Strong Jahn-Teller Research Articles

Multimode Luminescence Tailoring in PMA4Na(In,Sb)Cl8 Organic-inorganic Hybrid Metal Halide via Rigid Benzene Ring Induced Local Lattice Distortion.

Low dimensional organic-inorganic hybrid metal halide materials have attracted extensive attention due to their superior optoelectronic properties. However, low photoluminescence quantum yields (PLQYs) caused by parity-forbidden transition hinder their further application in optoelectronic devices. Herein, a novel yellow-emitting PMA4Na(In,Sb)Cl8 (C7H10N+, PMA+) low-dimensional OIMHs single crystal with a PLQY as high as 88 % was successfully designed and synthesized, originating from the fact that the doping of Sb3+ effectively relaxes the parity-forbidden transition by strong spin-orbit (SO) coupling and Jahn-Teller (JT) interaction. The as-prepared crystal shows an efficient dual emission peaking 495 and 560 nm at low temperature, which are ascribed to different levels of 3P1→1S0 transitions of Sb3+ in [SbCl6]3- octahedral caused by JT deformation. Moreover, wide-range luminescence tailoring from cyan to orange can be achieved through adjusting excitation energy and temperature because of flexible [SbCl6]3- octahedral in the PNIC lattice. Based on a relative stiff lattice environment, the 560 nm yellow emission under 350 nm light excitation exhibits abnormal anti-thermal quenching from 8 to 400 K owing to the suppression of non-radiative transition. The multimode luminescence regulation enriches PMA4Na(In,Sb)Cl8 great potential in the field of optoelectronics such as temperature sensing, low temperature anti-counterfeiting and WLED applications.

Tunable energy bandgap of Fe-doped (Bi, Li) co-substituted barium titanate

In this work, a polycrystalline Ba0.96(½ Bi, ½ Li)0.04Ti(1-x) FexO3; (0 ≤ x ≤ 0.08) ceramics have been synthesised using a solid-state reaction method. The prepared systems were explored to detect the impact of Fe substitution on the energy bandgap of the ceramics. XRD patterns confirmed that there is a structural phase transition from tetragonal (P4 mm) to hexagonal (P63/mmc) phase as the concentration of Fe increases. Rietveld refinement was performed to obtain the lattice information. Furthermore, Raman spectroscopic analysis confirmed the structural information obtained from XRD study. The average bond length variations, strain evolutions, crystallite size, and theoretical density have been calculated from the structural analysis. It is found that the lower Fe concentration with the tetragonal phase showed a strong Jahn-Teller effect. Meanwhile, the higher concentration of Fe led to phase transition to hexagonal phase with fewer structural distortions. The optical band gap species were investigated through UV-Vis. Following the onset of defects induced by acceptor ions, an exciting band gap reduction up to 2.09 eV for the sample with x = 0.08 was attained. ESR and PL spectroscopies analyses showed that in the hexagonal phase region more defects are formed giving rise to promoting band gap narrowing. Furthermore, the ac conductivity analysis indicates the appearance of defect levels due to the formation of oxygen vacancies. This study demonstrates that the right choice of Fe content in the host material can tune the energy band gap significantly in the BLBTF system and may be exploited in photovoltaics in the visible region.

Fast and Facile Microwave Synthesis of Cubic CuFe2O4 Nanoparticles for Electrochemical CO2 Reduction

Electrochemical CO2 reduction is a promising strategy for the sustainable synthesis of carbon-based chemicals or fuels such as CO, or CH4.[1] Copper-based materials are of special interest due to their broad substrate scope. While copper itself is the first known metal catalyst able to directly yield CH4, CO, methanol, or C2+ products such as ethanol can also be obtained, e.g. from oxide derived catalysts.[2][3] Another interesting copper oxide is the spinel CuFe2O4, due to its versatile electronic and magnetic properties. Those can be tuned via variations in the cation distribution between octahedral and tetrahedral sites.[4] Changes in the cation distribution, or degree of inversion, can additionally lead to a structural transformation between the cubic and tetragonal form, due to the strong Jahn-Teller effect of Cu2+. Both degree of inversion and structure are strongly dependant on the synthesis conditions.[4]In this contribution we introduce a fast microwave-assisted solvothermal synthesis of cubic CuFe2O4. Very short synthesis times of 1 min and low temperatures of 120 °C in ethylene-glycol/water mixtures could be realised, without a loss of phase-purity and no necessity for subsequent thermal treatment.[5] The degree of inversion was found to decrease with increasing synthesis time, whereas the particle size and crystallinity was almost independent of the synthesis conditions. The influence of the synthesis conditions – and thus material properties – on the activity in electrochemical CO2 reduction to CO in 0.1 M KHCO3 was investigated. The best activity was obtained for CuFe2O4 with an intermediate degree of inversion of approx. 0.75, together with a large crystallite size and micro-strain. Since hydrogen was produced as the only side-product, CuFe2O4 can be an interesting candidate for the production of syngas.

