Tetragonal State Research Articles

Spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2As2

We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe$_2$As$_2$, which has a tetragonal-to-orthorhombic structural distortion at $T_s$ and antiferromagnetic (AF) phase transition at $T_N$ with ordered moments along the orthorhombic $a$-axis below $T_s\approx T_N\approx 136$ K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with $M_a=M_b=M_c$, where $M_a$, $M_b$, and $M_c$ are spin excitations polarized along the $a$, $b$, and $c$-axis directions of the orthorhombic lattice, respectively. On cooling towards $T_N$, significant spin excitation anisotropy with $M_a>M_b\approx M_c$ develops below 3 meV with a diverging $M_a$ at $T_N$. The in-plane spin excitation anisotropy in the tetragonal phase of BaFe$_2$As$_2$ is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.

Creating a low-symmetry insulating, ferroelectric, and antiferromagnetic material from a high-symmetrical metallic ferromagnet via defect engineering: The case of LaBaCo2O5+δ compounds

First-principles calculations are performed to investigate the properties of defect-free $\mathrm{LaBaC}{\mathrm{o}}_{2}{\mathrm{O}}_{6}$ materials as well as $\mathrm{LaBaC}{\mathrm{o}}_{2}{\mathrm{O}}_{5}$ films possessing ordered oxygen vacancy structures. These calculations predict that the defect-free $\mathrm{LaBaC}{\mathrm{o}}_{2}{\mathrm{O}}_{6}$ crystallizes in a tetragonal state with an axial ratio smaller than unity, and is metallic and ferromagnetic as consistent with the literature. In contrast, the $\mathrm{LaBaC}{\mathrm{o}}_{2}{\mathrm{O}}_{5}$ film adopts a low-symmetry monoclinic state having an axial ratio larger than 1, is insulating and even ferroelectric, as well as exhibits a G-type antiferromagnetic ordering. Our calculations further reveal the origin of these defect-induced changes in properties. Moreover, further experiments we conducted demonstrate the possibility to grow highly epitaxial $\mathrm{LaBaC}{\mathrm{o}}_{2}{\mathrm{O}}_{5}$ films with ordered oxygen vacancies, via reduction treatments, and confirm some of our predictions. These findings can open a new avenue for the design and synthesis of multiferroics via defect engineering.

Anisotropy of infrared-active phonon modes in the monodomain state of tetragonal SrTiO3 (110)

With infrared and terahertz ellipsometry we investigated the anisotropy of the infrared active phonon modes in SrTiO3 (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at T* = 105 K. In particular, we show that the anisotropy of the oscillator strength of the so-called R-mode, which becomes weakly infrared active below T*, is a valuable indicator for the orientation of the structural domains. Our results reveal that a mono-domain state with the tetragonal axis (c-axis) parallel to the [001] direction can be created by applying a moderate uniaxial stress of about 2.3 MPa along the [1-10] direction (with a mechanical clamp). The resulting, intrinsic splitting of the infrared-active phonon modes is reported.

On the rich magnetic phase diagram of (Ni, Co)–Mn–Sn Heusler alloys

We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)–Mn–Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations.

Electric-field-induced tetragonal phase in [Pb(Mg1/3Nb2/3)O3]0.68–[PbTiO3]0.32 by Raman scattering

ABSTRACTThe paper describes investigation of the electric-field-induced phase transition in PMN-xPT single crystals with x = 0.32 by means of detailed polarized Raman scattering measurements. Field-induced tetragonal state was reached by applying an electric field up to 30 kV/cm along pseudo-cubic direction at a temperature slightly below that of the zero-field rhombohedral–tetragonal phase transition (TRT). In this field-induced tetragonal state, the angular dependencies of the polarized Raman spectra were recorded and compared with the similar data available for the rhombohedral single-domain and multidomain states.

