Cooperative Jahn-Teller Distortion Research Articles

Sodium and Manganese Stoichiometry of P2-Type Na2/3 MnO2.

To realize a reversible solid-state Mn(III/IV) redox couple in layered oxides, co-operative Jahn-Teller distortion (CJTD) of six-coordinate Mn(III) (t2g (3) -eg (1) ) is a key factor in terms of structural and physical properties. We develop a single-phase synthesis route for two polymorphs, namely distorted and undistorted P2-type Na2/3 MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3 MnO2 delivers 216 mAh g(-1) as a 3 V class positive electrode, reaching 590 Wh (kg oxide)(-1) with excellent cycle stability in a non-aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na2/3 MnO2 . Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3 MnO2 , providing a new insight into utilization of the Mn(III/IV) redox couple for positive electrodes of Na-ion batteries.

Sodium and Manganese Stoichiometry of P2‐Type Na2/3MnO2

AbstractTo realize a reversible solid‐state MnIII/IV redox couple in layered oxides, co‐operative Jahn–Teller distortion (CJTD) of six‐coordinate MnIII (t2g3–eg1) is a key factor in terms of structural and physical properties. We develop a single‐phase synthesis route for two polymorphs, namely distorted and undistorted P2‐type Na2/3MnO2 having different Mn stoichiometry, and investigate how the structural and stoichiometric difference influences electrochemical reaction. The distorted Na2/3MnO2 delivers 216 mAh g−1 as a 3 V class positive electrode, reaching 590 Wh (kg oxide)−1 with excellent cycle stability in a non‐aqueous Na cell and demonstrates better electrochemical behavior compared to undistorted Na2/3MnO2. Furthermore, reversible phase transitions correlated with CJTD are found upon (de)sodiation for distorted Na2/3MnO2, providing a new insight into utilization of the MnIII/IV redox couple for positive electrodes of Na‐ion batteries.

Cooperative Jahn-Teller effect and the role of strain in the tetragonal-to-cubic phase transition in Mg x Cu1 - x Cr2O4.

Temperature and composition dependences of the I41/amd → [Formula: see text] phase transition in the Mg x Cu1 - x Cr2O4 spinel solid solution, due to the melting of the cooperative Jahn-Teller distortion, have been studied by means of single-crystal X-ray diffraction. Crystals with x = 0, 0.10, 0.18, 0.43, 0.46, 0.53, 1 were grown by flux decomposition methods. All crystals have been refined in the tetragonal I41/amd space group except for the Mg end-member, which has cubic symmetry. In Mg x Cu1 - x Cr2O4 the progressive substitution of the Jahn-Teller, d9 Cu2+ cation with spherical and closed-shell Mg2+ has a substantial effect on the crystal structure, such that there is a gradual reduction of the splitting of a and c unit-cell parameters and flattening of the tetrahedra. Single-crystal diffraction data collected in situ up to T = 1173 K show that the tetragonal-to-cubic transition temperature decreases with increasing Mg content. The strength of the Cu-Cu interaction is, in effect, modulated by varying the Cu/Mg ratio. Structure refinements of diffraction data collected at different temperatures reveal that heating results in a gradual reduction in the tetrahedron compression, which remains significant until near the transition temperature, however, at which point the distortion of the tetrahedra rapidly vanishes. The spontaneous strain arising in the tetragonal phase is large, amounting to 10% shear strain, et, and ∼ 1% volume strain, Vs, in the copper chromite end-member at room temperature. Observed strain relationships are consistent with pseudoproper ferroelastic behaviour ([Formula: see text] ∝ Vs ∝ [Formula: see text], where qJT is the order parameter). The I41/amd → [Formula: see text] phase transition is first order in character for Cu-rich samples and then evolves towards second-order character. Although a third order term is permitted by symmetry in the Landau expansion, this behaviour appears to be more accurately represented by a 246 expansion with a change from negative to positive values of the fourth-order coefficient with progressive dilution of the Jahn-Teller cation.

Understanding the Effect of Synthesis Conditions on the Electrochemical Performance of the Mn3+/Mn4+ Redox Couple in LiNi0.5Mn1.5O4.

