Orthorhombic State Research Articles

Low-temperature magnetism of Ln2Ba (Ln = Nd, Er, Ho) furoate-based polymeric chains: Slow relaxation, magnetic anisotropy and interactions

The synthesis, structure and magnetothermal properties of three new lanthanide 1D polymeric complexes, {[Ln2Ba(α-fur)8(H2O)4]}n for Ln = Nd (1), Er (2) and {[Ho2Ba(α-fur)8(H2O)4]⋅2H2O}n (3), based on carboxylic α-fur = C4H3OCOO ligands is reported. The α-furoate ligands consolidate zigzag chains formed by Ln2 dimers separated by Ba ions. Ab initio calculations, in combination with the fit of experimental static magnetization and heat capacity, predict the magnetic ground state, energy levels and magnetic interactions in these heteronuclear nanomagnets. Nd2Ba (1) presents two different coordination sites for Nd, with an orthorhombic magnetic ground state. Nd ions are coupled along the chain through a weak antiferromagnetic (AF) interaction J″/kB = −0.08 K. Er(III) ions in Er2Ba (2) present a highly axial ground state, forming magnetic dimers with an interaction of J′/kB = −8.6 K, while interdimer coupling along the chain is J″/kB = −0.28 K. The Ho2Ba (3) complex consists of a highly anisotropic quasi-doublet with a ΔHo/kB = 0.7 K gap. Non-Kramers Ho ions form magnetic dimers within the Ho2Ba cluster, coupled by an AF intradimer interaction J′/kB = −2.5 K. The three complexes exhibit in-field slow relaxation of the magnetization: 1 relaxes through an Orbach process at high temperatures [Ueff/kB = 60(1) K] evolving to quantum tunneling below 3 K [τQT = 0.05(1) s]; 2 exhibits a rapid Orbach-like process [τ0 = 8(6)⋅10–8 s and Ueff/kB = 10(2) K] and 3 shows a direct process [τ = 0.4(1) s].

Magnetic-field-induced transitions and the inverse magnetocaloric effect at liquid helium temperatures in the Mn1-xCoxNiGe system (0.15 ≤ x < 0.80)

Magnetic field-induced phase transitions found in the Mn1-xCoxNiGe system (0.15 ≤ x < 0.8) at T = 5 K may differ in nature and magnitude of the saturation magnetization of the initial weakly magnetic phase. These differences also affect the phenomena accompanying the transitions, specifically, the inverse magnetocaloric effect (MCE). In the current work, the mechanisms of these phenomena are analyzed within the framework of the magnetostructural model, which takes into account the difference in the limiting values of the magnetic order parameters in competing orthorhombic and hexagonal crystal structures. As follows from the analysis performed, the features observed in Mn1-xCoxNiGe can be explained by the realization at low temperatures of a metastable orthorhombic crystal state in magnetically ordered phases with a stable high-temperature hexagonal crystal structure.

Pressure-induced phase transitions of CsSnBr3 perovskite from first-principles calculations

High-pressure structure transition of nontoxic all-inorganic MHP CsSnBr3 was fully explored up to 15 GPa using an advanced structure search technique CALYPSO combined with first-principles calculations. Besides the known orthorhombic Pnma ground state phase, two high-pressure semiconducting Cmcm and P21/m phases of CsSnBr3 were firstly uncovered above 2.37 and 6.8 GPa, respectively. Both phase transitions of the Pnma → Cmcm at 2.37 GPa and Cmcm → P21/m at 6.8 GPa were characterized as first order with a volume reduction of 4.7% and 10.8%. The occurrences of high-pressure Cmcm and P21/m phases follow the enhanced distortions of Sn–Br polyhedrons and increased coordination of Sn atoms from 6 to 8 at elevated pressures. Compared to the direct band gap of the ambient-pressure Pnma phase, the Cmcm and P21/m phases exhibit a larger indirect band gap of 2.347 and 3.143 eV, respectively, originating from the movement away from the Fermi level of conduction bands driven by the twisting of Sn–Br polyhedrons under pressure. The light absorption performances of two high-pressure phases in comparison with the Pnma phase were studied by the calculated optical absorption coefficients.