Pressure-induced metallization and amorphization inVO2(A)nanorods

A metallic state enabled by the metal-insulator transition (MIT) in single crystal $\mathrm{V}{\mathrm{O}}_{2}(\mathrm{A})$ nanorods is demonstrated, which provides important physical foundation in experimental understanding of MIT in $\mathrm{V}{\mathrm{O}}_{2}$. The observed tetragonal metallic state at \ensuremath{\sim}28 GPa should be interpreted as a distinct metastable state, while increasing pressure to \ensuremath{\sim}32 GPa, it transforms into a metallic amorphous state completely. The metallization is due to V $3d$ orbital electrons delocalization, and the amorphization is attributed to the unique variation of V-O-V bond angle. A metallic amorphous $\mathrm{V}{\mathrm{O}}_{2}$ state is found under pressure, which is beneficial to explore the phase diagram of $\mathrm{V}{\mathrm{O}}_{2}$. Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials.

On the Theory of Tetragonality of Martensite Crystals Surrounded with Elastic Matrix

The paper is dedicated to the study of thermodynamic stability of tetragonal and cubic states of dilute Fe–C interstitial solid solutions. The combination of the thermodynamic theory and atomistic simulations results was used. This approach allowed us to analyze a widespread theory of carbon ordering in martensite crystal lattice enclosed in an elastic matrix developed by A.G. Khachaturyan. The key parameter of the theory is λ0, the strain interaction parameter. The value of λ0 calculated by A.G. Khachaturyan for a free martensite crystal (2.73 eV/atom ) yields the critical concentration of carbon ccrit=0,55 wt. % for room temperature. In fact, according to the experimental works this concentrations is close to 0.25 wt. %. A.G. Khachaturyan offered an improved theory of carbon ordering based on the assumption that decreasing the sizes of crystals along z axis will cause elastic resistance from surrounding crystals. The stresses arising when the martensite crystal is enclosed in an elastic matrix causes the appearance of a “tail” of order parameter at concentrations below the ccr for free crystal, which explained the discrepancy between Khachaturian’s theory and the experimental data. However our analysis shows the absence of this “tail” that means incorrect calculation of λ0 parameter. Over the last ten years the calculations of the strain interaction parameter λ0 have been made. The values of the parameter λ0 range from 5 to 10 eV/atom, in contrast to 2.73 eV/atom as defined by A.G. Khachaturyan. This fact has a great effect on the estimates of the critical carbon concentration at room temperature. This concentration becomes close to 0.2 wt. %, the value previously indicated by G.V. Kurdjumov. The reasons of abnormal tetragonality observed in the 0.2–0.6 wt. %C range are also considered.

Formation of the C-type Orbital-Ordered State in the Simple Perovskite Manganite Sr1-xSmxMnO3

ABSTRACTThe simple perovskite manganite Sr1-xSmxMnO3 (SSMO) has been reported to have a highly-correlated electronic system for eg-electrons in a Mn ion. According to the previous studies, the C-type orbital-ordered (COO) state with the I4/mcm symmetry was found to be formed from the disordered-cubic (DC) state on cooling. The feature of the COO state is that its crystal structure involves both the Jahn-Teller distortion to orbital ordering and the R25-type rotational displacement of oxygen octahedra. Because of the involvement of both the distortion and the displacement, their competition should be expected in the formation of the COO state. However, the detailed features of the competition have not been understood yet. Thus, the crystallographic features of the COO state in SSMO have been examined by x-ray powder diffraction and transmission electron microscopy. It was found that, when the Sm content increased from x = 0 at room temperature, the DC state changed into the COO state with the tetragonal symmetry around x = 0.13. The notable feature of the COO state is that the state is characterized by a nanometer-scaled banded structure consisting of an alternating array of two tetragonal bands. One tetragonal band consisted of the COO state involving both the Jahn-Teller distortion and the R25-type rotational displacement. But, there was only the latter displacement in the other, the state of which could be identified as a disordered tetragonal (DT) state. Based on this, it is understood that the COO-state formation from the DC state should take place via the appearance of the DT state, which may involve fluctuations of the C-type orbital ordering.

