Structure of Silver Nanoparticles Embedded in Glass

The structural characteristics of silver nanoparticles embedded in glass by various routes of fabrication were studied in detail using high-resolution electron microscopy to find out if they are influenced by interaction with the surrounding glass matrix. Besides the formation conditions, the strength of the interaction between metal and glass governs the size-dependent changes of lattice spacings in such nanoparticles. However, determination of these changes is not straightforward because of complicated particle configurations and the interference nature of the lattice imaging technique. Imaging of lattice plane fringes and careful diffractogram analysis allowed the exclusion of any kind of tetragonal lattice distortion or transformation to hexagonal lattice type that may be deduced at first sight. Instead, the formation of twin faults in these nanoparticles turned out to be the essential structural feature and the main source of confusion about the lattice structure observed. The variety of particle forms is comparable to particles supported on oxide carriers. It is composed of single-crystalline particles of nearly cuboctahedron shape, particles containing single twin faults, multiple twinned particles containing parallel twin lamellae, and multiple twinned particles composed of cyclic twinned segments arranged around axes of 5-fold symmetry. The more twin planes involved in the particle composition, the more complicated is the interpretation of lattice spacings and lattice fringe patterns due to superposition of several twin segments.

Interdependence between the magnetic properties and lattice parameters of Ni–Mn–Gamartensite

The temperature dependences of both the saturation magnetic field values and the powderdiffraction patterns of two Ni–Mn–Ga tetragonal martensites were studied experimentally.A reasonable agreement between the temperature dependences of the lattice parametersdetermined from the x-ray diffraction data and obtained from the magnetization curves isobserved. This experimental finding demonstrates the linear dependence of the magneticanisotropy constant on the tetragonal distortion of the cubic crystal lattice arising in thecourse of the martensitic transformation, and hence supports the macroscopic andmicroscopic theories by L’vov et al (1998 J. Phys.: Condens. Matter 10 4587) and Enkovaaraet al (2002 Phys. Rev. B 65 134422), respectively, which predict this dependence.

X‐Ray Investigations of the intrinsic Carbon‐Incorporation during the MOVPE Growth of AlxGa1‐xAs

AbstractWe have investigated the intrinsic C‐incorporation during the metalorganic vapour‐phase epitaxy of GaAs, (Al,Ga)As, and AlAs. The carbon from the methyl radicals of trimethylgallium and trimethylaluminium has been used as intrinsic carbon source and the necessary growth conditions (low growth temperatures and low V/III‐ratios) have been determined. Under these conditions we observed a reduction of the Al‐incorporation in (Al,Ga)As resulting from a change of surface kinetics at low temperatures. We have used the tensile tetragonal lattice distortion due to the C‐incorporation for determination of the carbon concentration. This has been done by investigating the symmetric (004) reflection peaks of GaAs and the strained epitaxial layer by high resolution x‐ray diffraction. The resulting carbon concentrations showed a good agreement with the hole concentrations determined by Hall measurements and spectroscopic ellipsometry.

Strain analysis in ultrathin silicide layers in Fe∕CsCl–FeSi57∕Fe sandwiches

Epitaxially stabilized iron monosilicide films with the CsCl structure (B2-FeSi) have been investigated by conversion electron Mössbauer spectroscopy and x-ray diffraction. A detailed investigation of the elastic strain in these metastable layers is presented. Using hyperfine interaction information the tetragonal distortion of the silicide lattice could be quantified for layers as thin as 14Å. A general tendency for strain relaxation with increasing layer thickness is observed.

Temperature variation of the tetragonality in ordered PtFe alloy

Tetragonal lattice distortion of the PtFe alloy was studied by neutron diffraction as a function of temperature. The tetragonality (1– c / a ) increases with increasing temperature up to the Curie temperature ( T C =750 K). Data analysis based on the hard sphere model in the (1 1 1) plane leads to the shrink of the atomic volume of Fe with increasing temperature. The invar-like effect at high temperature would exert a significant influence on the magnetic anisotropy energy of this system.

NMR study of ionic shifts and polar ordering in the relaxor ferroelectricPb(Sc1/2Nb1/2)O3

