Jahn-Teller Ions Research Articles

Manifestation of quantum rotor orbital excitations in Raman spectra of Jahn-Teller crystal LaMnO3

Materials, consisting of Jahn-Teller (JT) ions, such as cuprates and manganites, display many outstanding properties, including high temperature superconductivity and colossal magnetoresistance. There, the role of JT effect, although widely recognized, is still elusive. Here we show that these materials have vibronic excitations, related to local deformations rotating around JT ions in the dynamic limit, arising from linear electron-vibrational coupling in the “Mexican hat” potential profile. Their energy depends on total angular momentum, which is quantized, as in quantum rotors. We found them in the representative JT compound of orthorhombic manganites, LaMnO3. Since the “Mexican hat” potential energy surface is double-valued, they show up near the ground and excited states of JT ions. Recently, by using spectroscopic ellipsometry technique, we showed that they appear in the excited state in the form of sidebands, accompanying the electron transition between the JT split orbitals at neighboring Mn3+ ions. Here, by using Raman scattering technique, we show that they also exist near the ground state. The found quantum rotor excitations may play an important role in many unusual properties observed in these materials.

Interaction of spin-orbital-lattice degrees of freedom: Vibronic state of the corner-sharing-tetrahedral frustrated spin system HoBaFe4O7 by dynamical Jahn-Teller effect

A powder inelastic neutron-scattering study of ${\mathrm{HoBaFe}}_{4}{\mathrm{O}}_{7}$ (HBFO) revealed characteristic magnetic excitations associated with geometrical spin frustration. Dispersionless excitations in energy ($\ensuremath{\omega}$) and wave-vector ($Q$) space are observed at several discrete energies. Some of them can be attributed to crystal-field excitations of ${\mathrm{Ho}}^{3+}$ in octahedral symmetry, but the others are explained instead by the vibronic state of the ${\mathrm{Fe}}^{2+}$ dynamical Jahn-Teller effect with orbit-spin coupling, indicating interaction among the spin, the orbital, and the lattice degree of freedom. The antiferromagnetic HBFO lattice has cubic symmetry, and both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ reside on corner-sharing tetrahedra with a number ratio of 3:1. Even though ${\mathrm{Fe}}^{2+}$ is a Jahn-Teller active ion in tetrahedral symmetry, the system does not exhibit any static lattice distortion to the lowest temperature studied (4 K). The observed excitations can be understood by considering the dynamical interaction among spin-orbital-lattice degrees of freedom, indicating that spin fluctuation due to the frustration effect induces the dynamical Jahn-Teller effect, although in most cases a Jahn-Teller active ion ${\mathrm{Fe}}^{2+}$ takes the static Jahn-Teller effect in a magnetic oxide system.

Contribution from structural Jahn–Teller ions to the elastic and ferroelectric properties of lithium niobate and lithium tantalate

Elastic and ferroelectric characteristics of single crystals of lithium niobate and tantalate are investigated in a wide range of temperatures by complex acoustooptic means. The contribution from Jahn–Teller NbO6 and TaO6 systems to the characteristics of elastic moduli, ultrasonic attenuation, and nonlinear optical coefficients is analyzed using a new phenomenological model. It is hypothesized that the displacement of Nb5+ and Ta5+ ions, which exhibit the second-order Jahn–Teller effect along trigonal axis C, and the subsequent ordering of octahedral, result in unusual elastic and ferroelectric properties.