Research in rechargeable lithium ion battery (LIBs) systems has recently focussed on improving the operating voltage, specific capacity as well as safety. In this regard, nickel substituted spinel (LiNi0.5Mn1.5O4, LNM) is attractive as a cathode with an operating voltage of ~4.7V vs. Li/Li+. Nickel substitution in spinel LiMn2O4 was initially performed with the aim of stabilizing the material against phase transformation that arose due to co-operative Jahn-Teller distortion of the lattice. But it was discovered that the Ni2+/Ni4+ redox couple was active at a potential of ~4.7 V in a reversible manner[1]. This generated intense interest in this material. Although the material has proven to be robust and maintains structural stability during cycling, the existing electrolytes are unable support such high operating voltages and continue to remain a hurdle in the development of batteries based on this spinel. In this work, we are interested in understanding another redox couple based on Mn3+/Mn4+ which operates at ~2.8V vs Li/Li+ [1,2]. This redox couple has not been explored in detail and understandably so due to the attractive high voltage characteristics offered by the nickel redox couple. But the operating potential (~2.8V vs Li/Li+) makes it an ideal candidate as anode for aqueous rechargeable lithium battery systems (which would operate at ~ -0.24V vs SHE). Furthermore, the effect of co-operative Jahn-Teller distortion that occurs due to the average manganese valence reducing below 3.5 is minimized to a significant extent compared to the counterpart spinel LiMn2O4. It would be possible to insert approximately 0.75 mols of lithium before the average manganese valence reaches 3.5. This would give it a capacity of roughly 110 mAh.g-1. However there are only a few studies that investigate systematically the electrochemical performance of the Mn3+/Mn4+ redox couple in LNM. In this work, we have investigated the effect of synthesis method and conditions on the electrochemical performance of LNM while storing and extracting lithium using the manganese redox couple in non-aqueous electrolytes. It was found that synthesis conditions had a significant effect on the specific capacity that could be obtained for the manganese redox couple. Lower temperatures of synthesis resulted in higher capacities. This is in contrast to the performance of the nickel redox couple where higher temperatures offered better capacities. Cyclic voltammetry, galvanostatic cycling and impedance measurements were performed to understand the origin of the differences in the behaviour. More results will be presented at the meeting.

Correlations induced orbital ordering and cooperative Jahn–Teller distortion in the paramagnetic insulator KCrF3

We investigate the origin of the orbital ordering in the paramagnetic phase of KCrF$_3$. All previous studies described structural parameters of the paramagnetic phase using a magnetic ordering in the compound. Our simulations of real paramagnetic KCrF$_3$ were performed within an approach combining density functional theory and dynamical mean field theory (DFT+DMFT). As a result, it was found that the experimentally observed cooperative Jahn-Teller effect is successfully described in a lattice relaxation calculation for structure without any long-range magnetic ordering. It is established that the existence of the orbital ordering even in undistorted perovskite structure clearly confirms the electronic origin of the orbital ordering in KCrF$_3$.

Interplay Between Structural, Jahn–Teller, and Magnetic States of Slightly Doped Lanthanum Manganites

By combining the results of elastic moduli, electrical resistivity, and magnetization measurements for La\(_{1-x}\)Sr\(_{x}\)MnO\(_{3}\) (x = 0.125, x = 0.15, and x = 0.175), we have constructed a phase diagram that describes the structural, magnetic, transport properties and the relationships among them as a function of the composition and temperature (140–340 K). The local, intermediate, and cooperative Jahn–Teller distortions of the octahedral structural units MnO\(_{6}\) have been studied. It is common for these distortions to be observed using probes of intermediate structures (domains or super-cells), but they are absent in the averaged crystallographic structure. In the cooperative Jahn–Teller distorted phase, the macroscopic sample length is temperature dependent. We presume that the structural transitions from the cooperative Jahn–Teller phase to the charge ordering phase at low temperatures (150 and 180 K at x = 0.125 and x = 0.15) are due to the increase in the spontaneous magnetization with the conservation of the local deformations of separate octahedra. The agreement between the types of the orbital ordering and the local, intermediate, and Jahn–Teller cooperative distortions of octahedra was established.