Annealing-Dependent Morphotropic Phase Boundary in the BiMg0.5Ti0.5O3-BiZn0.5Ti0.5O3 Perovskite System.

The annealing behavior of (1-x)BiMg0.5Ti0.5O3–xBiZn0.5Ti0.5O3 [(1-x)BMT–xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT–BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT–BZT system promising for the development of environmentally friendly piezoelectric ceramics.

Two-dimensional ferroelectricity induced by octahedral rotation distortion in perovskite oxides

Two-dimensional (2D) ferroelectricity has attracted extensive attention since its discovery in the monolayers of van der Waals materials. Here we show that 2D ferroelectricity induced by octahedral rotation distortion is widely present in the perovskite bilayer system through first-principles calculations. The perovskite tolerance factor plays a crucial role in the lattice dynamics and ground-state structure of the perovskite monolayers and bilayers, thus providing an important indicator for screening this hybrid improper ferroelectricity. Generally, the ferroelectric switching via an orthorhombic twin state has the lowest energy barrier. Epitaxial strain can effectively tune the ferroelectric polarization and ferroelectric switching by changing the amplitude of octahedral rotation and tilt distortion. The increasing compressive strain causes a polar to nonpolar phase transition by suppressing the tilt distortion. The cooperative effect of octahedral distortion at the interface with the substrate can reduce the energy barrier of the reversing rotation mode and can even change the lowest-energy ferroelectric switching path.

Spin-lattice coupling in an epitaxial NdFeO3 thin film

Rare-earth orthoferrite RFeO3 materials such as NdFeO3 are strongly studied because of their fascinating magnetic properties and their potential applications. Here, we show the successful epitaxial synthesis of parasitic-free NFO thin film by pulsed laser deposition on (001)-SrTiO3. High-resolution X-ray diffraction shows a coherent growth and a tetragonal-like structure of a tensile strained 80 nm thick NFO film in contrast with the bulk orthorhombic state. Room temperature magnetometry indicates a bulk-like antiferromagnetic state for the NFO film. Temperature-dependent X-ray diffraction and magnetometry highlight a significant spin-lattice coupling at the Néel Temperature while a new magneto-structural instability is discovered around 250 °C that needs further investigation.

Ab initio thermochemistry study of polymorphism in the Si2N2(NH) analog of Si2N2O

We present a systematic density functional theory (DFT) study of the structural, bonding and electronic properties of the Si2N2(NH) polymorphs. This recently synthesized compound is a direct analog of Si2N2O in which the oxygen is replaced by an NH group. We show that a local-density approximation (LDA) description in the static lattice approximation gives a good account of the lowest energy orthorhombic phase of both the parent oxide and imide analog. In this work we extend our systematic comparisons between Si2N2O and Si2N2(NH) to denser tetragonal and monoclinic polymorphs. For Si2N2(NH) we find that the tetragonal and monoclinic polymorphs have energies 0.8 and 1.8 eV higher than (respectively) that of the orthorhombic ground state, similar to their Si2N2O counterparts (0.6 and 1.4 eV, respectively). We elucidate differences in chemical binding in Si2N2O and Si2N2(NH) polymorphs using Bader charge analysis and conclude that the imide systems possess slightly less ionic character than their oxide counterpart. Using the modified Becke-Johnson (MBJ) meta-LDA approach we find that orthorhombic Si2N2O and Si2N2NH have direct gaps of 5.65 and 5.75 eV, respectively, in excellent agreement with experiment. Relative to the orthorhombic Si2N2O (5.65 eV, direct), the band gaps in the denser polymorphs increase and become indirect (tetragonal: 5.72 eV, monoclinic: 6.06 eV). We find more complex trends in Si2N2NH in which the band gaps are also indirect in character, but decrease to 3.91 eV in the tetragonal phase, and then increase to 4.18 eV (Γ → D) in the monoclinic case.

