Tetragonal Lattice Distortion Research Articles

Compositional behavior of heat capacity and magnetic properties in double spinel ferrites

The purpose of the work is to identify the effects of the substitution of paramagnetic ions in ferromagnetic ferrites by diamagnetic components of zinc and aluminum. The following isomorphic spinel series are the object of review: Me(1-x)ZnxFe2O4 (Me – Ni, Co, Li, Cu, Mg) and Li0.5Fe2.5-xAlxO4 (x= 0-2.5). The single-phased samples were synthesized by ceramic procedure. Heat capacity near standard temperature Cp (298.15) was studied using calorimeter methods: drop calorimeter and DSC. Magnetic parameters – magnetic moments (M) and Curie temperatures (Tc) - were determined. All Me-Zn ferrite systems are characterized by dome-shaped correlations between heat capacity Cp(298) and composition (x), which have maximum values in the equimolecular region (x=0.4-0.6). This fact coincides with the nature of the compositional dependence of magnetic moments and indicates a significant effect of the excess magnetic contribution on the total heat capacity. A noticeable influence of the cooperative Jahn-Teller effect and the associated tetragonal distortion of the cubic lattice of Cu-Zn ferrites with a low Zn content (x<0.2) is observed, which causes overestimated values of the heat capacity of these compositions. Curie temperatures reveal the rectilinear decrease with increasing diamagnetic Zn and Al content in ferrite systems. The results are presented by graphs and tables.

Extremely High Ferromagnetic Resonance Frequency Induced by Triclinic Lattice Distortion in Epitaxial FeCo/MgAl2O4 (001) Films

Theoretically, tetragonal lattice distortion of FeCo epitaxial films can result in a very large in‐plane magnetic anisotropy field, leading to an extremely high ferromagnetic resonance (FMR) frequency. Herein, thin films are epitaxially grown on (001) MgAl2O4 single‐crystal substrates. A triclinic lattice distortion with , instead of a tetragonal one, is found in the FeCo films. The cubic symmetry breaking leads to a deviation of easy axes from the directions, forming a distribution of magnetic moments with a strong perpendicular magnetic anisotropy (PMA) along the out‐of‐plane [001] directions and a deviation of the in‐plane components from the ([10 100]) directions. The effective field of the former is as high as 1.5–2.5 T, enough to overcome the thin film shape anisotropy, while that of the latter stays at a low value of around 0.05 T. The strain‐induced PMA gradually relaxes to in‐plane for thicker films with a strained sublayer remaining. As a result, an extremely high out‐of‐plane FMR frequency over 40 GHz is achieved, accompanied by a lower in‐plane FMR frequency around 8 GHz. This study provides a possible approach to prepare self‐biased soft magnetic films with extremely high‐resonance frequency for applications in microwave‐integrated circuits.

Metallic ferrimagnetism and magnetic domain structure in (001) NiCo2O4 films grown at various temperatures

Epitaxial 31-nm-thick (001) NiCo2O4 films exhibiting metallic and ferrimagnetic properties at room temperature were grown on (001) MgAl2O4 substrates at various deposition temperatures. θ/2θ scan and reciprocal space mapping X-ray diffraction measurements indicated that tetragonal lattice distortion in the (001) NiCo2O4 films increased with the deposition temperature. Metallic ferrimagnetism and perpendicular magnetic anisotropy (PMA) were observed for (001) NiCo2O4 films grown at deposition temperatures below 350 °C. Remarkably, the ferrimagnetic transition temperature continuously decreased with increasing deposition temperature up to 350 °C, whereas images of the magnetic domains indicated that the density of the nucleated small domains during the early stage of magnetic reversal increased with the deposition temperature. Consequently, the PMA of the (001) NiCo2O4 films at room temperature was optimized using a deposition temperature of approximately 320 °C. The results of this study indicate that the deposition temperature is a useful parameter for adjusting the PMA and magnetic domain structure of NiCo2O4 films by inducing variations in structural distortion and metallic ferrimagnetism.