Variations in structure and electrochemistry of iron- and titanium-doped lithium nickel manganese oxyfluoride spinels

Doping of cathode materials can considerably improve electrochemical performance and stability. Here, the high-voltage LiNi0.5Mn1.5O4 spinel is used as a candidate material. It is high-voltage cycling at a potential of approximately 4.7 V and the ability to host 2 eq. Li, thus leading to a theoretical capacity of 294 mAh g−1, that makes this material interesting. In order to improve stability and electronic conductivity, the spinel is doped with titanium and iron.Cycling in a voltage range of 2.0–5.0 V leads to a cooperative Jahn-Teller distortion accompanied by a phase transformation from cubic to tetragonal symmetry. This causes a severe capacity fade. To improve capacity retention, the as-prepared spinel is post-doped with fluorine. Influence of different fluorine amounts in LiNi0.5Mn1.4Fe0.1Ti0.027O4−xFx (x = 0–0.3) on the capacity and stability is analyzed. The initial capacities decrease with increasing fluorine content but the low voltage capacity is stabilized. Best electrochemical results are obtained with a fluorine content of x = 0.15. Furthermore, an additional redox couple is found. The intensity of this depends on the fluorine content. It is assumed that manganese, either in the tetrahedral sites or in octahedral sites, bound to fluorine lead to a higher voltage.

Gate control of ferromagnetic insulating phase in lightly-doped La0.875Sr0.125MnO3−δ film

The electric field effect on the lightly doped La0.875Sr0.125MnO3−δ (LSMO) thin film in electric double-layer transistors was investigated by measuring transport properties of the film under various gate voltages. It was found that the positive gate bias leads to an increase of the charge-orbital ordering (COO) transition temperature and a decrease of the Curie temperature TC, indicating the suppression of ferromagnetic metal (FMM) phases and preference of COO/ferromagnetic insulator (FMI) with the hole depletion by gate bias. Such different electric field effects can be ascribed to the weakening of the ferromagnetic interaction and enhancement of Jahn-Teller (JT) distortion caused by the transformation of JT inactive Mn4+-ions to JT active Mn3+-ions. Moreover, a step-like increase in the high temperature region of the ρ-T curve, which is related to the transition of cooperative JT distortion, was found to develop with increasing the positive bias, indicating that the cooperative JT distorted phase is stabilized by the depletion of holes in LSMO film. These results demonstrate that the modulation of holes via electric field strongly affects the balance between energy gains of different interactions and thus produce different effects on the competing FMI, FMM, and cooperative JT distorted phases in LSMO film.

Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered ${\mathrm{LaSrMnO}}_{4}$ with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar $Pca{2}_{1}$ space group where small polar displacements of ${d}^{4}\phantom{\rule{0.28em}{0ex}}{\mathrm{Mn}}^{3+}$ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making ${\mathrm{LaSrMnO}}_{4}$ a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between $A$ cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local ${e}_{g}$ orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional ${\mathrm{MnO}}_{2}$ sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.

Jahn-Teller, Polarity, and Insulator-to-Metal Transition in BiMnO3at High Pressure

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53GPa and, between 37 and 53GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3.

Oxygen‐deficiency‐induced suppression of JT distortion and stabilization of charge ordering in La0.2Sr0.8MnO3−δ

Structural phase‐transition studies employing low‐temperature transmission electron microscopy and low‐temperature X‐ray diffraction have been carried out on nearly stoichiometric (δ = 0.01) and on off‐stoichiometric (δ = 0.12) versions of La0.2Sr0.8MnO3−δ manganite. The nearly stoichiometric La0.2Sr0.8MnO3−δ undergoes a cubic to Jahn–Teller distorted C‐type antiferromagnetic tetragonal phase transition at 260 K. For off‐stoichiometric La0.2Sr0.8MnO3−δ the Jahn–Teller distorted tetragonal phase transition is totally suppressed and the basic perovskite lattice remains cubic but shows a charge‐ordered phase. Stabilization of charge‐ordered phase in the absence of cooperative Jahn–Teller distortion has been attributed to Coulomb repulsion and Hunds coupling energy. The off‐stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic, and calorimetric measurements. A smooth crossover from variable‐range hopping transport to a power‐law dependence of resistivity (ρ) on temperature (T), i.e., ρ = CT−α has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.

