Tetragonal State Research Articles

Probing the Mott insulating behavior of Ba2MgReO6 with DFT+DMFT

We investigate the interplay of spin-orbit coupling, electronic correlations, and lattice distortions in the 5d1 double perovskite Ba2MgReO6. Combining density-functional theory (DFT) and dynamical mean-field theory (DMFT), we establish the Mott insulating character of Ba2MgReO6 in both its cubic and tetragonal paramagnetic phases. Despite substantial spin-orbit coupling, its impact on the formation of the insulating state is minimal, consistent with theoretical expectations for d1 systems. We further characterize the electronic properties of the cubic and tetragonal phases by analyzing spectral functions and local occupations in terms of multipole moments centered on the Re sites. Our results confirm the presence of ferroically ordered z2 quadrupoles in addition to the antiferroic x2−y2-type order. We compare two equivalent but complementary descriptions in terms of either effective Re-t2g frontier orbitals or more localized atomiclike Re-d and O-p orbitals. The former maps directly on a physically intuitive picture in terms of nominal d1 Re cations, while the latter explicitly demonstrates the role of hybridization with the ligands in the spin-orbit splitting and the formation of the charge quadrupoles around the Re sites. Finally, we compare our DFT+DMFT results with a previous DFT+U study of the tetragonal paramagnetic state. We find good qualitative agreement for the dominant charge quadrupoles, but also notable differences in the corresponding spectral functions, underscoring the need for more comparative studies between these two methods. Published by the American Physical Society 2024

Pressure-induced morphotropic phase boundary and critical points in Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 at room temperature

We find rhombohedral-tetragonal morphotropic phase boundary showing high piezoelectricity in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 subjected to hydrostatic pressure based on a Landau-Ginzburg-Devonshire phenomenological modeling. Room-temperature Curie transitions driven by moderate pressures were investigated by examining evolution of free energy landscape and equations for determining critical pressures are derived. Monoclinic state was induced when applying [111]-oriented electric field to originally metastable rhombohedral state coexisting with the pressure-stabilized tetragonal state within a narrow pressure window. High piezoelectricity found at the morphotropic phase boundary can be further enhanced under electric field.

Strain and electric field induced low-symmetry states in Pb0.1Sr0.9TiO3

Polarization rotation and occurrence of monoclinic and triclinic states were investigated in Pb0.1Sr0.9TiO3. For this composition, the rhombohedral ground state may coexist with a tetragonal state close in free energy and become sensitive to electric and mechanical stimuli. The polarization path under an electric field and biaxial strain involves a monoclinic state, while for uniaxial strain the transition involves both monoclinic and triclinic states. Longitudinal and shear piezoelectric constants are enhanced in the induced monoclinic and tetragonal states, respectively. Low-temperature secondary peaks in the literature-reported dielectric spectra can be explained as polycrystalline average of dielectric tensor components of the low-symmetry structures.

Quantification of polarization distribution from polarizing light microscopy images of ferroelectric single crystals

Qualification of polarization can be realized either on a macroscopic scale as an average property by P-E hysteresis measurements or on a nano/micro scale by piezoelectric force microscopy, transmission electron microscopy, scanning electron microscopy, and so on. However, visualization and qualification of polarization distribution in the micron to millimeter scale is still a challenge. Polarizing light microscopy (PLM) is often used in the study of ferroelectric domain structures mainly for domain patterns. A phenomenon called “chromatic polarization” has been observed in transparent ferroelectric crystals by using a crossed-PLM system viewed with white light, which contains rich information about local polarization distribution. In this study, an automatic full-angle light intensity detection (AFALID) algorithm combined with colorimetry is developed to analyze the distribution of nonuniform local spontaneous polarization distribution in transparent ferroelectric single crystals. Temperature-dependent spontaneous polarizations from the color analysis for PMN-0.36PT single crystals with single tetragonal domain state are in good coincidence with those extracted from temperature-dependent hysteresis loops and pyroelectric current measurements. We further apply this method to quantify the nonuniform domain distributions with nano-indentations. This non-contact and non-destructive characterization can provide fast and automatic detection of polarization distributions in ferroelectric materials.