Unveiling the Local Structure of Cu2+ Ions from d-Orbital Splitting. Application to K2ZnF4:Cu2+ and KZnF3:Cu2+

This work investigates why Jahn-Teller (JT) transition-metal ions like Cu2+ (d9) or Mn3+ (d4) with octahedral coordination exhibit low-symmetry distortions of the CuF64- (or MnF63-) octahedron, when Cu2+ replaces Zn2+ either in the cubic perovskite KZnF3 or in the layer perovskite K2ZnF4. The aim is to establish correlations between the splitting Δe of the parent Oh eg(x2 - y2, 3z2 - r2) d orbitals and the low-symmetry local distortion ρ, allowing us to unravel the local structure of Cu2+ introduced as an impurity into such perovskites. However, the question of whether tetragonal a1g - b1g splitting Δe of Cu2+ is proportional to ρ or Δe contains additional contributions from the rest of the lattice (crystal anisotropy) is controversial. Here we show that Δe depends linearly on ρ. High-pressure experiments and compound series involving JT ions give support for the proposed scenario and provide correlations relating ρ and Δe in Cu2+ and Mn3+ systems, from which the electron-vibration coupling for the E⊗e JT effect is obtained experimentally.

Protonation State of MnFe and FeFe Cofactors in a Ligand-Binding Oxidase Revealed by X-ray Absorption, Emission, and Vibrational Spectroscopy and QM/MM Calculations.

Enzymes with a dimetal-carboxylate cofactor catalyze reactions among the top challenges in chemistry such as methane and dioxygen (O2) activation. Recently described proteins bind a manganese-iron cofactor (MnFe) instead of the classical diiron cofactor (FeFe). Determination of atomic-level differences of homo- versus hetero-bimetallic cofactors is crucial to understand their diverse redox reactions. We studied a ligand-binding oxidase from the bacterium Geobacillus kaustophilus (R2lox) loaded with a FeFe or MnFe cofactor, which catalyzes O2 reduction and an unusual tyrosine-valine ether cross-link formation, as revealed by X-ray crystallography. Advanced X-ray absorption, emission, and vibrational spectroscopy methods and quantum chemical and molecular mechanics calculations provided relative Mn/Fe contents, X-ray photoreduction kinetics, metal-ligand bond lengths, metal-metal distances, metal oxidation states, spin configurations, valence-level degeneracy, molecular orbital composition, nuclear quadrupole splitting energies, and vibrational normal modes for both cofactors. A protonation state with an axial water (H2O) ligand at Mn or Fe in binding site 1 and a metal-bridging hydroxo group (μOH) in a hydrogen-bonded network is assigned. Our comprehensive picture of the molecular, electronic, and dynamic properties of the cofactors highlights reorientation of the unique axis along the Mn-OH2 bond for the Mn1(III) Jahn-Teller ion but along the Fe-μOH bond for the octahedral Fe1(III). This likely corresponds to a more positive redox potential of the Mn(III)Fe(III) cofactor and higher proton affinity of its μOH group. Refined model structures for the Mn(III)Fe(III) and Fe(III)Fe(III) cofactors are presented. Implications of our findings for the site-specific metalation of R2lox and performance of the O2 reduction and cross-link formation reactions are discussed.

The role of Jahn–Teller ions in the creation of domain structures in lithium niobate

Using tightly focused laser beams, features of space charge fields (∼107 V/m) are studied through the photoionization of doped Jahn–Teller Fe2+ ions in LiNbO3 single crystals. These fields can be used for selective formation of the inverse domain state following the additional application of a field with a strength below the coercive field. The characteristics of laser-induced domains and periodic domain structures are studied by laser-acoustic means.

Molecular geometric phase from the exact electron-nuclear factorization

The Born-Oppenheimer electronic wavefunction $\Phi_R^{BO}(r)$ picks up a topological phase factor $\pm 1$, a special case of Berry phase, when it is transported around a conical intersection of two adiabatic potential energy surfaces in $R$-space. We show that this topological quantity reverts to a geometric quantity $e^{i\gamma}$ if the geometric phase $\gamma = \oint \mathrm{Im} \langle \Phi_R |\nabla_{\mu} \Phi_R\rangle \cdot d\mathbf{R}_{\mu}$ is evaluated with the conditional electronic wavefunction $\Phi_R(r)$ from the exact electron-nuclear factorization $\Phi_R(r)\chi(R)$ instead of the adiabatic function $\Phi_R^{BO}(r)$. A model of a pseudorotating molecule, also applicable to dynamical Jahn-Teller ions in bulk crystals, provides the first examples of induced vector potentials and molecular geometric phase from the exact factorization. The induced vector potential gives a contribution to the circulating nuclear current which cannot be removed by a gauge transformation. The exact potential energy surface is calculated and found to contain a term depending on the Fubini-Study metric for the conditional electronic wavefunction.

