Jahn-Teller Transition Research Articles

First-order phase transition inUO2:235Uand17ONMR study

A nuclear magnetic resonance (NMR) study is reported for a $5f$-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, ${\mathrm{UO}}_{2},$ where we have performed magnetic actinide ${(}^{235}\mathrm{U})$ as well as nonmagnetic ligand ${(}^{17}\mathrm{O})$ NMR studies in a $5f$-electron system. The observed ${}^{235}\mathrm{U}$ hyperfine interaction is consistent with an axially symmetric $5f$-wave-function character for ${\mathrm{U}}^{4+}$ below a first-order transition temperature ${T}_{\mathrm{N}}=30.8\mathrm{K}.$ Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the ${}^{17}\mathrm{O}$ spin-echo decay provide evidence of a lattice distortion just below ${T}_{\mathrm{N}},$ which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between $5f$ quadrupoles and lattice distortion, the behavior of the $5f$ quadrupoles can be investigated using uranium NMR, while the lattice distortion can be studied with the nonmagnetic ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear $3\mathit{k}$-type structure is the most likely, because either the $1\mathit{k}$ or $2\mathit{k}$ would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates ${T}_{1}^{\ensuremath{-}1}$ at both sites show a ${T}^{7}$ behavior below ${T}_{\mathrm{N}},$ which suggests the presence of gapless excitations due to strong magnon-phonon coupling.

Induced Jahn-Teller transition in DyPO4 caused by a change in ground state upon level crossing

The transition caused by a change in the ground state of a rare-earth ion upon level crossing, an analogue of the induced Jahn-Teller transition in rare-earth compounds, is observed. It is shown that a jumpwise change in the Jahn-Teller interactions of α and γ symmetries, caused by a change in the corresponding quadrupole moments in DyPO4 upon level crossing, diminishes the critical field by ∼20 kOe and leads to a sharper change in magnetization M and differential magnetic susceptibility dM/dH near the crossover point.

Order-disorder in the Jahn-Teller transition ofLaMnO3:A Raman scattering study

The high-$T$ ``orthorhombic-cubic'' Jahn-Teller (JT) transition of a polycrystalline sample of ${\mathrm{LaMnO}}_{3}$ is studied by Raman scattering. A ${\mathrm{CaMnO}}_{3}$ ceramic sample is used as a reference compound. Dramatic broadening of the ${\mathrm{LaMnO}}_{3}$ Raman modes are observed as T approaches the JT transition temperature ${(T}_{\mathrm{JT}})$ from below. The strong T dependence of the phonon linewidth suggests that considerable lattice disorder is already present at temperatures well below ${T}_{\mathrm{JT}}.$ Our results indicate that the JT transition in ${\mathrm{LaMnO}}_{3}$ is of order-disorder type and is driven by thermally activated ${e}_{g}$ orbital disorder of the ${\mathrm{Mn}}^{3+}$ ion in the ${\mathrm{MnO}}_{6}$ octahedra.

Study of the ferrodistorsive orbital ordering in NaNiO2 by neutron diffraction and submillimeter wave ESR

NaNiO2 has been studied by neutron-powder diffraction, magnetic susceptibility and submillimeter wave ESR. The monoclinic structure at room temperature is characterised by a ferrodistorsive orbital ordering due to the Jahn-Teller (JT) effect of the Ni3+ ions in the low spin state. NaNiO2 undergoes a structural transition at around 480 K, above which the orbital ordering disappears. The high temperature phase is rhombohedral with the layered -NaFeO2 structure ( space group). The magnetic susceptibility exhibits hysteresis and we observe a change of the Curie-Weiss law parameters above the JT transition. The anisotropy of the g-factor at 200 K can be attributed to the JT effect which favours the orbital occupation. Finally, the interplay between the magnetic and structural properties of NaNiO2 and Li1-xNi1+xO2 is discussed.

Electron paramagnetic resonance studies of Cu2+ ions in KZnClSO4·3H2O: an observation of Jahn–Teller distortion

EPR studies of Cu2+ ions doped in KZnClSO4·3H2O single crystals have been studied at different temperatures. Two magnetically inequivalent Cu2+ sites have been identified. The Cu2+ ions in the lattice are found to be in rhombic symmetry. The temperature variation studies of polycrystalline EPR spectra reveal that Cu2+ ions exhibit dynamic and static Jahn–Teller effects above and below the Jahn–Teller transition temperature, respectively. The temperature-dependant spin-Hamiltonian parameters were evaluated and discussed.

Pressure effect on the cooperative Jahn–Teller transition in PrCu 2

We have studied pressure dependence on the cooperative Jahn–Teller specific heat in the orthorhombic PrCu 2 using a clamp-type pressure-cell. The pressure in the clamp-type pressure-cell is determined from the superconducting transition temperatures of Sn (999.99%) at liquid-helium temperatures. The pressure is ineffective on the specific heat for PrCu 2 up to 0.8 GPa below 5 K. Above 5 K, obvious depression in the specific heat peak is observed as pressure increases. The Jahn–Teller transition temperature T C increases with increasing pressure.