Phonon softening and slowing-down of charge density wave fluctuations in BaNi2As2

BaNi$_2$As$_2$ is a nonmagnetic analogue of the iron pnictide superconductors, and in its tetragonal state exhibits an incommensurate charge-density-wave (IC-CDW) and a sizable elastoresistance. In this work, phonons in BaNi$_2$As$_2$ associated with the IC-CDW and uniform in-plane lattice distortions are investigated using high-resolution inelastic X-ray scattering. The in-plane transverse acoustic phonons reveal no softening at temperatures where the elastoresistance increases strongly, indicating the latter to be electronically driven. Systematic phonon measurements reveal the IC-CDW occurs in two stages upon cooling: underdamped phonons first soften to zero energy well above the IC-CDW ordering temperature, then the resulting quasielastic IC-CDW fluctuations gradually slow down and coalesce into the static IC-CDW order. A possible origin for our observations is the IC-CDW in tetragonal BaNi$_2$As$_2$ being uniaxial, which provides an additional Ising degree of freedom favorable for disordered IC-CDW modulations, and accounts for the elastoresistance through a weak coupling to the lattice.

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.

Absence of precursor incommensurate charge order in electronic nematic Ba0.35Sr0.65Ni2As2

Recent discoveries of charge order and electronic nematic order in the iron-based superconductors and cuprates have pointed towards the possibility of nematic and charge fluctuations playing a role in the enhancement of superconductivity. The Ba1-xSrxNi2As2 system, closely related in structure to the BaFe2As2 system, has recently been shown to exhibit both types of ordering without the presence of any magnetic order. We report single crystal X-ray diffraction, resistance transport measurements, and magnetization of \BaSrLate, providing evidence that the previously reported incommensurate charge order with wavevector $(0,0.28,0)_{tet}$ in the tetragonal state of \BaNi~vanishes by this concentration of Sr substitution together with nematic order. Our measurements suggest that the nematic and incommensurate charge orders are closely tied in the tetragonal state, and show that the $(0,0.33,0)_{tri}$ charge ordering in the triclinic phase of BaNi2As2 evolves to become $(0,0.5,0)_{tri}$ charge ordering at $x$=0.65 before vanishing at $x$=0.71.

Photonic bandgap engineering in (VO2) n /(WSe2) n photonic superlattice for versatile near- and mid-infrared phase transition applications

The application of the photonic superlattice in advanced photonics has become a demanding field, especially for two-dimensional and strongly correlated oxides. Because it experiences an abrupt metal–insulator transition near ambient temperature, where the electrical resistivity varies by orders of magnitude, vanadium oxide (VO2) shows potential as a building block for infrared switching and sensing devices. We reported a first principle study of superlattice structures of VO2 as a strongly correlated phase transition material and tungsten diselenide (WSe2) as a two-dimensional transition metal dichalcogenide layer. Based on first-principles calculations, we exploit the effect of semiconductor monoclinic and metallic tetragonal state of VO2 with WSe2 in a photonic superlattices structure through the near and mid-infrared (NIR–MIR) thermochromic phase transition regions. By increasing the thickness of the VO2 layer, the photonic bandgap (PhB) gets red-shifted. We observed linear dependence of the PhB width on the VO2 thickness. For the monoclinic case of VO2, the number of the forbidden bands increase with the number of layers of WSe2. New forbidden gaps are preferred to appear at a slight angle of incidence, and the wider one can predominate at larger angles. We presented an efficient way to control the flow of the NIR–MIR in both summer and winter environments for phase transition and photonic thermochromic applications. This study’s findings may help understand vanadium oxide’s role in tunable photonic superlattice for infrared switchable devices and optical filters.

FeSe and the Missing Electron Pocket Problem

The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating thedxyorbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.

Unraveling peculiar magnetism and band topology in Mn3Sb

Magnetic, pseudogap, topological, magnetostructural, and elastic behaviors of Mn3Sb have been unraveled. The ferrimagnetism (FIM) is described by localized and delocalized electron magnetism resulting in different magnetic moments on Mn atoms, confirming the neutron diffraction data. The identified magnetostructural properties are due to the non-equivalent Mn atoms in its lowest symmetry structure. The electronic structure is also unique due to variable valance states of Mn atoms. The magnetic moment (4.10 μB) of non-equivalent Mn1 atom is antiparallely aligned with the magnetic moments (2.34 μB) of Mn2 and Mn3 atoms. The estimated Curie temperature, TC, is higher than the room temperature, which may have above the room temperature applications in spintronic devices. The band structure and density of states (DOS) show the characteristics of band topology (opening of a gap in Dirac-like band features) and pseudo-gap, respectively, around the Fermi level. While expanding the unit cell, the tetragonal FIM ground state transforms to the cubic primitive FIM phase, however, the contraction transforms it to the L12 ferromagnetic (FM) phase. The estimated elastic constants, bulk to shear modulus ratio, and elastic anisotropy factor indicate that Mn3Sb exhibits mechanically stable, ductile, and anisotropic behaviors, respectively.