Potential energy surface as a key to understanding the structure and properties of short‐living radical ions of cyclic organic molecules

The review proposed summarizes the results of investigations on the adiabatic potential energy surfaces (PESes) for the radical ions of some derivatives of highly symmetric organic molecules such as benzene and cyclohexane. The results obtained show that the main feature of the PESes of highly symmetric Jahn–Teller ions, namely conical intersection, may persist for their low‐symmetric derivatives. Hence, their PESes have a pseudorotational shape resulting from the intersection avoidance. A distinctive feature of radical anions of fluorine containing aromatic compounds is the planar structure disturbance due to the vibronic coupling of the ground π and low‐lying excited σ states. The data on the PES structure including the positions and relative energies of its extrema, curvature in their vicinity, and stationary point interrelations provide the foundation for understanding the spectral properties and reactivity of radical ionic species. Examples of applying the PES study results to experimental data interpretation are given. © 2015 Wiley Periodicals, Inc.

Phase Stabilization of Fe Substituted SmMn2O5: SmFeMnO5

The SmFeMnO5 phase has been stabilized using mechanical milling and respective high temperature sintering of the oxides in stoichiometric ratio. The crystal structure is studied by Rietveld analysis of the X-ray diffraction data obtained at room temperature. Analysis is done by considering Fe3+ ions at the Mn3+ sites in the parent SmMn2O5 structure and the results indicate the formation of SmFeMnO5. A change in bond angles and bond distances is obtained due to the reduction in the percentage of Mn3+ Jahn–Teller ions. Isothermal magnetization study at room temperature reveal ferromagnetic nature of the material with a saturation magnetization of 6.57 emu/g. A linear decrease in resistivity with temperature is obtained and the activation energy is calculated from Arrhenius plot. Impedance studies show decrease in dielectric constant values with frequency and temperature due to Fe substitution in the SmMn2O5 structure.

Kondo Effect of a Jahn–Teller Ion Vibrating in a Cubic Anharmonic Potential

We discuss the Kondo effect in a spinless two-orbital conduction electron system coupled with anharmonic Jahn-Teller vibration by employing a numerical renormalization group technique. When a temperature $T$ is decreased, we encounter a plateau of $\log 3$ entropy due to quasi-triple degeneracy of local low-energy states, composed of vibronic ground states and the first excited state with an excitation energy of $\Delta E$. Around at $T$$\approx$$\Delta E$, we observe an entropy change from $\log 3$ to $\log 2$. This $\log 2$ entropy originates from the rotational degree of freedom of the vibronic state and it is eventually released due to the screening by orbital moments of conduction electrons, leading to the Kondo effect of a Jahn-Teller ion. The Kondo temperature is explained by the effective $s$-$d$ model with anisotropic exchange interactions.

Chaos in Jahn–Teller Rattling

We unveil chaotic behavior hidden in the energy spectrum of a Jahn-Teller ion vibrating in a cubic anharmonic potential as a typical model for rattling in cage-structure materials. When we evaluate the nearest-neighbor level-spacing distribution $P(s)$ of eigenenergies of the present oscillator system, we observe the transition of $P(s)$ from the Poisson to the Wigner distribution with the increase of cubic anharmonicity, showing the occurrence of chaos in the anharmonic Jahn-Teller vibration. The energy scale of the chaotic region is specified from the analysis of $P(s)$ and we discuss a possible way to observe chaotic behavior in the experiment of specific heat. It is an intriguing possibility that chaos in nonlinear physics could be detected by a standard experiment in condensed matter physics.