First-principles study of orbital ordering in cubic fluoride KCrF3

Comprehensive first-principles calculations are performed to provide insight into the intriguing physical properties of the ternary cubic fluoride KCrF3. The electronic structures exhibit a prominent dependence on the effective local Coulomb interaction parameter Ueff. The ground state of the cubic phase is a ferromagnetic (FM) half-metal with Ueff equal to 0, 2, and 4 eV, whereas the insulating A-type antiferromagnetic (A-AFM) state with concomitant homogeneous orbital ordering is more robust than the FM state for Ueff exceeding 4 eV. We propose that the origin of the orbital ordering is purely electronic when the cooperative Jahn—Teller distortions are absent in cubic KCrF3.

Multiferroic (ferroelastic/ferromagnetic/ferrimagnetic) aspects of phase transitions in RCo2 Laves phases

Magnetic phase transitions in RCo2 Laves phases with R as a rare earth element are accompanied by changes in crystallographic space group. For purely structural transitions they would be described as improper ferroelastic and therefore fulfil the condition for multiferroic phase transitions in combining two out of three properties, ferro/antiferromagnetism, ferroelectricity and ferroelasticity. Here lattice parameter data from the literature and new measurements of elastic and anelastic properties, by resonant ultrasound spectroscopy, for NdCo2 and ErCo2 have been analysed from this perspective. The temperature dependence of symmetry-breaking shear strains is consistent with the cubic ↔ tetragonal transition in NdCo2 being close to tricritical in character and the cubic ↔ rhombohedral transition in ErCo2 being first order. Elastic softening and acoustic loss within the stability ranges of the ferroelastic phases can be understood in terms of a combination of intrinsic softening due to strain/order parameter coupling and ferroelastic twin-wall motion. Softening ahead of the transitions does not fit with standard macroscopic descriptions of dynamic effects from other systems but, rather, in the case of NdCo2, might be attributed to the involvement of a second zone centre order parameter related to a separate instability driven by cooperative Jahn–Teller distortions. In ErCo2, acoustic loss in the temperature interval above the transition point is discussed in terms of a possible tweed microstructure associated with strain coupling to local magnetic ordering. The overall multiferroic behaviour can be understood in terms of a single magnetic order parameter (irrep of magnetic space group ) which couples with a structural order parameter (irrep or ). The coupling is linear/quadratic which, in the case of two separate instabilities, causes them to combine in a single multiferroic phase transition.

Magnetism in CdV2O4 from First Principles

The nature of the orbital ordering in the frustrated spinel CdV2O4 was examined on the basis of first-principles electronic structure calculations. GGA predicts the ferromagnetic half-metallic ground state while GGA+U shows antiferromagnetic Mott insulator with a staggered dxy 1dxz 1 (V2) and dxy 1dyz 1 (V1) orbital order. The magnetic moments were found to be close to the experimental results. Results show that electron correlation and co-operative Jahn-Teller distortions play a significant role in determining the orbital ordering. Keywords: Oxide, magnetism, electronic structure, orbital ordering.

Laser-induced voltage (LIV) enhancement of La2/3Sr1/3MnO3 films with Ag addition

La2/3Sr1/3MnO3:Ag x (LSMO:Ag x , x=0, 0.04, 0.08, 0.10, 0.20, 0.30) films were grown on vicinal cut LaAlO3 (LAO) substrates by pulsed laser deposition technique (PLD). It is found that laser-induced voltage (LIV) of LSMO:Ag x films is improved and enhanced by Ag addition. With x increasing, figure of merit (F m ) and anisotropic Seebeck coefficient (ΔS) of LSMO:Ag x (x=0.08) films reach the maximum values of 28.25 mV/ns and 0.38 μV/K, respectively. The results suggest that the LIV enhancement of LSMO:Ag x films is due to the anisotropy of the Seebeck tensor, which is produced by long range cooperative Jahn–Teller distortions with Ag addition.