Properties of (001) NaNbO3 films under epitaxial strain: A first-principles study

First-principles calculations are performed to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely, $\mathrm{Na}\mathrm{Nb}{\mathrm{O}}_{3}$ (NNO), and are subject to epitaxial strain. In particular, an energy-versus-misfit strain phase diagram is constructed and reveals the existence of three different ground states for different strain regimes. For large compressive strain and up to moderate tensile strain, a monoclinic $Cc$ phase occurred, with its polarization and axis of antiphase tilting both rotating within a ($\overline{1}10$) plane with the magnitude of the strain. For large tensile strain, a ferroelectric orthorhombic state of $Pmc{2}_{1}$ symmetry emerges with polarization lying along the [110] in-plane direction together with an octahedral tilting adopting the ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$ pattern and an antiferroelectric vector associated with the reciprocal zone-border $X$ point. Finally, in between and for a narrow region of strain, a complex ground state is found. It has orthorhombic $Pca{2}_{1}$ symmetry, for which a complex tilting pattern coexists with polarization pointing along the [001] out-of-plane direction and antiferroelectric displacement associated with the $\mathrm{\ensuremath{\Delta}} k$ point located halfway between the zone-center $\mathrm{\ensuremath{\Gamma}}$ and zone-border $X$ point. Ferroelectric, antiferroelectric, and antiferrodistortive properties are also reported and discussed as a function of misfit strain.

The Sphingosine and Acyl Chains of Ceramide [NS] Show Very Different Structure and Dynamics That Challenge Our Understanding of the Skin Barrier.

The lipid phase of the uppermost human skin layer is thought to comprise highly rigid lipids in an orthorhombic phase state to protect the body against the environment. By synthesizing sphingosine‐d28 deuterated N‐lignoceroyl‐d‐erythro‐sphingosine (ceramide [NS]), we compare the structure and dynamics of both chains of that lipid in biologically relevant mixtures using X‐ray diffraction, 2H NMR analysis, and infrared spectroscopy. Our results reveal a substantial fraction of sphingosine chains in a fluid and dynamic phase state at physiological temperature. These findings prompt revision of our current understanding of the skin lipid barrier, where an extended ceramide [NS] conformation is preferred and a possible domain structure is proposed. Mobile lipid chains may be crucial for skin elasticity and the translocation of physiologically important molecules.

Determination of twinning path from broken symmetry: A revisit to deformation twinning in bcc metals

Deformation twinning in crystals is one of the major strain carrier during plastic deformation, which plays a critical role in determining the mechanical properties of metals and alloys. One of the key issues to understand deformation twinning mechanisms is the determination of deformation path, which is critical for the calculations of twinning strain and critical shear stress. This work provides a new perspective on deformation twinning by systematically investigating the relationship between symmetry breaking and deformation path, with the purpose of establishing a theoretical foundation to identify the broken symmetries associated with twinning and predicting the twinning modes using group theory and graph theory. From a physical point of view, deformation twins can be regarded as one type of the so-called topological defects that are induced by symmetry-breaking. Taking BCC crystals as an example, we demonstrate how the presence of intermediate high symmetry states in the deformation strain space (strain calculated through lattice distortion) along the twinning path, such as FCC, HCP or orthorhombic state, can lead to different characteristic twinning modes. Our predictions not only agree well with classical theoretical analyses and experimental observations in BCC metals and alloys, but also reveal the origin of recently observed high-index twinning modes (such as {5 8 11} and {3 9 10} twins). This work may open a new avenue for analyzing deformation twinning through the symmetry breaking along twinning pathways.

Macroscopic and Microscopic Structures of Cesium Lead Iodide Perovskite from Atomistic Simulations

AbstractA first‐principles‐based effective Hamiltonian is developed and employed to investigate finite‐temperature structural properties of a prototype of perovskite halides, that is CsPbI3. Such simulations, when using first‐principles‐extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic upon heating, unlike measurements that reported the occurrence of an intermediate long‐range‐tilted tetragonal P4/mbm phase in‐between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first‐principle methods can be trusted to mimic hybrid perovskites, can be resolved by “only” changing one short‐range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic phase is highly locally distorted and develops strong transverse antiphase correlation between first‐nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling.