Superstructure of atomic displacements in cubic compounds Zn0.9Ni0.1S and Zn0.7Ni0.3O

The fine structure of cubic compounds Zn0.9Ni0.1S and Zn0.7Ni0.3O has been discussed. At room temperature, their diffraction patterns contain, along with intense Bragg reflections of the face-centered cubic phase, superstructure diffuse maxima demonstrating the ordering of short-wavelength deformations due to Jahn-Teller nickel ions.

Fine structure of the cubic semiconductor compound Zn0.9Ni0.1S

The fine structure of a single crystal of the Zn0.9Ni0.1S cubic compound synthesized by the chemical transport method has been investigated at room temperature using thermal neutron diffraction. It has been found that the diffraction patterns of this compound, along with strong Bragg reflections of the facecentered cubic phase, include a system of diffuse maxima, which indicate the occurrence of a local deformation of the cubic structure by the Jahn-Teller nickel ions. Results of this experiment have been compared with the previously obtained data for the lightly doped Zn1 − xNixSe single crystal (x = 0.002).

Equation of state and a high-pressure structural phase transition in the gillespite-structured phase Ba0.5Sr0.5CuSi4O10

The equation of state and the crystal structure of the gillespite-structured phase Ba 0.5 Sr 0.5 CuSi 4 O 10 have been determined at seventeen pressures, ambient to 6.5 GPa, using powder neutron diffraction. At ~4.45 GPa, a structural phase transition is observed from the low-pressure structure, in space group P 4/ ncc , to a high-pressure structure in space group P 42 1 2. No experimental evidence was found for the presence of weak superlattice reflections with wave vector (2π/3 a , 2π/3 a , 0) that have recently been observed in the high-pressure, single-crystal X-ray diffraction patterns of the isostructural compounds BaCuSi 4 O 10 and BaCrSi 4 O 10 , and it should be noted that the possibility remains that the crystal structure reported here may represent the average of a similar commensurately ordered phase. The isothermal bulk modulus of the low-pressure phase, determined using a third-order Birch-Murnaghan equation of state, is 81.6(8) GPa with an initial pressure derivative of −3.3(3). The bulk modulus of the high-pressure phase was found to be smaller, estimated as 59.4 GPa. The phase transition appears to be driven by the need to relax the strong over-bonding of the alkaline-earth site, but apparently not that associated with the square-planar copper sites, which remain significantly over-bonded in the high-pressure phase. The phase transition equates to the loss of the centres of symmetry in the low-pressure phase, and results in marked differences in the tilting of the symmetry-independent Si 2 O 7 polyhedral groups, and in the aplanarity of the associated bonded CuO 4 square-planar groups. From the limited data that exists on isostructural compounds of gillespite, the low-pressure structure appears to be unstable at volume reductions of 4–6 %, however the structure of the high-pressure phase seems to depend on whether or not the cation in the square-planar site is a Jahn-Teller ion.

Effect of high-pressure/high-temperature processing on V, Mn, Fe, Co, and Ni substitutions for copper in the structure of CaCu3V4O12

X-ray diffraction data demonstrate that high-pressure (7.0–8.0 GPa), high-temperature (700–1100°C) processing enables partial V, Mn, Fe, Co, and Ni substitutions for copper in the structure of the double perovskite CaCu3V4O12. The lattice parameter of CaCu3 − xNixV4O12 (a = 7.294–7.298 A) exceeds that of CaCu3V4O12 (a = 7.2845 A) even though the Ni2+ ion (0.69 A) is smaller than the Cu2+ ion (0.72 A). This may be due to lattice distortion caused by the presence of two Jahn-Teller ions (Cu2+ and Ni2+). The oxides CaCu2CoV4O12 and CaCu2FeV4O12 are shown to have metallic conductivity.