Ca2MnO3X (X = Cl, Br) Oxyhalides with 1-Dimensional Ferromagnetic Chains of Square-Planar S = 2 Mn3.

Mixed anion oxyhalides with the formula Ca2MnO3X (X = Cl, Br) are synthesized using solid-state reaction methods. These two materials crystallize in a novel structure type due to the small ionic radius of Ca and the strong Jahn-Teller effect of Mn3+. The resulting structure (space group Cmcm) contains one-dimensional chains of MnO4 square planes, with an angle of ∼120° between neighboring planes. At low temperatures, the two materials adopt magnetic arrangements, with ferromagnetic chains coupled antiferromagnetically. On applying a magnetic field, both materials experience spin-flop transitions.

A new Mn-based layered cathode with enlarged interlayer spacing for potassium ion batteries

Layered Mn-based cathode (KxMnO2) has attracted wide attention for potassium ion batteries (PIBs) because of its high specific capacity and energy density. However, the structure and capacity of KxMnO2 cathode are constantly degraded during the cycling due to the strong Jahn-Teller effect of Mn3+ and huge ionic radius of K+. In this work, lithium ion and interlayer water were introduced into Mn layer and K layer in order to suppress the Jahn-Teller effect and expand interlayer spacing, respectively, thus obtaining new types of K0.4Mn1-xLixO2·0.33H2O cathode materials. The interlayer spacing of the K0.4MnO2 increased from 6.34 to 6.93 Å after the interlayer water insertion. X-ray photoelectron spectroscopy studies demonstrated that proper lithium doping can effectively control the ratio of Mn3+/Mn4+ and inhibit the Jahn-Teller effect. In-situ X-ray diffraction exhibited that lithium doping can inhibit the irreversible phase transition and improve the structural stability of materials during cycling. As a result, the optimal K0.4Mn0.9Li0.1O2·0.33H2O not only delivered a higher capacity retention of 84.04 % compared to the value of 28.09 % for K0.4MnO2·0.33H2O, but also maintained a greatly enhanced rate capability. This study provides a new opportunity for designing layered manganese-based cathode materials with high performance for PIBs.

Spintronic State‐Induced Interface Reconfiguration of 2H‐Perovskite Related Oxides for Pseudocapacitance Increase

AbstractPerovskite oxides are able to realize pseudocapacitive energy storage through oxygen anion intercalation. Herein, it is demonstrated that, based on the three 2H‐perovskite related oxides Sr6Co5O15‐x, Sr5Co4O12‐x, and Sr5Co3NiO12‐x as the research objects, metal oxyhydroxides generated through surface reconfiguration during electrochemical processes can contribute extra capacity besides the oxygen‐anion intercalation. This is because the electron spin state controlled by the transition metal valence state in the pseudo trigonal prisms for the 2H‐perovskite structure will affect the Jahn–Teller (JT) distortion. The strong JT distortion of Co2+ and Ni3+ would elongate metal‐O bonds, thereby helping the formation of metal oxyhydroxides on the surface of the pristine material. Among the three 2H‐perovskite related oxides, the specific capacity of Sr5Co3NiO12‐x having the most Co2+ and Ni3+ contents showed the highest capacity increase. This study will provide a promising pathway to develop advanced perovskite‐like oxide pseudocapacitive materials.