Room temperature phase superposition as origin of enhanced functional properties in BaTiO3 - based ceramics

BaSnxTi1-xO3 (x = 0, 0.05) ceramics with orthorhombic/tetragonal phases at room temperature were comparatively investigated to understand the role of phase composition on their functional properties. With respect to the values of BaTiO3, the switching polarization, permittivity peak, tunability and piezoelectric coefficients are enhanced by doping with 5 % Sn onto Ti4+ sites. The orthorhombic polymorph amount is larger in the doped ceramic and explains its higher switching polarization. High field poling favors the orthorhombic phase in both compositions; this polymorph becomes predominant in the BaSn0.05Ti0.95O3 ceramic. Landau-based calculations developed for ceramics with randomly oriented grains predicted the stability of variable amounts of orthorhombic/tetragonal phases around room temperature and explain the field-induced predominant orthorhombic state, mostly in BaSn0.05Ti0.95O3. Due to the twelve allowed spontaneous polarization directions, the orthorhombic state is responsible for the enhanced polarization, tunability and piezoelectric properties with respect to the tetragonal state with six polarization possible orientations.

Synthesis of La0.5Ca0.5−x□xMnO3 nanocrystalline manganites by sucrose assisted auto combustion route and study of their structural, magnetic and magnetocaloric properties

Perovskite manganite La0.5Ca0.5-x xMnO3 (LCMO) nanomaterials were elaborated using the sucrose modified auto combustion method. Rietveld refinements of the X-ray diffraction patterns of the crystalline structure confirm a single-phase orthorhombic state with Pbnm space group (No. 62). The Ca-vacancies were voluntarily created in the LCMO structure in order to study their influence on the magnetic behaviour in the system. The magnetic susceptibility was found to be highly enhanced in the sample with Ca-vacancies. Paramagnetic-to-ferromagnetic phase transition was evidenced in both samples around 254 K. This transition is, characterized by a drastic jump of the susceptibility in the sample with Ca-vacancies. The maximum of entropy change, observed for both compounds at magnetic field of 6T was 2.30 J kg-1K-1 and 2.70 J kg-1K-1 for the parent compound and the lacunar one respectively. The magnetocaloric adiabatic temperature change value calculated by indirect method was 5.6 K and 5.2 K for the non-lacunar and Ca-vacancy compound, respectively. The Ca-lacunar La0.5Ca0.5-x xMnO3 (x=0.05) reported in this work demonstrated overall enhancement of the magnetocaloric effect over the LCMO. The technique used to elaborate LCMO materials was beneficial to enhance the magnetocaloric effect and magnetic behaviour. Therefore, we conclude that this less costly environmentally friendly system can be considered as more advantageous candidate for magnetic refrigeration applications then the commonly Gd-based compounds.

Distinct pressure evolution of coupled nematic and magnetic orders in FeSe

FeSe, despite being the structurally simplest compound in the family of iron-based superconductors, shows an astoundingly rich interplay of physical phenomena including nematicity and pressure-induced magnetism. Here, we present a microscopic study of these two phenomena by high-energy x-ray diffraction and time-domain M\"ossbauer spectroscopy on FeSe single crystals over a wide temperature and pressure range. The topology of the pressure-temperature phase diagram is a surprisingly close parallel to the well-known doping-temperature phase diagram of BaFe2As2 generated through partial Fe/Co and Ba/Na substitution. In FeSe with pressure p as a control parameter, the magneto-structural ground state can be tuned from "pure" nematic - paramagnetic with an orthorhombic lattice distortion - through a strongly coupled magnetically ordered and orthorhombic state to a magnetically ordered state without an orthorhombic lattice distortion. The magnetic hyperfine field increases monotonically over a wide pressure range. However, the orthorhombic distortion initially decreases under increasing pressure, but is stabilized by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature Tc (occuring at p = 6.8 GPa), the orthorhombic distortion suddenly disappears and FeSe remains tetragonal down to the lowest temperature measured. Analysis of the structural and magnetic order parameters suggests an independent origin of the structural and magnetic ordering phenomena, and their cooperative coupling leads to the similarity with the canonical phase diagram of iron pnictides.