Improvement of the physical properties of novel (1 − y) Co0.8Cu0.2Fe2O4 + (y) SrTiO3 nanocomposite

Magnetoelectric (ME) nanocomposites (1−y) Co0.8Cu0.2Fe2O4+(y) SrTiO3 (y=40, 50, 60, 80 and 100%) were prepared by standard ceramic method. Phase formation was checked using X-ray diffraction analysis. Both saturation magnetization (Ms) and Curie temperature (TC) decrease with increasing SrTiO3 content. Temperature dependence of the dielectric constant reveals two maxima, one about 550K corresponds to non-stoichiometry and lattice distortions while the second around 900K corresponds to the Curie temperature (TC). The large value of ME output is due to the strain induced by lattice distortion in the ferrite phase by Jahn–Teller ions like Cu. Hence, Jahn–Teller effect in the ferrite leads to polarization in the piezoelectric phase.

Properties of mechanically activated oxide NdMnO3 + δ

Analysis of the behavior of mechanically activated oxides opens additional possibilities for studying their phase states and properties and also for controlling their functional characteristics. The rareearth metal- manganese-oxygen systems were chosen as objects of investigation because of the special interest in them in view of the colossal magnetoresistance effect and the promises of using materials based on these systems in electrochemical and catalytic applications. The fact that these oxides exhibit the cooperative Jahn-Teller effect significantly simplifies the theoretical interpre� tation of experimental results. The Jahn-Teller distor� tion and the Jahn-Teller transition temperature are directly related to the concentration of Jahn-Teller ions. This allows one to obtain more information on the changes in the nonstoichiometry and the size effects caused by the mechanical activation of oxide. Quite intriguing is also the possibility of relating the interactions between mechanically activated oxides and a gaseous medium to the diversity of Jahn-Teller phase states. In this context, the main purpose of this work was to study specific features of structural phase transfor� mations, isotope exchange, and redox processes in metastable states of the rareearth metal-Mn-O ( NdMnO 3+ δ) system that are caused by its mecha� nochemical activation.

Orbital order in La0.5Sr1.5MnO4: Beyond a common local Jahn-Teller picture

The standard way to find the orbital occupation of Jahn-Teller (JT) ions is to use structural data, with the assumption of a one-to-one correspondence between the orbital occupation and the associated JT distortion, e.g. in O6 octahedron. We show, however, that this approach in principle does not work for layered systems. Specifically, using the layered manganite La0.5Sr1.5MnO4 as an example, we found from our x-ray absorption measurements and theoretical calculations, that the type of orbital ordering strongly contradicts the standard local distortion approach for the Mn3+O6 octahedra, and that the generally ignored long-range crystal field effect and anisotropic hopping integrals are actually crucial to determine the orbital occupation. Our findings may open a pathway to control of the orbital state in multilayer systems and thus of their physical properties.

Relationship between orbital structure and lattice distortions in Jahn-Teller systems

Analyzing the orbital structure and lattice distortions in the CE phase of half-doped manganites, we demonstrate that the usual approach directly relating the orbital occupation of Jahn-Teller ions to the displacements of neighboring ligands may be misleading. For the correct identification of orbital structure, it is necessary to take self-consistently into account the electron-lattice interactions, kinetic energy of charge carriers, and crystal-field effects. In certain situations, e.g. in the CE phase of single-layered manganite La$_{0.5}$Sr$_{1.5}$MnO$_4$, the type of orbital ordering strongly deviates from that, which one would deduce from the local lattice distortions.

Giant ultrasonic attenuation in ZnSe doped with Cr and its possible application in crystal characterization

AbstractUltrasonic experiment was done at the frequency of 54 MHz on ZnSe:Cr2+ crystal with concentration of the dopand 3.8 × 1018 cm−3. Giant peak of attenuation of slow shear wave propagating along 〈110〉 crystallographic direction was observed at low temperatures (about 13 K). The peak proved to be due to relaxation process in the Jahn–Teller system. Magnitude of the peak is proportional to the concentration of the dopand. This circumstance forms the basis for a new non‐destructive method of characterization of the diluted crystals, which particularly could be useful for determining the low concentration of the Jahn–Teller ions. Estimation showed that the concentration of the dopand can be registered in ZnSe:Cr2+ crystal of the order of 1016 cm−3. Other Jahn–Teller ions prospective for such characterization are suggested.