Platonic and Archimedean bodies as the basis of the structure of self-accommodating complexes of martensite crystals in alloys with shape memory effects

The aim of the work is to analyze the relationship of the architecture of self-accommodation complexes (SC) with the lattice syngony of martensite crystals. Self-accommodating complexes consist of a set of pairwise twinned domains — crystals of martensite belonging to crystallographically equivalent variants of the orientation relationship between the lattices of austenite and martensite. The simplest SC are calculated for tetragonal, orthorhombic, rhombohedral and monoclinic distortion of the cubic lattice of austenite. It is shown that complete self-accommodation is possible only in complexes containing simultaneously all variants of the orientation relation. The issue of external faceting of complexes is discussed. The reason for the formation of SC is the minimization of elastic energy, i.e. the appearance regulated by the energy of the interphase boundary. On the other hand, if the outer surface of the SC is a polyhedron, then its symmetry should "fit"into the anisotropy of the elastic properties of austenite. For reasons of symmetry, it is clear that the polyhedron must be correct and have the same symmetry elements as the cubic lattice of austenite, while the axes of symmetry of the cubic lattice of austenite must coincide with the axes of symmetry of the polyhedron. Similar polyhedra are some of the bodies of Platon and Archimedes, which have axes of symmetry of the 2nd, 3rd and 4th order. A number of examples calculated in the work confirms the possibility of the existence of complexes in the form of these polyhedra.

Определение когерентности интерфейса твердорастворной и интерметаллидной фазыв монокристаллических литейных сплавах на основе интерметаллида Ni3Al рентгенодифрактометрическим методом

The paper considers splitting of the solid solution gamma phase diffraction peaks in the nickel heat-resistant alloys, including alloys based on the Ni3Al intermetallic compound with the heterophase structure. It is shown that the effect of splitting the solid solution gamma phase diffraction peaks is missing in the alloys based on the Ni3Al intermetallic compound with heterophase structure, in contrast to the nickel heat-resistant alloys. It was established that splitting of the solid solution gamma phase diffraction peaks in the diffraction pattern was caused by tetragonal distortion of the gamma phase crystal lattice (solid solution) exposed to the interphase stresses. Only under condition of the interphase boundary coherence exposed to the interfacial stresses the tetragonal distortion of the gamma phase crystal lattice (solid solution) appeared. Therefore, a conclusion could be made based on the absence of splitting the gamma phase diffraction peaks that the interphase boundary (interface) coherence was missing in alloys based on the Ni3Al intermetallic compound with the heterophase structure.

Origin of heterogeneous magnetostriction: A case study of nanoprecipitates-bearing Fe-Mo alloys

Ferromagnets with structural nanoheterogeneities have recently been found to exhibit large magnetostriction, as reported in a considerable number of Fe-based alloys soluted with non-magnetic elements. Unlike homogeneous ferromagnets, where their large magnetostriction stems from strong spin-orbit coupling, however origin of the large heterogeneous magnetostriction is still in debate. In this work, we performed a comparative study between the cubic supersaturated solid solution Fe96Mo4 and its aged counterpart with low-symmetry nanoprecipitates. The results showed that the saturation magnetostriction can be enhanced from 19 ppm to 40 ppm by 110% through introducing coherent low-symmetry nanoprecipitates, despite of reduced saturation magnetization and weakened net magnetocrystalline anisotropy. Further structural characterizations revealed that the coherent nanoprecipitates result in an apparent tetragonal lattice distortion of the cubic matrix, i.e., the spontaneous magnetostriction constant (lattice strain) is enlarged. These results indicate that the heterogeneous magnetostriction cannot merely be attributed to the spin-orbit coupling, but also is closely related to the lattice distortion induced by the coherent low-symmetry nano-heterogeneities.