Orbital ordering in La0.5Sr1.5MnO4: Density functional and exact diagonalization studies

The nature of orbital ordering and type of the Jahn-Teller (JT) distortion in the doped manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$ has been a topic of long standing debate, with both experiments and theory supporting opposite views. With the help of ab initio density functional and exact diagonalization studies we have investigated the energetics of the cooperative JT distortion and orbital ordering in this system. The density functional calculations yield a ${x}^{2}\ensuremath{-}1/{y}^{2}\ensuremath{-}1$ (rod-type) orbital order as well as a Jahn-Teller distortion where the octahedral axes on the plane are elongated along the chain direction and compressed perpendicular to it in the $ab$ plane (type I). The essential physics of the problem is captured by a single zigzag chain model, which was studied with a tight-binding Hamiltonian and exact diagonalization on a finite chain, which help unravel the reason for the rod-type orbital order. The main conclusions from the latter studies are that: (i) The Coulomb correlation effects are relatively unimportant due to the low electron density in the ${e}_{g}$ states, (ii) the kinetic energy term (hopping along the ferromagnetic zigzag chain) favors the rod-type orbital order, and (iii) a significant noncubic crystal field, proposed earlier and found here from our DFT calculations, helps stabilize the rod-type orbital order even further.

Influence of the Jahn-Teller effect on the structural, magnetic, and electrical properties of lightly doped manganites

Features of the structural, magnetic and electric properties in lightly doped La1−x Sr x MnO3 (0.12 < x < 0.18) manganites are studied. Agreement between the types of orbital ordering and the local cooperative Jahn-Teller distortion octahedra is established. The interaction between the Jahn-Teller effect and magnetic ordering is described.

Crystal and Magnetic Structures and Physical Properties of a New Pyroxene NaMnGe2O6 Synthesized under High Pressure

A new pyroxene compound, NaMnGe(2)O(6), has been synthesized at 3 GPa and 800 °C and fully characterized by X-ray single-crystal diffraction, neutron powder diffraction, and measurements of magnetization and specific heat. NaMnGe(2)O(6) crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) Å, b = 8.7507(18) Å, c = 5.5724(11) Å, and β = 105.64(3)° at 153 K. A cooperative Jahn-Teller distortion is formed by an ordering of the longest Mn-O bonds between two neighboring octahedra along the chain direction. This feature distinguishes NaMnGe(2)O(6) from other pyroxene compounds without Jahn-Teller active cations and suggests that the Jahn-Teller distortion competes with the intrinsic local distortion in the pyroxene structure. No orbital order-disorder transition has been found up to 750 K. Like other alkali-metal pyroxenes with S > (1)/(2), NaMnGe(2)O(6) (S = 2) was found to undergo a long-range antiferromagnetic (AF) ordering at T(N) = 7 K due to intrachain and interchain exchange interactions. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, >60%, has been removed above T(N) as a result of strong spin correlations within the quasi-one-dimensional Mn(3+)-spin chains. The Reitveld refinement of neutron powder diffraction data gives a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned mainly along the c-axis with a small component along both a- and b-axes.

Mesoscopic mean-field theory for spin-boson chains in quantum optical systems

We present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E⊗β) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent.

Structural transitions and transport-half-metallic ferromagnetism in LaMnO3at elevated pressure

By means of hybrid density functional theory we investigate the evolution of the structural, electronic, and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO3 under pressure. We predict a transition from a low-pressure antiferromagnetic (AFM) insulator to a high-pressure ferromagnetic (FM) transport half metal (tHM), characterized by a large spin polarization (approximate to 80-90%). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the Pnma orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S = 2, m(Mn) = 3.7 mu(B)) to low-spin (S = 1, m(Mn) = 1.7 mu(B)) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite.