Change In Lattice Symmetry Research Articles

Research of the impact of uniaxial tension on the thermoelectric properties of NbCoX (X=Ge, Sn, Pb) compounds

Half-Heusler materials have been confirmed as excellent thermoelectric materials. To simulate the stress conditions in engineering applications, we adopt uniaxial tensile strain to assess the impact of lattice changes on NbCoX (X=Ge, Sn, Pb) compound's thermoelectric performance. The research reveals that when NbCoX compounds are stretched by 1.5 %, the material almost does not decrease the power factor but significantly reduces thermal conductivity, thereby enhancing the ZT value. At a carrier concentration of 1E22 cm−3 and a temperature of 873 K, the predicted ZT values for p-type NbCoSn compound reach 0.359 and 0.354 after stretching by 1.5 % in the x-y and z directions, respectively. Compared to the previous original values, this improvement is approximately 1.5 times, highlighting the significant role of tensile strain in enhancing thermoelectric performance. This may be attributed to changes in compound lattice symmetry induced by strain, leading to increased defects and interfaces, which complicate phonon transport paths, resulting in a decrease in the average phonon mean free path and consequently lowering thermal conductivity. Additionally, with the increase in atomic number of Ge, Sn, and Pb, their thermoelectric figure of merit also increases continuously, with ZT values remaining higher under stretching than compression. Such research provides a new avenue for designing and optimizing high-performance thermoelectric materials and offers insights into strain engineering for future exploration of thermoelectric materials.

Hidden-vortex quantum droplets in an optical lattice

We explore discrete quantum droplets with hidden-vortex structures in a deep two-dimensional optical lattice containing a Bose-Bose mixture. Using the extended Gross-Pitaevskii equation, and considering local interactions and Lee-Huang-Yang corrections, we affirm the stable existence of these droplets, which are identified by identical density patterns and opposing vortices in both components. We extensively study the droplets with lower topological charges, focusing on their morphological changes. Despite the expansion of internal boundaries within the vortex mode with increased charge, the lattice symmetry and phase change limit the boundary growth, leading to a unique mode pattern degeneracy exclusive to discrete systems. By fixing the norm of the states (representing the particle number of the system), we identify multiple degenerate modes with topological charges up to 5, plotting their chemical potential curves and providing a theoretical explanation for the energy changes linked to topological charge. Our simulations confirm these outcomes, validating our discoveries.

Ferroelectric–Ferroelastic Transitions in (Na0.5Bi0.5)TiO3-BaTiO3 Single Crystals

The comprehensive understanding of (Na0.5Bi0.5)TiO3-BaTiO3 (NBT-BT) lattice structure is highly desired to develop lead-free ferroelectric materials. However, most of the previous studies focused on the improvement of piezoelectric properties at room temperature, and many structural puzzles are left unclear. In this work, the lattice structure of a ferroelastic phase and the ferroelectric-ferroelastic transitions in both rhombohedral NBT and tetragonal NBT-8%BT single crystals are investigated in detail. Our results illustrate the complex process of the ferroelectric-ferroelastic transition of NBT. The variation of Ti-O modes and oxygen octahedra modes clearly indicates the gradual change of lattice symmetry from R3c to P4bm during a wide temperature range between 170 and 350 °C. A ferroelectric-ferroelastic transition is also confirmed in tetragonal NBT-8BT for the first time, and the lattice symmetry of P4bm is found to be maintained during the ferroelastic stage. This work reveals the lattice evolutions of the ferroelectric-ferroelastic transition of NBT-BT crystals and provides new insights for understanding the ferroelasticity and the evolution of phonon modes in a lead-free relaxor.

Structure Effect on Pseudocapacitive Properties of A2LaMn2O7 (A = Ca, Sr)

Herein, the effect of structure on pseudocapacitive properties in alkaline conditions is demonstrated through the investigation of isoelectronic oxides Ca2LaMn2O7 and Sr2LaMn2O7, where the difference in ionic radii of Ca2+ and Sr2+ leads to a change in structure and lattice symmetry, resulting in an orthorhombic Cmcm structure for the former and a tetragonal I4/mmm structure for the latter. While calcium and strontium do not make a direct contribution to the near‐surface faradaic processes that are essential to the pseudocapacitive properties, their effect on the structure leads to a change in the oxygen intercalation process and the associated pseudocapacitive energy storage. It is shown that Sr2LaMn2O7 has a significantly greater specific capacitance than Ca2LaMn2O7. In addition, the former shows a considerably higher‐energy density compared to the latter. Furthermore, these materials show highly stable energy‐storage properties, and retain their specific capacitance over 10 000 cycles of charge–discharge in a symmetric pseudocapacitive cell. Importantly, these findings show the structure–property relationships, where a change in the structure and lattice symmetry can result in a significant change in pseudocapacitive charge–discharge properties in isoelectronic systems.