Deciphering the electrocatalysis essence of cobalt diselenide in lithium-sulfur electrochemistry from crystal-phase engineering

Polysulfide shuttle effect and sluggish sulfur reaction kinetics severely impede the cycling stability and sulfur utilization of lithium-sulfur (Li-S) batteries. Precisely designing effective electrocatalysts to accelerate sulfur conversion kinetics and suppress polysulfide migration is urgently needed. Generally, electrocatalytic activity is highly dependent on the electronic structures of catalysts, while the crystal phases play a pivotal role in governing the electronic structures. However, the “crystal phase - electronic structure - catalytic capability” relation chain of Li-S batteries catalysts usually lacks clear elucidation. Herein, crystal-phase engineering is reported by regulating phases of CoSe2 to enhance the electrochemical performance of Li-S batteries. Experimental investigations and atomic level analyses reveal that the orthorhombic CoSe2 can trap and accelerate the conversion of sulfur species more effectively than cubic one. Electronic structure analysis further demonstrates that the superior electrocatalytic activity is originated from the strong Jahn-Teller distortion. As a result, the Li-S batteries with orthorhombic CoSe2 modified separator exhibit a high initial capacity of 1390.7 mAh g−1 at 0.1C and outstanding rate performance with a specific capacity of 738.4 mAh g−1 at 3C. Moreover, even at a high sulfur loading of 10.09 mg cm−2, a favorable initial areal capacity of 12.0 mAh cm−2 is achieved at 0.05C, and the battery still maintains at 7.9 mAh cm−2 over 50 cycles under 0.1C. This work may provide a new strategy for effective catalyst design in Li-S batteries.

Ground-state structure, orbital ordering and metal-insulator transition in double-perovskite PrBaMn2O6

In recent years, A-site ordered half-doped double-perovskite manganites $\rm RBaMn_2O_6$ (R=rare earth) have attracted much attention due to their remarkable physical properties and a prospect of application as magnetoresistance, multiferroic, and oxygen storage materials. The nature of the ground state in ${\rm RBaMn_2O_6}$ as well as sequence of phase transitions taking place at cooling are not yet well understood due to complexity in both experimental and theoretical studies. Here we address the origin of the ground-state structure in PrBaMn$_2$O$_6$ as well as its electronic and magnetic properties. Utilizing GGA+U approach and specially designed strategy to perform structural optimization, we show that the system has two competing AFM-A and AFM-CE magnetic structures with very close energies. The AFM-A structure is a metal, while AFM-CE is an insulator and the transition to the insulating state is accompanied by the charge Mn$^{3+}$/Mn$^{4+}$, and orbital $3x^2-r^2$/$3y^2-r^2$ orderings. This orbital ordering results in strong cooperative Jahn-Teller (JT) distortions, which lower the crystal symmetry. Our findings give a key to understanding contradictions in available experimental data on ${\rm PrBaMn_2O_6}$ and opens up the prospects to theoretical refinements of ground-state structures in other ${\rm RBaMn_2O_6}$ compounds.

Suppressing Jahn-Teller distortion and phase transition of K0.5MnO2 by K-site Mg substitution for potassium-ion batteries

In the development of cathode materials for potassium-ion batteries, K0.5MnO2 cathode has shown great potential due to its high capacity and high energy density. However, it suffers from poor cycle stability due to the strong Jahn-Teller effect of Mn3+ and structure degradation. Here, by introducing Mg into the potassium layer, we design a new layered P3-type K0.5Mg0.15[Mn0.8Mg0.05]O2 cathode material. By using X-ray diffraction and absorption techniques, it is revealed that Mg ions in potassium layer can not only suppress the Jahn-Teller effect and prevent phase transition effectively, but also enhance the kinetics and thermodynamic properties of the cathode materials, thus greatly improve the cycle stability and rate capability. The strategy may open a new opportunity to design layered cathode materials for high performance potassium-ion batteries.

Effective tuning of magnetic anisotropy in distorted pentagonal bipyramidal Ni(II) complexes via substitution of axial coligands.

A series of Ni(II) complexes with pyridine-based macrocyclic ligand L (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) with general formula [Ni(L)(X)2]0/2+ (X = Br- (1), I- (2), CH3CN (3), NCS- (4), imidazole (5)) was prepared and thoroughly investigated. X-ray molecular structures confirmed pentagonal bipyramidal geometry for all studied complexes with a strong Jahn-Teller distortion in the pentagonal equatorial plane and significantly elongated Ni-O distance(s) with a decrease of this distortion by varying axial coligands (CH3CN > Br- > I- > NCS- > imidazole). Direct current magnetic measurements revealed the easy-axis type of magnetic anisotropy with negative as well as positive axial zero-field-splitting parameter D ranging from +6.8 to -14.5 cm-1, which remains not affected in the halogenido series Cl- → Br- → I-, but which increases in the series with N-axial ligands in order CH3CN → NCS- → imidazole. Theoretical calculations helped to elucidate (i) the final coordination numbers 6 + 1 for 1 and 2, and 5 + 2 for 2-5, (ii) the pattern of splitting of d-orbitals, contributions of excited states to the final D-values and their final signs, and (iii) the complexity in the variation of the D and E parameters with elongation of axial bond distances in such strongly distorted systems. The studied complexes did not show any alternating magnetic susceptibility signal, but it was clearly documented that the magnetic anisotropy of the pentagonal bipyramidal Ni(II) complexes can be modulated/tuned by variation of axial coligands. Nevertheless, great care has to be taken for symmetry of the equatorial ligand field.