Ligand Exchange and Impurity Doping in 2D CdSe Nanoplatelet Thin Films and Their Applications

AbstractThe effects of halide‐ligand exchange and Cu and Ag doping are studied on structural, optical, and electrical properties of four monolayer CdSe nanoplatelet (NPL) and NPL thin films. Combinational study shows that NH4Cl‐treatment on CdSe NPL and NPL thin films show tetragonal lattice distortion of NPL, side‐to‐side attachment between NPLs, bathochromic shift in absorption spectra, and complete quenching of band‐edge and dopant‐induced emissions. First‐principle calculations reveal that Cl creates states below valence band maximum while Ag and Cu dopants create acceptor‐like states, explaining the change of their optical property. Field‐effect transistors are fabricated to investigate the effect of doping and reduced interplatelet distance on electrical properties of CdSe NPL thin films, demonstrating Cu and Ag dopants mitigate n‐type character of CdSe NPL thin films. Temperature‐dependent electrical characterization is conducted to further understand charge transport behavior depending on the existence of dopants. This work provides scientific information on the influence of surface chemistry and impurity doping on quantum confined semiconductors and new directions for the design of high‐performance nanomaterial‐based electronic and optoelectronic devices.

Effect of A-site off-stoichiometry on the microstructural, structural, and electromechanical properties of lead-free tetragonal 0.80Na0.5Bi0.5TiO3–0.20BaTiO3 (NBT–20BT) piezoceramic

A-site off-stoichiometry has been an efficient method to enhance the electromechanical properties of lead-free Na0.5Bi0.5TiO3(NBT)-based piezoceramics. In this work, we have reported the effect of Na/Bi off-stoichiometry on the microstructural, structural, and electromechanical properties of the tetragonal 0.80Na0.5Bi0.5TiO3–0.20BaTiO3 (NBT–20BT) ceramic. The maximum piezoresponse ~ 115 pC/N is obtained for 4 mol% Na-deficient (20BT–Na46) and 2 mol% Bi-excess (20BT–Bi52) compositions. It is a 25% increment over the piezoresponse of the stoichiometric composition (20BT–Na50 ~ 90 pC/N). The enhancement in piezoresponse has been attributed to an optimized presence of polar-structural heterogeneity/disorder in the poled ceramic matrix and an optimized distortion of the long-range tetragonal ferroelectric phase. It is established that A-site off-stoichiometry is influencing the polar-structural heterogeneity/disorder and tetragonal lattice distortion via the grain size.

Excess Solute Carbon and Retained Tetragonality in Tempered Fe-0.6C-1Mn Martensite and the Effect of Silicon Addition

The tetragonality and carbon distribution in tempered Fe-0.6C-1Mn martensite were investigated by X-ray diffraction and atom probe tomography to elucidate strain relaxation in the tetragonal lattice during tempering and its relationship with the solubility of excess carbon in martensite. Even though tetragonality (c/a) decreased with an increase in the tempering temperature, it persisted at low levels up to 400 °C. Si addition suppressed the decrease in tetragonality at 400 °C by inhibiting recovery in the dislocated matrix. Such persistence implies that dislocation migration is crucial for the complete release of tetragonal lattice strain at such a temperature, in addition to the decrease in the amount of solute carbon in martensite. A low level of tetragonality was observed for martensite containing carbon in the solid solution below the critical value of ~ 0.2 mass pct, at which a bcc structure was predicted. The amount of solute carbon after tempering was linearly correlated with tetragonality in the solute carbon content range of 0.07 to 0.6 mass pct, and the correlation coefficient was similar to those for as-quenched auto-tempered martensite and bainitic ferrite; these results indicate that the amount of excess carbon is simply determined by the amount of tetragonal lattice distortions remaining after carbide precipitation and recovery.