Unusual first-order magnetic phase transition and large magnetocaloric effect in Nd2In

A large magnetocaloric effect with its maximum near the boiling point of natural gas occurs in a rare-earth intermetallic compound ${\mathrm{Nd}}_{2}\mathrm{In}$. While behaviors of physical properties indicate that paramagnetic-ferromagnetic transformation supporting the large magnetocaloric effect is first order in nature, temperature-dependent crystallographic study reveals no changes in lattice symmetry and lack of discontinuities either in phase volume or lattice parameters. The borderline first-order nature of phase transformation in ${\mathrm{Nd}}_{2}\mathrm{In}$ is markedly different from conventional first-order magnetic transitions occurring in other members of the family -- isostructural ${\mathrm{Pr}}_{2}\mathrm{In}$ and nonisostructural ${\mathrm{Eu}}_{2}\mathrm{In}$.

Observation of magnetic field-induced second magnetic ordering and peculiar ferroelectric polarization in L-type ferrimagnetic Fe2(MoO4)3

${\mathrm{Fe}}_{2}{({\mathrm{MoO}}_{4})}_{3}$ was synthesized and characterized as a prototype of L-type ferrimagnets (L-FiM) with an ordering temperature ${T}_{\mathrm{N}1}\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ using magnetic susceptibility $(\ensuremath{\chi})$, specific heat $({C}_{\mathrm{p}})$, and dielectric $({\ensuremath{\varepsilon}}^{\ensuremath{'}})$ anomaly. Two remarkable findings are magnetic field $(H)$ induced (i) an additional magnetic phase transition at ${T}_{\mathrm{N}2}$ below ${T}_{\mathrm{N}1}$; and (ii) the emergence of flexible ferroelectric polarization $(P)$ with the tuning parameters $H$ and $T$ below ${T}_{\mathrm{N}2}$. Thus, the H-T phase diagram for spin-induced type-II multiferroics was established. In contrast to some known multiferroics with a critical field-induced spin-flip $P$ below ${T}_{\mathrm{N}1}$, the observed multiferroic nature is exotic. The origin of the profound multiferroic nature may be hidden in the complex $T$-and $H$-dependent spin and lattice structures. Consequently, the schematic picture of $H$-induced possible change of lattice symmetry and conical spin structure has been proposed. More experimental and theoretical works providing solid evidence and interpretations have been suggested to explore these peculiar multiferroic phenomena.

Photochromic and Electric Field-Regulating Luminescence in High-Transparent (K,Na)NbO3-Based Ferroelectric Ceramics with Two-Phase Coexistence.

Rare earth Tb3+ doped (K0.465Na0.465Li0.07)(Nb0.93Bi0.07)O3 (KNNLB-x%Tb) lead-free transparent ferroelectric ceramics were designed and prepared. The effects of Tb3+ on phase structure, microstructure, optical transmittance, photoluminescence, and photochromic behaviors were studied. Although two ferroelectric phases coexist, the optical transmittance can reach the high value of 74% in the visible light region because of the fine grains, dense ceramic microstructure, large optical energy band gap, and relatively high symmetry of coexisting ferroelectric phases. In addition, Tb3+ works as a luminescent center, and the reversible photochromic modulation is achieved by alternate stimulation of illumination and heat treatment. Meanwhile, the luminescence contrast can be improved under in situ electric field stimulation due to the easy change of lattice symmetry in coexisting ferroelectric phases. The generation of color centers after illumination and the local crystal field around the luminescent center caused by in situ electric field contributes to above phenomena. These ceramics exhibit the great potential in optical data storage and anticounterfeiting.

Unconventional charge density wave and photoinduced lattice symmetry change in the kagome metal CsV3Sb5 probed by time-resolved spectroscopy

The spin-orbit entangled quantum states in 4d/5d compounds, e.g., the Jeff=1/2 and Jeff=3/2 states, have attracted great interests for their unique physical roles in unconventional superconductivity and topological states. Here, the key role of tetragonal distortion is clarified, which determines the ground states of 4d1/5d1 systems to be the Jeff=3/2 one (e.g. K2NbCl6) or S=1/2 one (e.g. Rb2NbCl6). By tuning the tetragonal distortion via epitaxial strain, the occupation weights of dxy/dyz/dxz orbitals can be subtly modulated, competing with the spin-orbit coupling. Consequently, quantum phase transitions between S=1/2 state and Jeff=3/2 state, as well as between different Jeff=3/2 states, can be achieved, resulting in significant changes of local magnetic moments. Our prediction points out a reliable route to engineer new functionality of Jeff states in these quantum materials.