Insight into the effects of oxygen vacancy on the toluene oxidation over α-MnO2 catalyst

In order to clarify the role of oxygen vacancy (OV), five α-MnO2 catalysts with abundant OVs are fabricated via a novel and facile redox-precipitation approach and employed to the toluene oxidation in air. The concentration of OVs in α-MnO2 catalysts is regulated via the alkyl chain length of alcohols, and its correlation with catalytic performances is scientifically investigated based on various characterization technologies and density functional theory (DFT) calculation. The α-MnO2-C2 catalyst exhibits excellent catalytic activity (T90 = 217 °C), stability, and water resistance for toluene oxidation in air. The OVs can induce the new bandgap states (BGS), which upshift the antibonding orbitals relative to the Fermi level (Ef), eventually favoring the formation of adsorbed active oxygen species. Furthermore, the OVs cause an increase in the amount of Mn3+, resulting in the elongated Mn–O bonds due to the strong Jahn-Teller effect of Mn3+. Therefore, the synergistic effects of OVs benefit toluene oxidation through L-H and MvK mechanisms over the prepared α-MnO2-Cx catalysts. This work reveals the important role of OVs in the promotion of toluene catalytic oxidation activity and also may provide new insights for the design of high-performance VOCs oxidation elimination catalyst.

Electronic Coherences Steer the Strong Isotope Effect in the Ultrafast Jahn-Teller Structural Rearrangement of Methane Cation upon Tunnel Ionization.

We report on fully quantum electronic-nuclear dynamics following sudden ionization from the neutral in the three lowest electronic states of the CH4+ and CD4+ cations. There is a strong Jahn-Teller effect in the Franck-Condon region, and we employ two nuclear degrees of freedom that span the internal coordinates involved in the Jahn-Teller coupling. The initial state results from tunneling ionization by a strong IR field which coherently pumps the three lowest states of the cation, D0, D1, and D2. The quantum dynamical simulations show that a strong isotope effect occurs when the ionization significantly accesses the D2 state of the cation in the Franck-Condon region. The computed isotope effect is larger than expected on the basis of the effective mass ratio. The strong effect is due to fast oscillations of the electronic coherences between the D2 and the D1 and D0 electronic states and their modulation by the nonadiabatic couplings before a significant onset of nuclear motion. The magnitude of the effect is similar to the one that we previously reported for a sudden photoionization process. A strong isotope effect has been observed in high harmonic spectroscopy studies of the very short time dynamics Jahn-Teller structural rearrangement of the methane cation upon sudden ionization.

Tb3+,Mn3+ co-doped La2Zr2O7 nanoparticles for self-referencing optical thermometry

In optical thermometry based on fluorescence intensity ratio (FIR) of two thermally coupled energy states, the sensitivity and signal discriminability are limited by the energy gap between two thermally coupled excited states. Here, we have employed the strategy of combining rare earth ion Tb3+ and transition metal ion Mn3+ with completely different thermal-quenching behaviors to obtain high thermometric sensitivity using uniform La2Zr2O7:Tb3+,Mn3+ nanoparticles (NPs) synthesized by a molten salt synthesis method. We have proposed a model to describe the electronic transitions of Mn3+ ion doped in the compound. Based on Tanabe-Sugano diagram, the excitation peaks centered at 345 and 450 nm are assigned to the 5E → 3E and 1E transitions. Due to the strong Jahn-Teller effect, the 5E ground state is split into two components, namely 5E′ and 5E΄΄. The excitation at upper 3E state is followed by two emission bands to the split ground state (3E → 5E′ and 5E″) which are dominated upon heating. Additionally, two transitions from lower excited level of 1E to the split ground states are possible (1E → 5E′ and 5E″) which are strengthened upon cooling. Mn3+ ion featuring d-d transitions and Tb3+ with f-f transitions have high and low temperature dependencies, which allow their use as luminescence probe and reference, respectively. The emissions intensity ratio of Tb3+ at 543 nm to Mn3+ at 719 nm is explored over the temperature range of 98–338 K. The maximum relative sensitivity value from our La2Zr2O7:Tb3+,Mn3+ NPs is obtained as high as 1.82%·K−1 with a minimum temperature uncertainty of ~0.27K at 298 K. The temperature-dependent FIR has also shown repeatable results which indicate that the La2Zr2O7:Tb3+,Mn3+ NPs are a promising material for dual activator ratiometric optical thermometry.