Reversible tuning of the collapsed tetragonal phase transition inCaFe2As2by separate control of chemical pressure and electron doping

Single crystals of Ca(Fe1-xRux)2As2 (0<x<0.065) and Ca1-yLay(Fe0.973Ru0.027)2As2 (0<y<0.2) have been synthesized and studied with respect to their structural, electronic and magnetic properties. The partial substitution of Fe by Ru induces a decrease of the c-axis constant leading for x<0.023 to a suppression of the coupled magnetic and structural (tetragonal to orthorhombic) transitions. At x_cr=0.023 a first order transition to a collapsed tetragonal (CT) phase is found, which behaves like a Fermi liquid and which is stabilized by further increase of x. The absence of superconductivity near x_cr is consistent with truly hydrostatic pressure experiments on undoped CaFe2As2. Starting in the CT regime at x=0.027 we investigate the additional effect of electron doping by partial replacement of Ca by La. Most remarkably, with increasing y the CT phase transition is destabilized and the system is tuned back into a tetragonal ground state at y>0.08. This effect is ascribed to a weakening of interlayer As-As bonds by electron doping. Upon further electron doping filamentary superconductivity with Tc of 41 K at y=0.2 is observed.

Spin reorientation driven by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides

In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.

Ordering of Carbon Atoms in Free Martensite Crystals and When Enclosed in Elastic Matrix

The thermodynamic stability of tetragonal and cubic states of dilute Fe-C interstitial solid solutions is studied combining results of atomistic simulations with the thermodynamic theory. This approach allowed us to analyze the widespread Khachaturyan’s theory of carbon ordering in martensite crystal lattice enclosed in an elastic matrix. The key parameter of the Khachaturyan’s theory is λ 0, the strain interaction parameter. The value of λ 0 calculated by A.G. Khachaturyan for a free martensite crystal (2.73 eV/atom) yields the critical concentration of carbon c cr = 0.55 wt pct for room temperature. In fact, this concentration is close to 0.25 wt pct. A.G. Khachaturyan offered an improved theory of carbon ordering based on the assumption that decreasing the sizes of crystals along z axis will cause elastic resistance from surrounding crystals. The stresses arising when the martensite crystal is enclosed in an elastic matrix cause the appearance of a “tail” of order parameter at concentrations below the c cr for free crystal, which explained the discrepancy between Khachaturian’s theory and the experimental data. Our analysis shows the absence of this “tail.” Over the last 10 years, the calculations of the strain interaction parameter λ 0 have been made. The values of the parameter λ 0 range from 5 to 10 eV/atom, in contrast to 2.73 eV/atom as defined by A.G. Khachaturyan. This fact has a great effect on the estimates of the critical carbon concentration at room temperature. This concentration becomes close to 0.2 wt pct, the value previously indicated by G.V. Kurdjumov. The reasons of abnormal tetragonality observed in the 0.2 to 0.6 wt pct C range are considered.

Electronic nematic correlations in the stress-free tetragonal state ofBaFe2−xNixAs2

We use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe$_{2-x}$Ni$_x$As$_2$ iron pnictides in the external stress free detwinned state. Using a specially designed in-situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial strained tetragonal state of BaFe$_{2-x}$Ni$_x$As$_2$ below a temperature $T^\ast$, previously identified as a signature of the electronic nematic phase, is also present in the stress free tetragonal phase below $T^{\ast\ast}$ ($<T^\ast$). By carrying out neutron scattering measurements on BaFe$_2$As$_2$ and BaFe$_{1.97}$Ni$_{0.03}$As$_2$, we argue that the resistivity anisotropy in the stress free tetragonal state of iron pnictides arises from the magnetoelastic coupling associated with antiferromagnetic order. These results thus indicate that the local lattice distortion and nematic spin correlations are responsible for the resistivity anisotropy in the tetragonal state of iron pnictides.

Muon spin rotation study of the magnetic structure in the tetragonal antiferromagnetic state of weakly underdoped Ba1−xKxFe2As2

With muon spin rotation (μSR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba1−xKxFe2As2 single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-Q magnetic structure with c-axis–oriented moments which has a vanishing magnetic moment on half of the Fe sites.