Influence of the BaTiO3 addition to K0.5Na0.5NbO3 lead-free ceramics on the vacancy-like defect structure and dielectric properties

A study on the induced changes in the vacancy-like defect structure and the dielectric properties of K0.5Na0.5NbO3 ceramics by the addition of different amounts, between 0% and 7%, of BaTiO3 is presented. The samples were prepared by a mechanochemically activated solid-state reaction method. The structural evolution due to the orthorhombic to the tetragonal phase transition of the KNN doped samples was followed using X-ray diffraction and Raman spectroscopy. The use of positron annihilation lifetime spectroscopy allowed to identify the defect structure at sub-nanometric scale and the nature of the vacancy-like defects generated by the phase transition. The obtained results are discussed considering the lattice symmetry change and the defects structure formed due to the replacement of alkaline and niobium ions by barium and titanium ones. Additionally, changes in the dielectric properties are discussed in terms of the structural modifications of the different KNN-based ceramics involving different kinds of vacancy-like defects.

Ferrimagnetic 120∘magnetic structure inCu2OSO4

We report magnetic properties of a 3d$^9$ (Cu$^{2+}$) magnetic insulator Cu2OSO4 measured on both powder and single crystal. The magnetic atoms of this compound form layers, whose geometry can be described either as a system of chains coupled through dimers or as a Kagom\'e lattice where every 3rd spin is replaced by a dimer. Specific heat and DC-susceptibility show a magnetic transition at 20 K, which is also confirmed by neutron scattering. Magnetic entropy extracted from the specific heat data is consistent with a $S=1/2$ degree of freedom per Cu$^{2+}$, and so is the effective moment extracted from DC-susceptibility. The ground state has been identified by means of neutron diffraction on both powder and single crystal and corresponds to a $\sim120$ degree spin structure in which ferromagnetic intra-dimer alignment results in a net ferrimagnetic moment. No evidence is found for a change in lattice symmetry down to 2 K. Our results suggest that \sample \ represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored.

Temperature-Stimulus Responsive Ferroelastic Molecular–Ionic Crystal: (C8H20N)[BF4

Molecular–ionic crystals, which are built of organic cations and discrete anionic components with AMX4 stoichiometry, easily induce structural order–disorder transitions. These phenomena can generate interesting ferroic properties. Herein, we have synthesized and characterized a novel AMX4-type crystal, (C8H20N)[BF4] (1). Thermal analysis, (DSC, TGA) indicates one structural phase transition (PT) at 257/259 K (cooling/heating, I ↔ II). The structure of 1 has been solved at two temperatures, 120 and 298 K, with single-crystal X-ray diffraction. The high-temperature phase I (1-HT) shows a tetragonal structure with the space group I4/mmm, whereas the low-temperature phase II (1-LT) is monoclinic with the space group P21/a. The PT belongs, thus, to the improper ferroelastic species 4/mmmF2/m with a 4-fold multiplication of the unit cell. All four possible ferroelastic domains of phase II have been identified in polarized light microscopy. The monoclinic deformation has been shown to give rise to a characteristic parquet-like texture preserving integrity of 1. The molecular mechanism of PT involving changes in the cationic and anionic dynamics has been postulated based on measurements of the 1H and 19F NMR spin–lattice relaxation times, T1, and second-moment, M2, and of the dielectric responses. The ac and dc conductivity measurements have been used to determine the transport properties of the charge carriers in 1. The intrinsic features of molecular–ionic crystals, such as the dynamics of molecular motions, structural instability with a lattice-symmetry change, formation of domain patterns, etc., provide a crystal with unique ferroelastic properties. This type of crystal may be useful as an functional material.