A Janovec‐Kay‐Dunn‐Like Behavior at Thickness Scaling in Ultra‐Thin Antiferroelectric ZrO2 Films

AbstractOriginally based on phenomenological observations, the Janovec–Kay–Dunn (JKD) scaling law has been historically used to describe the dependence of the ferroelectric coercive fields (Ec) on a critical length scale of the material, wherein the film thickness (t) is considered the length scale, and Ec ∝ t−2/3. Here, for the first time, a JKD‐type scaling behavior is reported in an antiferroelectric material, using the ultra‐thin films of prototypical flourite‐structure binary oxide, zirconia. In these films, a decrease in the ZrO2 layer thickness from 20 nm to 5.4 nm leads to an increase in critical fields for both nonpolar‐to‐polar (Ea), and polar‐to‐nonpolar (Ef) transitions, accompanied by a decrease in the average crystallite size, and an increase in the tetragonal distortion of the non‐polar P42/nmc ground state structure. Notably, the ‐2/3 power law as in the JKD law holds when average crystallite size (d), measured from glancing‐incident X‐ray diffraction, is considered as the critical length scale—i.e., Ea, Ef ∝ d−2/3. First principles calculations suggest that the increase of tetragonality in thinner films contributes to an increase of the energy barrier for the transition from the non‐polar tetragonal ground state to the field‐induced polar orthorhombic phase, and in turn, an increase in Ea critical fields. These results suggest a de‐stabilization of the ferroelectric phase with a decreasing thickness in antiferroelectric ZrO2, which is contrary to the observations in its fluorite‐structure ferroelectric counterparts. With the recent interests in utilizing antiferroelectricity for advanced semiconductor applications, our fundamental exposition of the thickness dependence of functional responses therein can accelerate the development of miniaturized, antiferroelectric electronic memory elements for the complementary metal‐oxide‐semiconductor based high‐volume manufacturing platforms.

Structural behaviour of BiFeO3/SrRuO3 superlattices: An X-ray diffraction and Raman spectroscopy investigation

Epitaxial BiFeO 3 /SrRuO 3 superlattices have been grown by pulsed laser deposition on a (001) oriented LaAlO 3 substrate and probed by X-ray diffraction and Raman spectroscopy . To investigate the structural competition between rhombohedral BiFeO 3 and orthorhombic SrRuO 3 the total thickness of all SLs was kept constant and the bilayer thickness (period) Λ was varied. The interlayer strain effects are therefore tuned from large strain effects (short Λ period) to quasi-relaxed structure (large Λ). A complementary investigation using X-ray diffraction and phonon dynamics hints to change from a rhombohedral to a tetragonal structure in the superlattices with the increase of the interlayer strain effect. • In depth structural resolution of BiFeO3/SrRuO3 superlattices. • Group theoretical analysis superlattices raman spectra. • Strain induced tetragonal state in superlattices.

Insights into the Rich Polymorphism of the Na+ Ion Conductor Na3PS4 from the Perspective of Variable-Temperature Diffraction and Spectroscopy.

Solid electrolytes are crucial for next-generation solid-state batteries, and Na3PS4 is one of the most promising Na+ conductors for such applications, despite outstanding questions regarding its structural polymorphs. In this contribution, we present a detailed investigation of the evolution in structure and dynamics of Na3PS4 over a wide temperature range 30 < T < 600 °C through combined experimental–computational analysis. Although Bragg diffraction experiments indicate a second-order phase transition from the tetragonal ground state (α, P4̅21c) to the cubic polymorph (β, I4̅3m) above ∼250 °C, pair distribution function analysis in real space and Raman spectroscopy indicate remnants of a tetragonal character in the range 250 < T < 500 °C, which we attribute to dynamic local tetragonal distortions. The first-order phase transition to the mesophasic high-temperature polymorph (γ, Fddd) is associated with a sharp volume increase and the onset of liquid-like dynamics for sodium-cations (translational) and thiophosphate-polyanions (rotational) evident by inelastic neutron and Raman spectroscopies, as well as pair-distribution function and molecular dynamics analyses. These results shed light on the rich polymorphism of Na3PS4 and are relevant for a range host of high-performance materials deriving from the Na3PS4 structural archetype.