The UC2−x – Carbon eutectic: A laser heating study

The UC2−x – carbon eutectic has been studied by laser heating and fast multi-wavelength pyrometry under inert atmosphere.The study has been carried out on three compositions, two of which close to the phase boundary of the UC2−x – C miscibility gap (with C/U atomic ratios 2 and 2.1), and one, more crucial, with a large excess of carbon (C/U = 2.82). The first two compositions were synthesised by arc-melting. This synthesis method could not be applied to the last composition, which was therefore completed directly by laser irradiation. The U – C – O composition of the samples was checked by using a combustion method in an ELTRA® analyser. The eutectic temperature, established to be 2737 K ± 20 K, was used as a radiance reference together with the cubic – tetragonal (α → β) solid state transition, fixed at 2050 K ± 20 K. The normal spectral emissivity of the carbon-richer compounds increases up to 0.7, whereas the value 0.53 was established for pure hypostoichiometric uranium dicarbide at the limit of the eutectic region. This increase is analysed in the light of the demixing of excess carbon, and used for the determination of the liquidus temperature (3220 K ± 50 K for UC2.82). Due to fast solid state diffusion, also fostered by the cubic – tetragonal transition, no obvious signs of a lamellar eutectic structure could be observed after quenching to room temperature. The eutectic surface C/UC2−x composition could be qualitatively, but consistently, followed during the cooling process with the help of the recorded radiance spectra. Whereas the external liquid surface is almost entirely constituted by uranium dicarbide, it gets rapidly enriched in demixed carbon upon freezing. Demixed carbon seems to quickly migrate towards the inner bulk during further cooling. At the α → β transition, uranium dicarbide covers again the almost entire external surface.

Persistence of ferroelectricity above the Curie temperature at the surface ofPb(Zn1/3Nb2/3)O3−12%PbTiO3

Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer (``skin'') that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at ${T}_{\mathrm{c}}=200{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of $\mathrm{N}{\mathrm{b}}^{5+}$---a small ion with ${d}^{0}$ orbital occupancy---stabilizes the ferroelectricity of the skin layer.

Temperature dependent properties and poling effect of K4CuNb8O23 modified (Na0.5K0.5)NbO3 lead free piezoceramics

Lead free piezoelectric ceramics (Na0.5K0.5)NbO3 modified by 4% mol. K4CuNb8O23 (abbreviated NKN:4KCN hereinafter) contain moderate piezoelectric constant d33 ∼ 100 pC N−1 and large mechanical quality factor Qm &amp;gt; 1000, showing possible replacement of the lead-based ones (Chen et al., J Appl. Phys. 102, 104109 (2007)). In terms of practical use, however, the temperature stability of NKN:4KCN is not clear to date. We made a systematic investigation on the properties versus temperature of NKN:4KCN to evaluate whether it can be practically used. In the range from room temperature (RT ∼ 25 °C) to 100 °C, the ferroelectricity of poled NKN:4KCN material is nearly temperature independent, remanent polarization Pr is about 27.6 ±1 μC cm−2. When the as-studied NKN:4KCN ceramics were thermal depolarized in temperature range from RT to 450 °C, piezoelectric constant d33 changed little, retaining about 99 pC N−1, 77 ± 3 pC N−1, from RT to 150 °C, 200 °C to 350 °C, respectively. The poled NKN:4KCN material showed higher orthorhombic to tetragonal phase transition temperature (TO−T ∼ 200 °C) compared to unpoled sample (TO−T ∼ 194 °C). Moreover, this kind of lead free material displayed negative temperature coefficient of frequency (TCF) and positive TCF in orthorhombic and tetragonal phase state, respectively. The TCF was about −360 ppm K−1 in the range from RT to 125 °C, close to some lead-based commercial ones. The significance of this work lies in evaluating whether such a material can be practically used or not. We believe such a material might be the most promising candidate for replacing lead-based ones in some areas in the future.