Matching conflicting oxidation conditions and strain accommodation in perovskite epitaxial thin-film ferroelectric varactors

Perovskite oxide materials of the general chemical formula ABO3 are a rich playground for epitaxial stacks of different functional layers for novel device applications. In the example of a tunable metal–insulator–metal ferroelectric varactor (tunable capacitor) made from the highest conducting perovskite SrMoO3 as an electrode and the tunable dielectric Ba0.5Sr0.5TiO3 (BST), we show how the extremely conflicting oxidation potentials can be conciliated in a fully functional heterostructure. Controlling the growth kinetics by the substrate temperature, oxygen pressure, and oxidation time, the formation of the non-conducting Mo6+ states can be effectively suppressed and the BST cation stoichiometry can be tuned. The cumulative impact of the cation nonstoichiometry, epitaxial strain, and oxygen deficiency in the BST films leads to the expansion of their c-axis lattice parameter via the formation of point defects. The dielectric permittivity of 440, the high tunability of 3.5, and the quality factor of 50 are achieved for the varactors at the frequency of 1 GHz. It turns out that the varactor performance is anti-correlated to the tetragonal lattice distortion of BST, which itself is interrelated to the oxidation conditions. The mechanism of the leakage current through oxygen deficient BST layers of the varactors is analyzed within the combined scenarios of the space-charge limited current and Poole–Frenkel field-assisted emission from traps. The achieved high capacitance per unit area of 0.04 pF/μm2 and moderate leakage currents of 0.025 μA/pF make these varactors suitable for applications in microwave microelectronic devices.

Ferroelectric and piezoelectric properties of (Bi1/2K1/2)(ZrxTi1−x)O3 lead-free ceramics

We report the effect of Zr substitution for Ti on the ferroelectric and piezoelectric properties of lead-free (Bi1/2K1/2)TiO3 (BKT) ceramics. Dense ceramics of (Bi1/2K1/2)(ZrxTi1−x)O3 with x up to 0.06 were fabricated by sintering of hydrothermally derived fine powders. The investigation of the crystal structure and dielectric properties revealed that the Zr substitution decreased the tetragonal lattice distortion of BKT and stabilized the relaxor state. The coercive electric field was found to be decreased monotonically with increasing the Zr content, while the largest remanent polarization of 26 μC cm−2 was observed at x = 0.04. As a result, the ceramic with x = 0.04 showed an enhanced piezoelectric d33 constant of 140 pC N−1.

Quasicubic model for metal halide perovskite nanocrystals.

We present an analysis of quantum confinement of carriers and excitons, and exciton fine structure, in metal halide perovskite (MHP) nanocrystals (NCs). Starting with coupled-band k · P theory, we derive a nonparabolic effective mass model for the exciton energies in MHP NCs valid for the full size range from the strong to the weak confinement limits. We illustrate the application of the model to CsPbBr3 NCs and compare the theory against published absorption data, finding excellent agreement. We then apply the theory of electron-hole exchange, including both short- and long-range exchange interactions, to develop a model for the exciton fine structure. We develop an analytical quasicubic model for the effect of tetragonal and orthorhombic lattice distortions on the exchange-related exciton fine structure in CsPbBr3, as well as some hybrid organic MHPs of recent interest, including formamidinium lead bromide (FAPbBr3) and methylammonium lead iodide (MAPbI3). Testing the predictions of the quasicubic model using hybrid density functional theory (DFT) calculations, we find qualitative agreement in tetragonal MHPs but significant disagreement in the orthorhombic modifications. Moreover, the quasicubic model fails to correctly describe the exciton oscillator strength and with it the long-range exchange corrections in these systems. Introducing the effect of NC shape anisotropy and possible Rashba terms into the model, we illustrate the calculation of the exciton fine structure in CsPbBr3 NCs based on the results of the DFT calculations and examine the effect of Rashba terms and shape anisotropy on the calculated fine structure.