Charge fluctuations across the pressure-induced quantum phase transition in EuCu2(Ge1−xSix)2

Pressurizing strategically selected compositions of the $\mathrm{EuC}{\mathrm{u}}_{2}{(\mathrm{G}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{i}}_{x})}_{2}$ series affords an opportunity for gaining microscopic insight into the ground-state properties and interplay between magnetism and valence fluctuations across a quantum critical point. This is investigated by way of systematic $^{151}\mathrm{Eu}$ M\"ossbauer spectroscopy measurements on $x=0$ and $x=0.5$ compositions in the series, pressurized up to 7 GPa including variable temperature scans in the range 300--4.2 K. In $\mathrm{EuC}{\mathrm{u}}_{2}\mathrm{G}{\mathrm{e}}_{2}$ the temperature and pressure dependences of the hyperfine interaction parameters indicate that both the magnetic and divalent state, $\mathrm{E}{\mathrm{u}}^{\ensuremath{\nu}+}$ where $\ensuremath{\nu}=2$, are stable up to 6--7 GPa, thus serving as a useful reference. Whereas in the $x=0.5$ composition which initially involves $\mathrm{E}{\mathrm{u}}^{2+}$, collapse of the magnetically ordered state is onset at \ensuremath{\sim}1.3 GPa and there is emergence of a nonmagnetic intermediate valence state coexisting with the magnetically ordered state. This regime of mixed states is a precursor of a quantum phase transition to a nonmagnetic homogeneous intermediate valence state $\ensuremath{\nu}\ensuremath{\sim}2.45$, across a quantum critical point at 3.6 GPa, suggesting a first-order phase transition. X-ray-diffraction pressure studies at 300 K up to 6 GPa of the $x=0.5$ composition indicate there is no change in lattice symmetry from the tetragonal $\mathrm{ThC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{i}}_{2}$-type structure. There are also no obvious discontinuities in pressure dependences of the lattice parameters upon evolving through the quantum critical point at 3.6 GPa. Increasing pressure changes the starting $\mathrm{E}{\mathrm{u}}^{2+}$ valence monotonically, until the mean valence attains its largest value $\ensuremath{\nu}\ensuremath{\sim}2.45$ indicative of enhanced charge fluctuations at the quantum critical point and plateaus thereafter. High-pressure resistance measurements at low temperatures down to 40 mK near the quantum critical point reveal no evidence for superconductivity.

Indications for Lifshitz transitions in the nodal-line semimetal ZrSiTe induced by interlayer interaction

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at $\approx$4.1 and $\approx$6.5 GPa. By pressure-dependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.

Cooperative elastic fluctuations provide tuning of the metal–insulator transition

Metal-to-insulator transitions1 driven by strong electronic correlations occur frequently in condensed matter systems, and are associated with remarkable collective phenomena in solids, including superconductivity and magnetism. Tuning and control of the transition holds the promise of low-power, ultrafast electronics2, but the relative roles of doping, chemistry, elastic strain and other applied fields have made systematic understanding of such transitions difficult. Here we show that existing data3-5 on the tuning of metal-to-insulator transitions in perovskite transition-metal oxides through ionic size effects provides evidence of large systematic effects on the phase transition owing to dynamical fluctuations of the elastic strain, which have usually been neglected6. We illustrate this using a simple yet quantitative statistical mechanical calculation in a model that incorporates cooperative lattice distortions coupled to the electronic degrees of freedom. We reproduce the observed dependence of the transition temperature on the cation radius in the well studied manganite7 and nickelate8 materials. Because elastic couplings are generally strong, we anticipate that these conclusions will generalize to all metal-to-insulator transitions that couple to a change in lattice symmetry.

Pressure-induced structural dimerization in the hyperhoneycomb iridateβ−Li2IrO3at low temperatures

A pressure-induced collapse of magnetic ordering in $\beta$-Li$_2$IrO$_3$ at $P_m\sim1.5- 2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at \emph{room temperature}, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $\beta$-Li$_2$IrO$_3$ under pressure at low temperatures ($T\leq50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\sim1.5-2$ GPa below 50 K. While a direct $Fddd \rightarrow C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{\rm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $\beta$-Li$_2$IrO$_3$ is the pressure-induced formation of Ir$_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{\rm{eff}}=1/2$ moments and related non-collinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $\beta$-Li$_2$IrO$_3$.