Dependence of Semiconductor to Metal Transition of VO2(011)/NiO{100}/MgO{100}/TiN{100}/Si{100} Heterostructures on Thin Film Epitaxy and Nature of Strain

We have studied semiconductor to metal transition (SMT) characteristics of VO2(011) thin films integrated epitaxially with Si(100) through NiO{001}/MgO{001}/TiN{001} buffer layers and correlated with the details of epitaxy and nature of residual stresses and strains across the VO2/NiO interface. Thin film epitaxy at both room and elevated temperatures is studied in detail by electron microscopy and in situ high‐temperature X‐ray diffraction techniques. The epitaxial relationship across the interface between monoclinic VO2 and NiO is determined to be (011)VO2||{100}NiO and [01]VO2||[001]NiO at room temperature. The epitaxial alignment at the temperature of growth where tetragonal VO2 is stable is determined as: (110)VO2||{100}NiO and [001]VO2||[100]NiO. A cube‐on‐cube crystallographic alignment is established across the NiO{100}/MgO{100}/TiN{100}/Si{100} interfaces. The misfit strains across the VO2/NiO interface at the growth temperature are calculated and the mechanism of strain relaxation is discussed. The out‐of‐plane orientation is found to be relaxed in both monoclinic and tetragonal states of VO2. It is shown that a compressive strain of 31.65% along the [001] direction of tetragonal VO2 is fully relaxed via matching of multiple domains. However, a small tensile misfit strain of about 5% along [10] direction cannot relax and remains in the lattice. This tensile residual strain leads to a compressive strain along [001] axis which, in turn, results in an SMT temperature slightly lower than that of freestanding strain‐free VO2. SMT characteristics of VO2(011) epilayers are assessed where an amplitude of near five orders of magnitude, and a hysteresis of less than 3.6 °C are obtained. This study introduces VO2/NiO thin film heterostructure integrated with silicon as a promising candidate for multifunctional devices with novel characteristics where a combination of sensing, manipulation, and response functions is needed.

Lasing behaviors upon phase transition in solution-processed perovskite thin films

In this paper, the temperature dependent lasing characteristics of solution-processed organic-inorganic halide perovskite CH3NH3PbI3 films have been demonstrated. The lasing temperature can be sustained up to a near room temperature at 260 K. Via the temperature dependent photoluminescence (PL) measurements, an emerged phase-transition band can be observed, ascribing to the crystalline structures changed from the orthorhombic to tetragonal phase states in the perovskites as a function of a gradual increase in the ambient temperature. The optical characteristics of the PL emission peaks and the anomalous shifts of the peak intensities are highly correspondent with the phase states in perovskites at different temperatures, showing a low-threshold lasing behavior at the phase transition. The laser cavities may be formed under multiple random scattering provided by the polycrystalline grain boundary and/or phase separation upon the phase transition. Since the threshold gain is potentially high in the random cavities, the large material gain exhibited by the solution-processed perovskite would be very promising in making practical laser devices.

Creating multiferroics with large tunable electrical polarization from paraelectric rare-earth orthoferrites

The quest for materials possessing both a magnetic ordering temperature above room temperature and a large electrical polarization is an important research direction in order to design novel spintronic and memory devices. Up to now, BiFeO3 and related systems are the only known compounds simultaneously possessing such characteristics. Here, first-principles calculations predict that another family of materials, namely epitaxial films made of rare-earth orthoferrites (RFeO3), can also exhibit such desired features. As a matter of fact, applying a large enough strain to these compounds, which are nominally paraelectric and have a high magnetic transition temperature, is predicted to render them ferroelectric, and thus multiferroic. At high compressive strain, the resulting ferroelectric phase of RFeO3 systems having large rare-earth ions is even a tetragonal state characterized by a giant polarization and axial ratio. For large tensile strain, two striking inhomogenous ferroelectric phases—including one never observed before in any perovskite—are further predicted as having significant polarization. A multiphase boundary also occurs, which may lead to optimization of properties or unusual features. Finally, many quantities, including electrical polarization and magnetic ordering temperature, are tunable by varying the epitaxial strain and/or chemical pressure.

Comment on: “Structural, elastic, and thermodynamic properties under pressure of the FeC in the martensitic phase: an ab-initio study”

Recently reported [Chihi T, Bouhemadou A, Bin-Omran S. High Press Res. 2013;33:572] results of density-functional theory calculations for a “martensitic” FeC with allegedly tetragonal body-centered ground state structure are shown to be compatible with a cubic rocksalt structure of that FeC.