${}^{207}\mathrm{Pb},$ ${}^{45}\mathrm{Sc},$ and ${}^{93}\mathrm{Nb}$ nuclear-magnetic-resonance (NMR) spectra of partially ordered relaxor ferroelectric ${\mathrm{PbSc}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_{3}$ (PSN) have been studied at temperatures between 77 K and 420 K. The relatively narrow ${}^{45}\mathrm{Sc}$ and ${}^{207}\mathrm{Pb}$ NMR spectral components, arising from chemically ordered regions of the crystal, show marked anomalies at ${T}_{c}\ensuremath{\simeq}$ 360 K, where ${T}_{c}$ is the temperature of the dielectric susceptibility maximum. The abrupt frequency shifts of these components below ${T}_{c}$ are related to the first-order structural phase transition. In the coexisting broad spectral components, originating from compositionally disordered regions, such anomalies are barely detectable; they were not observed at all in the ${}^{93}\mathrm{Nb}$ NMR spectra. Our NMR data are compatible with an assumption of large, up to 0.04 nm, displacements of lead ions and smaller, $\ensuremath{\sim}0.01\mathrm{nm},$ displacements of scandium ions along the four polar rhombohedral axes, while Nb ions remain in the centers of oxygen octahedra. ${}^{207}\mathrm{Pb}$ NMR measurements in poled crystals reveal that long-range polar ordering is established solely in the chemically ordered parts of the crystal, whereas in the disordered regions a mixed ferro-glass order sets in. At low temperatures, both ${}^{207}\mathrm{Pb}$ and ${}^{93}\mathrm{Nb}$ resonances detect, in addition to the ferroelectric rhombohedral, a tetragonal distortion of the PSN lattice. This distortion appears at ${T}_{c}$ and increases smoothly on decreasing temperature, possessing a second-order phase-transition character. The effective local symmetry of PSN at $T<250\mathrm{K}$ thus lowers from the axial rhombohedral to the nonaxial orthorhombic with lead atom shifts deviating considerably from the [111] polar axes. Structural data, obtained via NMR, are compared with similar data obtained from neutron and x-ray-diffraction experiments.

Temperature variation of the tetragonality in ordered PtFe alloy

Tetragonal lattice distortion of the PtFe alloy was studied by neutron diffraction as a function of temperature. The tetragonality (1–c/a) increases with increasing temperature up to the Curie temperature (TC=750K). Data analysis based on the hard sphere model in the (111) plane leads to the shrink of the atomic volume of Fe with increasing temperature. The invar-like effect at high temperature would exert a significant influence on the magnetic anisotropy energy of this system.

Phase Transformation and the Type of Lattice Distortion of Some Platinum‐Rich Phases Belonging to the Cu Family.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anisotropic in-plane strain in W-doped (Ba, Sr)TiO 3 thin films deposited by pulsed-laser deposition on (001)MgO

The structural and the microwave dielectric properties of BaxSr(1-x)TiO3 films (BST) with x=0.5, 0.6 and 0.7, containing 1 mol % W have been investigated. The films were grown by pulsed-laser deposition on MgO (001) substrates at a temperature of 720 °C in an oxygen pressure from 3 to 500 mTorr. The film structures were determined by X-ray diffraction. The lattice parameters were fitted to a tetragonal distortion of a cubic lattice. The out-of-plane lattice parameter (c) was calculated from the position of the (004) reflection. Using c, the in-plane lattice parameter, a, was calculated from the position of the (024) and (224) reflections. A deviation in the calculated values for a, beyond the systematic error, was found in the in-plane lattice parameter, suggesting an in-plane orthorhombic distortion (a, a’). Films with x=0.7 showed a minimum in-plane distortion due to a better lattice match with the substrate. The ratio of the in-plane and out-of-plane lattice parameters was calculated as a measure of the lattice distortion (a/c and a’/c). The dielectric properties of the films deposited were measured at room temperature at 2 GHz using gap capacitors fabricated on top of the dielectric film. For all Ba/Sr ratios investigated in the W-doped material, the dielectric Q (1/cosδ) was observed to be insensitive to the oxygen deposition pressure. A peak in the change in the dielectric constant, as a function of an applied electric field (0–80 kV/cm), was observed for films deposited in 50 mTorr of oxygen. The largest K-factor, K=(e(0)-e(V )/e(0)×Q(0)), for films deposited from a BST x=0.6 (1 mol % W-doped) target was observed in the film that had a minimum in-plane strain, where a∼a’ and c was greater than a and a’.

Ab initio study of elastic properties of Ir and Ir3X compounds

Elastic constants and moduli of face-centered cubic Ir and its L12 intermetallic compounds Ir3X (X=Ti, Ta, Nb, Zr, Hf, V) have been determined using ab initio density functional theory calculations within the generalized gradient approximation. With the tetragonal, trigonal, and isotropical lattice distortions, elastic constants C11, C12, C44, and bulk modulus B are derived from the second derivative of the total energy as a function of volume. The calculated Young’s modulus E, shear modulus G, Poisson’s ratio ν, and the ratio RG/B of G over B are then used to examine mechanical properties of Ir and Ir3X compounds. By analyzing RG/B and Cauchy pressure C12–C44, the brittle-ductile behavior of the materials is assessed. Based on the modulus difference ΔG between the γ matrix (Ir) and γ′ precipitates (Ir3X), the γ′ strengthening effect in the γ matrix is studied.