Influence of individual phases on the magnetoelectric coupling and electromechanical response in (1 − x)Ba0.83Ca0.10Sr0.07TiO3-xBiFeO3 (x = 0–0.30) multiferroic composites

BaTiO3 based composites have been widely utilized to achieve better ferroelectric and magnetoelectric characteristics. BiFeO3 is one of materials that exhibit superior magnetoelectric characteristics in single phase. In this work, (1 − x)Ba0.83Ca0.10Sr0.07TiO3-xBiFeO3 (x = 0–0.30) multiferroic composites have been synthesized using mechano-chemical activation method. The strategy is to achieve optimum ferroelectric and ferromagnetic properties without hampering the inherent dielectric and ferroelectric characteristics of Ba0.83Ca0.10Sr0.07TiO3. The synthesized composites have shown diphasic nature with homogeneous grains and clearly distinguishable grain boundaries. The composition with x = 0.10 exhibits superior electrical properties, which are attributed to optimum resistivity and increased tetragonal lattice distortion introduced by BiFeO3 addition. The modification in bond length and bond angle leads to a significant enhancement in ferromagnetic characteristics of prepared composites. The composites have shown a gradual increase in magnetoelectric characteristics and substantial DC magnetic field dependence. This work provides an optimum composition and feasible route to prepare high-performance magnetoelectric composites.

Site-Preference, Electronic, Magnetic, and Half-Metal Properties of Full-Heusler Sc2VGe and a Discussion on the Uniform Strain and Tetragonal Deformation Effects

A hypothetical full-Heusler alloy, Sc2VGe, was analyzed, and the comparison between the XA and L21 structures of this alloy was studied based on first-principles calculations. We found that the L21-type structure was more stable than the XA one. Further, the electronic structures of both types of structure were also investigated based on the calculated band structures. Results show that the physical nature of L21-type Sc2VGe is metallic; however, XA-type Sc2VGe is a half-metal (HM) with 100% spin polarization. When XA-type Sc2VGe is at its equilibrium lattice parameter, its total magnetic moment is 3 μ B , and its total magnetism is mainly attributed to the V atom. The effects of uniform strain and tetragonal lattice distortion on the electronic structures and half-metallic states of XA-type Sc2VGe were also studied. All the aforementioned results indicate that XA-type Sc2VGe would be an ideal candidate for spintronics studies, such as spin generation and injection.

Influence of hydrogen on electron-phonon coupling and intrinsic electrical resistivity in zirconium: A first-principles study

The paper presents the first-principle calculation of the electron-phonon coupling and the temperature dependence of the intrinsic electrical resistivity of the zirconium-hydrogen system with various hydrogen concentrations. The nature of the anomalous decrease in the electrical resistivity of the Zr-H system with the increase of hydrogen concentration (at the high concentrations of H/Zr$>$1.5) was studied. It was found that the hydrogen concentration, where the resistivity starts to decrease, is very close to the critical concentration of the $\delta-\varepsilon$ phase transition. It was shown that the tetragonal lattice distortion due to the $\delta-\varepsilon$ phase transition of the Zr-H system eliminates imaginary phonon frequencies and the strong electron-phonon coupling of the $\delta$ phase and, as a result, leads to the reduction of the electrical resistivity of the Zr-H system at a high hydrogen concentration.

Theory of Mn-Based High-Magnetization Alloys

The realization of the magnetic moment of Mn atoms in ferromagnetic alloys is investigated theoretically. This paper is partly motivated by the recent discovery of high-magnetization Fe–Co–Mn thin films by Snow et al. , who reported moments of up to 3.25 $\mu _{\text {B}}$ per atom. Structural and strain effects are discussed with particular emphasis on the distinction between tetragonal martensitic lattice distortions and Bain transitions in body-centered cubic, CsCl, and Heusler alloys. It is outlined how these lattice distortions affect the electronic structure of the alloys, and ab initio calculations are used to determine the net moment of cubic Fe6Co63Mn31. The calculated moment, $2.90~\mu _{\text {B}}$ per atom, is reasonably close to the experimental value.