The influence of cerium oxide content on the crack growth in zirconia ceramic materials for engineering applications

This article proposes a method of using cerium oxide and zirconium oxide to prepare ceramic materials for cold welding equipment in the production of lithium batteries. Injecting ceria and other metal oxides can transform zirconia from martensite into a stable four cubic lattice. By increasing the content of cerium oxide above 9%, the monoclinic crystal is transformed to tetragonal phase, and further increase to 10%–15% is beneficial to transform the tetragonal crystal state or the cubic crystal phase into a complete solid solution. When cooled from a high temperature (2293 °C) to room temperature, ceria-doped zirconia presented a tetragonal phase structure without changing into a monoclinic phase. It was also demonstrated that the crack tip at the front of the material has a shielding effect on the tetragonal phase. In particular, it was found that the flexural strength of the material reached 800 MPa, indicating that the developed zirconia-based ceramic composite material has the ability to replace diamond knives. The current work has confirmed the possibility of obtaining new ceramic materials with different characteristics by reasonably distributing the ingredients and appropriately controlling the content of cerium oxide or other metal oxides.

Broken C4 symmetry in the tetragonal state of uniaxial strained BaCo0.9Ni0.1S1.9

The authors describe a broken C4 lattice symmetry in the tetragonal phase of BaCo${}_{0.9}$Ni${}_{0.1}$S${}_{1.9}$.

Symmetry breaking in single-crystal SrTiO3 plates: EPR manifestations

Results of the electron paramagnetic resonance (EPR) studies of Fe3+ and Mn4+ impurity centers in high-quality single-crystal SrTiO3 plates with thickness are presented. Impurities served the paramagnetic probes of the crystal structure. Analysis of the collected data indicates that the symmetry of the crystal structure of thin (001) and (110) oriented strontium titanate plates above lowers in the bulk from the cubic to tetragonal. The structure of this high-temperature tetragonal state is different from that of the conventional low-temperature antiferrodistorsive phase and has not been reported for SrTiO3 before. It is shown that the effect does not originate from the residual stress; its magnitude and particular manifestations depend on plate orientation, surface quality and geometry of a sample. Peculiarities of the observed phenomenon are presented and the possible scenario of its realization is discussed.

Phase Transition and Electronic Structures of All-d-Metal Heusler-Type X2MnTi Compounds (X = Pd, Pt, Ag, Au, Cu, and Ni).

In this work, we investigated the phase transition and electronic structures of some newly designed all-d-metal Heusler compounds, X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L21 structures of these materials was studied, and we found that X2MnTi favors to feature the L21-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L21 structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L21 structure is no longer the most stable structure, and a more stable tetragonal L10 structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L21 to tetragonal L10 structure). This mechanism of L21-L10 structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L10 and L21, defined as ΔEM (ΔEM = ECubic-ETetragonal), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L10 state. We hope that our research can provide a theoretical guidance for future experimental investigations.

Электрополевой эффект в спектрах ЭПР центров Fe-=SUP=-3+-=/SUP=- и Mn-=SUP=-4+-=/SUP=- в тонких пластинках SrTiO-=SUB=-3-=/SUB=-

The results of the study of the electric field effect in the electron paramagnetic resonance spectra of the Fe3+ and Mn4+ impurity ions in thin oriented strontium titanate single-crystal plates are presented. The crystal structure of plates at T&gt; 105 K turned out to be tetragonal, different from both the cubic and the antiferrodistortive tetragonal, inherent to SrTiO3 at T &lt; 105 K. It is shown that for the studied centers, the electric field effect is quadratic in the applied electric field and has the same sign and order of magnitude, indicating the nonpolar origin of the observed high temperature tetragonal state. Analysis of the dependences of the axiality value on the applied field indicates that the electric field effect does not originate from the electrostriction and is due to the modification the wavefunction of the ground state of the impurity centers.

Quasiclassical Theory of C4-Symmetric Magnetic Order in Disordered Multiband Metals

Recent experimental studies performed in the normal state of iron-based superconductors have discovered the existence of the $C_4$-symmetric (tetragonal) itinerant magnetic state. This state can be described as a spin density wave with two distinct magnetic vectors ${\vec Q}_1$ and ${\vec Q}_2$. Given an itinerant nature of magnetism in iron-pnictides, we develop a quasiclassical theory of tetragonal magnetic order in disordered three-band metal with anisotropic band structure. Within our model we find that the $C_4$-symmetric magnetism competes with the $C_2$-symmetric state with a single ${\vec Q}$ magnetic structure vector. Our main results is that disorder promotes tetragonal magnetic state which is in agreement with earlier theoretical studies.

Creating, probing and confirming tetragonality in bulk FeNi alloys

Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L10) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.