In celebration of the 70thbirthday of Peter Klüfers

In celebration of the 70^thbirthday of Peter Klüfers

Octahedral distortion enhances exceptional oxygen catalytic activity of calcium manganite for advanced Zn-Air batteries

Calcium manganite (CMO) holds great potential for OER/ORR catalysis. But, the weak interaction between the oxygen intermediates and reaction sites leads to sluggish catalytic kinetics. Herein, to trigger the exceptional catalytic activity of CMO, we propose a thermal-induced MnO 6 octahedral distortion strategy to advance the absorption capability of intermediate reactants. Refined structural analysis and theoretical calculation reveal that the strong Jahn-Teller distortion of MnO 6 is capable of optimizing the surface electron redistribution and accelerating the electron transfer between the oxygen and Mn sites, which significantly boosts the intrinsic oxygen catalytic activities of the distorted CMO (D-CMO). Notably, the D-CMO functions as excellent bifunctional oxygen electrocatalysts and enables a high-performance solid-state Zn-air battery with satisfactory mechanical strength. Specifically, the as-assembled battery displays a remarkable open circuit voltage of 1.46 V and high peak power density of 149 mW cm −2 , even outstripping noble-metal-based counterparts. Modulating the octahedral units to ignite the intrinsic oxygen catalytic activity provides enlightening clues to design perovskite-type air cathodes. • A thermal-induced MnO 6 octahedral distortion strategy is proposed. • The octahedral distortion can optimize electron density and e g electron-filling states. • The octahedron-distortion CaMnO 3 (D-CMO) owns excellent oxygen catalytic activity. • The solid-state ZAB exhibits a high peak power density of 149 mW cm −2 .

A novel family of hepta-coordinated Cr(III) complexes with a planar pentadentate N3O2 Schiff base ligand: synthesis, structure and magnetism

A series of seven-coordinate pentagonal-bipyramidal (PBP) Cr(III) complexes with pentadentate pyridine-based ligands, 2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone), H2DAPMBH (H2LOCH3) or 2,6-diacetylpyridine bis(benzoylhydrazone), H2DAPBH (H2L) and different axial ligands have been prepared. The reaction of the H2LOCH3 with CrCl2·4H2O in methanol or CrCl3·6H2O in CH3CN led to a novel seven-coordinate pentagonal-bipyramidal (PBP) complex [Cr(HLOCH3)Cl2] (1) with the mono-deprotonated chelating ligand in the equatorial plane and two apical Cl atoms. Then, taking advantage of lability of the apical Cl ligands in 1, a number of PBP CrIII complexes with charged (viz. CH3O−, N3−, CN−, NCS−) and neutral (viz. CH3OH, H2O) apical ligands was obtained and characterized: [Cr(HLOCH3)Cl2]·4CHCl3 (1), [Cr(HLOCH3)Cl2]·CH3OH (1a), [Cr(HLOCH3)(H2O)Cl]PF6·CH3OH (2), [Cr(HL)(H2O)Cl]ClO4·0.25H2O (3), [Cr(HLOCH3)(H2O)2](NO3)2·H2O·C2H5OH (4), [Cr(LOCH3)(CH3OH)(OCH3)]·CH3OH (5), [Cr(HLOCH3)(NCS)2]·1.5H2O (6), [Cr(HLOCH3)(N3)2]·xH2O (7, x = 0.2, 8, x = 0, 9, x = 0, three phases in the same synthesis), [Cr(LOCH3)(N3)2][Na(CH3OH)2]·2CH3OH (10), [Cr(LOCH3)(CN)2][Na(H2O)(C2H5OH)] (11). Single crystal X-ray analysis reveals that all the complexes 1–11 have the PBP geometry with a pentadentate ligand in a form of [HLOCH3]− or [LOCH3]2− in the equatorial plane. The PBP complexes are prone to aggregate into dimers or polymers, either due to strong hydrogen bonds or due to the transformation of terminal ligands into bridging between different metallic centers. All complexes 1–11 exhibit considerable in-plane distortion of the CrN3O2 pentagon due to the shift of the CrIII ion from the central position, which is caused by the strong first-order Jahn-Teller (JT) effect for the high-spin 3d3 configuration in the PBP ligand field. The mechanism of JT distortions is rationalized in terms of DFT calculations. DC magnetic measurements indicate a high-spin (S = 3/2) ground state of complex [Cr(HLOCH3)Cl2]·4CHCl3 (1); theoretical analysis of its magnetic properties reveals negative zero-field splitting energy with the anisotropy parameter D = –1.8 cm−1 and weak dimer-like antiferromagnetic spin coupling J = –0.23 cm−1 between neighboring PBP units [CrIII(HLOCH3)Cl2] mediated by π-stacking of planar H2LOCH3 ligands.