Structural Features of PLZT Films

Lead zirconate titanate (PZT) films doped with lanthanum, Pb(1–х)Laх(Zr0.48Ti0.52) (х = 0, 0.02, 0.05, 0.08, or 0.01), have been investigated by electron microscopy and X-ray diffraction. Films were formed on Si–SiO2–TiO2–Pt substrates by chemical vapor deposition from a solution and annealed at temperatures T = 650 and 750°C. The main structural features of the films, differing them from undoped PZT films fabricated by the same method, have been established. It is found that doping with lanthanum delays the pyrochlore–perovskite transformation in the film bulk, i.e., in the regions distant from the film–substrate interface. The fraction of metastable pyrochlore phase increases with an increase in the La molar content in the films. The main reason for the delay is the deficit of lead in the intergranular perovskite space, especially in the upper part of the film. Annealing at T = 750°C reduces the content of pyrochlore phase but does not completely remove it, which was never observed for undoped PZT films. Doping with lanthanum leads to a change in the lattice period c and a tetragonal distortion of the perovskite lattice (c/a ratio). Hence, the [100] texture of the films obtained, in contrast to the typical [111] texture of PZT films, is due to the increase in the lattice mismatch between the film and platinum layer when lead atoms are replaced with lanthanum. Lattice distortions of “transrotational” character, whose value exceeds 160 deg/μm, are found to arise in growing crystals.

Local structural distortions in strained Ba0.5Sr0.5TiO3 thin films

The local atomic geometries in ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{TiO}}_{3}$ thin films grown on MgO(001) substrates have been determined by Ti $1s$ near-edge x-ray-absorption fine-structure measurements and density-functional-theory calculations. The accuracy of the atomic geometries predicted by density-functional theory is demonstrated by simulations of the near-edge x-ray-absorption fine-structure spectra using a Bethe-Salpeter treatment of the Ti $1s$ core-hole interaction in the films. Our results show that films with either $cga$ or $cla$ tetragonal lattice distortions have their polarization vectors rotated toward or away from the [001] film-normal or $c$-axis direction, respectively. Both distortions result locally in the monoclinic $r$ phase of the strain-phase diagram of Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)], and the polarizations are rotated significantly larger than the geometry would suggest.

Local inhomogeneous structural origin of giant magnetostriction in Fe-Ga alloys

Melt-spun ribbons and single crystal plates of Fe83Ga17 alloys were studied at both nano-scale and atomic-scale to investigate the structure origin of their giant magnetostriction. The presence of nano-precipitates in the melt-spun ribbons was verified using synchrotron x-ray diffraction and was validated by small angle scattering experiment. The nano-precipitates were found to be inhomogeneously distributed within the ribbons, as peak splitting clearly observed in the diffraction patterns disappeared into broad peaks when the synchrotron beam was focused onto different parts of the same sample. Further analysis using x-ray diffuse scattering and extended x-ray absorption fine structure indicates the existence of short-range ordering (SRO) Ga-Ga pairs along the (001) direction in both the ribbon and single-crystal alloys. All experimental evidence suggests that this modified D03-type SRO leads to both a short-range lattice strain and a long-range symmetry breaking tetragonal distortion of the A2 lattice. This gives rise to giant magnetostriction in the Fe-Ga alloys. In the melt-spun ribbons, the Ga-Ga pairs can aggregate into nano-sized precipitates under fast cooling and enhance the magnetostriction further.

MgGa2O4 spinel barrier for magnetic tunnel junctions: Coherent tunneling and low barrier height

Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. A tunnel magnetoresistance (TMR) ratio up to 121% at room temperature (196% at 4 K) was observed, suggesting a TMR enhancement by the coherent tunneling effect in the MgGa2O4 barrier. The MgGa2O4 layer had a spinel structure and it showed good lattice matching with the Fe layers owing to slight tetragonal lattice distortion of MgGa2O4. Barrier thickness dependence of the tunneling resistance and current-voltage characteristics revealed that the height of the MgGa2O4 barrier is much lower than that of an MgAl2O4 barrier. This study demonstrates the potential of Ga-based spinel oxides for MTJ barriers having a large TMR ratio at a low resistance area product.