Quantum-fluctuation-stabilized orthorhombic ferroelectric ground state in lead-free piezoelectric (Ba,Ca)(Zr,Ti)O3

We numerically investigate the phase diagram of the giant-piezoelectric (1−x) Ba(Zr0.2Ti0.8) O3−x (Ba0.7Ca0.3) TiO3 system, treating the ions either as classical objects (via classical Monte Carlo or CMC simulations) or quantum mechanically (via path-integral quantum Monte Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram, but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that “only” yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.

Presence and persistence of a highly ordered lipid phase state in the avian stratum corneum.

To survive high temperatures in a terrestrial environment, animals must effectively balance evaporative heat loss and water conservation. In passerine birds, cutaneous water loss (CWL) is the primary avenue of water loss at thermoneutral temperatures and increases slightly as ambient temperature increases, indicating a change in the permeability of the skin. In the stratum corneum (SC), the outermost layer of the skin, lipids arranged in layers called lamellae serve as the primary barrier to CWL in birds. The permeability of these lamellae depends in large part on the ability of lipid molecules to pack closely together in an ordered orthorhombic phase state. However, as temperature increases, lipids of the SC become more disordered, and may pack in more permeable hexagonal or liquid crystalline phase states. In this study, we used Fourier transform infrared spectroscopy to monitor the phase state of lipids in the SC of house sparrows (Passer domesticus) at skin temperatures ranging from 25 to 50°C. As temperature increased, lipids became slightly more disordered, but remained predominantly in the orthorhombic phase, consistent with the small increase in CWL observed in house sparrows as ambient temperature increases. These results differ considerably from studies on mammalian SC, which find a predominantly hexagonal arrangement of lipids at temperatures above 37°C, and the increased order in avian SC may be explained by longer lipid chain length, scarcity of cholesterol and the presence of cerebrosides. Our results lend further insight into the arrangement and packing of individual lipid molecules in avian SC.

Pressure-Induced Ferromagnetism due to an Anisotropic Electronic Topological Transition in Fe_{1.08}Te.

A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. We show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in Fe_{1.08}Te. Around 2GPa, the orthorhombic and incommensurate antiferromagnetic ground state of Fe_{1.08}Te is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a "type-0" transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of Fe_{1.08}Te is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.

Optical anisotropy in the electronic nematic phase of FeSe

At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at ${T}_{s}\ensuremath{\simeq}90$ K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for $T\ensuremath{\le}{T}_{s}$, which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity.

Comparison of iron-related recombination centers in boron, gallium, and indium doped silicon analyzed by defect parameter contour mapping

In this work, we are showing that iron (Fe) related defects in mono-silicon have very different recombination characteristics depending on the doping element employed. While the defect characteristics of the Fe in its dissociated state is comparably the same in the materials of investigation, the defect characteristics of the associated state vary considerably. By using, defect parameter contour mapping (DPCM), a newly developed method for analyzing temperature and injection dependent lifetime data, we have for the first time, been able to show that in the case of gallium doping it is the orthorhombic state of the Fe-acceptor complex that is dominating the lifetime.

Microscopic origins of the large piezoelectricity of leadfree (Ba,Ca)(Zr,Ti)O3

In light of directives around the world to eliminate toxic materials in various technologies, finding lead-free materials with high piezoelectric responses constitutes an important current scientific goal. As such, the recent discovery of a large electromechanical conversion near room temperature in (1−x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 compounds has directed attention to understanding its origin. Here, we report the development of a large-scale atomistic scheme providing a microscopic insight into this technologically promising material. We find that its high piezoelectricity originates from the existence of large fluctuations of polarization in the orthorhombic state arising from the combination of a flat free-energy landscape, a fragmented local structure, and the narrow temperature window around room temperature at which this orthorhombic phase is the equilibrium state. In addition to deepening the current knowledge on piezoelectricity, these findings have the potential to guide the design of other lead-free materials with large electromechanical responses.