Strong static and dynamic Jahn-Teller and pseudo-Jahn-Teller effects in niobium tetrafluoride.

We present a first-principles study of the static and dynamic aspects of the strong Jahn-Teller (JT) and pseudo-JT (PJT) effects in niobium tetrafluoride, NbF4, in the manifold of its electronic ground state, 2E, and its first excited state, 2T2. The complex topography of the full-dimensional multi-sheeted adiabatic JT/PJT surfaces is analyzed computationally at the complete-active-space self-consistent-field (CASSCF) and multireference second-order perturbation levels of electronic structure theory, providing a detailed characterization of minima, saddle points, and minimum-energy conical intersection points. The calculations reveal that the tetrahedral (Td) configuration of NbF4 undergoes strong JT distortions along the bending mode of e symmetry, yielding tetragonal molecular structures of D2d symmetry with Td → D2d stabilization energies of about 2000 cm-1 in the X̃2E state and about 6400 cm-1 in the Ã2T2 state. In addition, there exists strong X̃2E-Ã2T2 PJT coupling via the bending mode of t2 symmetry, which becomes important near the crossing seam of the X̃2E and Ã2T2 potential energy surfaces. A five-state five-mode JT/PJT vibronic-coupling Hamiltonian is constructed in terms of symmetry-invariant polynomial expansions of the X̃2E and Ã2T2 diabatic potential energy surfaces in the e and t2 bending coordinates. The parameters of the Hamiltonian are determined by a least-squares fit of its eigenvalues to the CASSCF ab initio data. The vibronic spectra and the time evolution of adiabatic electronic population probabilities are computed with the multi-configuration time-dependent Hartree method. The complexity of the spectra reflects the effects of the exceptionally strong E × e and T2 × e JT couplings and (E + T2) × (e + t2) PJT coupling. The time evolution of the populations of the adiabatic electronic states after the initial preparation of the Ã2T2 state reveals the femtosecond nonadiabatic dynamics through a multidimensional seam of conical intersection. These results represent the first study of the static and dynamical JT/PJT effects in the X̃2E and Ã2T2 electronic states of NbF4.

Unraveling the Orbital Physics in a Canonical Orbital System KCuF_{3}.

We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF_{3} using the Cu L_{3}-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu^{2+} dd orbital excitations. These high-energy modes display good agreement with the abinitio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF_{3}, which predominantly drives orbital ordering.

Pressure-induced transition from Jeff=1/2 to S=1/2 states in CuAl2O4

The spin-orbit entangled (SOE) Jeff-state has been a fertile ground to study novel quantum phenomena. Contrary to the conventional weakly correlated Jeff=1/2 state of 4d and 5d transition metal compounds, the ground state of CuAl2O4 hosts a Jeff=1/2 state with a strong correlation of Coulomb U. Here, we report that surprisingly Cu2+ ions of CuAl2O4 overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the Jeff=1/2 state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only S=1/2 state for pressure higher than Pc=8 GPa. This phase transition from the Mott insulating Jeff=1/2 to the S=1/2 states is a unique phenomenon and has not been reported before. Our study offers a rare example of the SOE Jeff-state under strong electron correlation and its pressure-induced transition to the S=1/2 state.

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO$_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn--O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn--O bond lengths, orbital order and consequently the charge localization and polarity are tunable via strain.