Improving thermoelectric performance of (Bi0.2Sb0.8)2(Te0.97Se0.03)3 via Sm-doping

We herein report the effects of Sm doping on the thermoelectric performance of (Bi0.2Sb0.8)2(Te0.97Se0.03)3 compounds prepared by a growth-from-the-melt spark-plasma-sintering procedure. In conjunction with micro-morphology and compositional analyses, the electrical conductivity, Seebeck coefficient, and thermal conductivity of (Bi0.2-xSmxSb0.8)2(Te0.97Se0.03)3 samples were measured as a function of temperature (298–473 K) and the Sm doping ratio (x = 0%, 0.2%, 0.4%, and 0.8%). We found that (i) there was no discernible crystal lattice symmetry change upon Sm doping; (ii) Sm doping generally increased the electric conductivity and practically retained the Seebeck coefficient, thereby yielding higher power factors; and (iii) specially, a minimum thermal conductivity ∼ 0.92 Wm−1K−1 was attained in the x = 0.4% sample at 423 K, resulting in a state-of-the-art ZT value ∼ 1.22, which is ∼25% higher than the maximum ZT value of pristine sample. Contrary to the customary view that Sm dopant adopts a trivalent state Sm3+, the variations of lattice constant pointed toward a lower valence state of Sm. The results demonstrated the efficacy and deepened our understanding of rare earth doping in thermoelectric study of Bi2Te3-based materials.

Crystalline and oxide phases revealed and formed on InSb(111)B

Oxidation treatment creating a well-ordered crystalline structure has been shown to provide a major improvement for III–V semiconductor/oxide interfaces in electronics. We present this treatment’s effects on InSb(111)B surface and its electronic properties with scanning tunneling microscopy and spectroscopy. Possibility to oxidize (111)B surface with parameters similar to the ones used for (100) surface is found, indicating a generality of the crystalline oxidation among different crystal planes, crucial for utilization in nanotechnology. The outcome is strongly dependent on surface conditions and remarkably, the (111) plane can oxidize without changes in surface lattice symmetry, or alternatively, resulting in a complex, semicommensurate quasicrystal-like structure. The findings are of major significance for passivation via oxide termination for nano-structured III–V/oxide devices containing several crystal plane surfaces. As a proof-of-principle, we present a procedure where InSb(111)B surface is cleaned by simple HCl-etching, transferred via air, and post-annealed and oxidized in ultrahigh vacuum.

Changes in cell parameters and improvement in photocatalytic activity of KNbO3 and NaNbO3 crystals via polarization

A representative ferroelectric tetragonal potassium niobate (TKN) and antiferroelectric orthorhombic sodium niobate (ONN) crystalline powder samples were polarized by using an external electric field. The changes in lattice distortion and crystal symmetry of these samples polarized were revealed. The photocatalytic activities of the samples before and after the polarization were evaluated by photocatalytic decomposition of water and photocatalytic reduction of Ag+. The results imply that the polarization of such kind of perovskite-type niobate samples using the external electric field is one of effective ways to improve the photocatalytic performance, whether ferroelectric TKN or antiferroelectric ONN. The exploration of mechanism suggests that the polarization can change the ferroelectricity of niobate powder crystals. Furthermore, the photocatalytic activity on different exposed facets of these niobate crystals was also evaluated by detecting Ag content on the surfaces of sample via photocatalytic reduction of Ag+. It is found that {001} facet is the highest active facet of TKN, and {101} facet of ONN crystals has the highest photocatalytic activity among the exposed facets. The photocatalytic activity on each exposed facet can be increased to a certain extent by polarization, and the influence of polarization on photocatalytic activity is greater than that of surface energy.

Tunable Quantum Spin Liquidity in the 1/6th-Filled Breathing Kagome Lattice.

We present measurements on a series of materials, Li_{2}In_{1-x}Sc_{x}Mo_{3}O_{8}, that can be described as a 1/6th-filled breathing kagome lattice. Substituting Sc for In generates chemical pressure which alters the breathing parameter nonmonotonically. Muon spin rotation experiments show that this chemical pressure tunes the system from antiferromagnetic long range order to a quantum spin liquid phase. A strong correlation with the breathing parameter implies that it is the dominant parameter controlling the level of magnetic frustration, with increased kagome symmetry generating the quantum spin liquid phase. Magnetic susceptibility measurements suggest that this is related to distinct types of charge order induced by changes in lattice symmetry, in line with the theory of Chen etal. [Phys. Rev. B 93, 245134 (2016)PRBMDO2469-995010.1103/PhysRevB.93.245134]. The specific heat for samples at intermediate Sc concentration, which have the minimum breathing parameter, show consistency with the predicted U(1) quantum spin liquid.

Long-Range Lattice Engineering of MoTe2 by a 2D Electride.

Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to ∼1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N]+·e- to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 × 1014 cm-2 and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metal-semiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N]+·e-. The combination of 2D electrides and layered materials yields a novel material design in terms of doping and lattice engineering.