Crystal structure evolution of Sm 0.6Sr 0.4MnO 3 in the temperature range 1.5–300 K

Neutron powder diffraction experiments on 154Sm 0.6Sr 0.4MnO 3 in the temperature range from 1.5 to 300 K have been performed using the high-resolution G4.2 (LLB, France) diffractometer. The material undergoes an insulator–metal transition at T∼120 K and exibits the giant magnetoresistance effect. The Jahn–Teller transition temperature is found to be close to 180 K. The temperature dependence of microstrain parameters related to anisotropic peak broadening is obtained. Magnetic scattering detected for T<130 K can be formally attributed to the canted ferromagnetic state. The consequences of our experimental results are discussed in the context of current ideas about structure and magnetotransport properties of manganites.

Anomalies in thermal expansion of DyVO4 induced by quadrupolar ordering

The thermal expansion of the DyVO4 crystal has been experimentally and theoretically investigated in the range of the Jahn-Teller structural phase transition. The manifestation of totally symmetric magnetoelastic interactions upon this transition has been studied for the first time. It is found that the temperature dependences of the unit-cell and thermal expansion parameters along the nonactive Jahn-Teller direction in the basal plane for the DyVO4 crystal exhibit characteristic magnetoelastic anomalies at T<Tc due to the ordering of quadrupole moments of Dy3+ ions. The magnetoelastic contributions of the totally symmetric eα1 and eα2 and symmetry-lowering eγ modes to the thermal expansion are calculated within the general crystal-field formalism. The total quadrupolar coefficient Gγ and magnetoelastic coefficient Bγ are determined from the spectroscopic and spontaneous deformation data. It is demonstrated that the thermal expansion of the DyVO4 crystal in the tetragonal and orthorhombic phases is well described in the framework of the unified model using a common set of interaction parameters for both phases.

Vibronic structure of alkoxy radicals via photoelectron spectroscopy

Ultraviolet photoelectron spectra of CH3O−, CH3CH2O−, (CH3)2CHO−, (CH3)3CO−, as well as CD3O− and CD3CD2O− are presented, providing improved electron affinities and new information on vibronic coupling in the corresponding neutral radicals. Jahn-Teller vibronic transitions are assigned to e vibrational modes of X̃ 2E CH3O. The excitation energy of the à 2A′ state of CH3CH2O with respect to the X̃ 2A″ state is observed directly at 355±10 cm−1. Vibronic coupling between these low-lying electronic states perturbs the observed vibronic levels. Features of the (CH3)2CHO− photoelectron spectrum are assigned. The splitting between the X̃ 2A′ and à 2A″ states of (CH3)2CHO is determined to be 1225±65 cm−1. Significant vibronic coupling is not observed in (CH3)2CHO. Vibrational assignments are made for the spectral features of (CH3)3CO and no Jahn-Teller effects are observed directly. Electron affinities (EAs) for the neutrals (RO) are (in eV); EA(CH3O)=1.572±0.004; EA(CD3O)=1.559±0.004; EA(CH3CH2O)=1.712±0.004; EA(CD3CD2O)=1.699±0.004; EA((CH3)2CHO)=1.847±0.004; EA((CH3)3CO)=1.909±0.004. Bond dissociation energies of ROH, D300(RO–H), along with ΔfH300(RO), ΔfH300(RO−), and ΔfH300(RO+) are derived.

EPR linewidths inLa1−xCaxMnO3:0&lt;~x&lt;~1

We have analyzed recent results for the electron paramagnetic resonance linewidths in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ and related materials over the range $0l~xl~1.$ In all cases studied, the linewidth away from magnetic and structural transitions can be fitted to the expression $[{\ensuremath{\chi}}_{0}(T)/\ensuremath{\chi}(T)]\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty}),$ where the first factor is the ratio of the Curie susceptibility ${\ensuremath{\chi}}_{0}(T)$ to the measured susceptibility $\ensuremath{\chi}(T)$ and the second factor is a temperature-independent constant. Formally exact and approximate expressions for $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ are derived. In the case of ${\mathrm{LaMnO}}_{3},$ the linewidth in the distorted ${\mathrm{O}}^{\ensuremath{'}}$ phase arises from the interplay of crystal field (single ion) anisotropy and antisymmetric (Dzialozhinsky-Moriya) exchange together with isotropic superexchange. Above the Jahn-Teller transition to the pseudocubic O phase, the single-ion anisotropy is much weaker and the linewidth arises primarily from the antisymmetric interactions. Similar behavior is also found in both phases of the less-distorted, oxygenated material ${\mathrm{LaMnO}}_{3.04}.$ In the case of ${\mathrm{CaMnO}}_{3},$ the distortions of the oxygen octahedra are also small and the antisymmetric interactions are again dominant. In the intermediate region $0lxl1,$ $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ decreases with increasing Ca (i.e., ${\mathrm{Mn}}^{4+})$ concentration in contrast to the Curie-Weiss temperature, which peaks near $x=1/3,$ reflecting the importance of the double-exchange mechanism. It is proposed that the absence of a signature of the double-exchange interaction in the infinite-temperature linewidth is a consequence of the fact that the time scale associated with the changes in Mn valence arising from double exchange is too long to affect the decay of the correlation function in $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ so that the linewidth is determined primarily by the superexchange interactions between the Mn ions.

Anti-Jahn-Teller polaron inLaMnO3

Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$ and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$. A simple model for this transition is studied. Without further adjustment, the model predicts the shape and stability of small (anti-Jahn-Teller) polarons which form when holes are doped into the material. This leads to a new description of the lightly doped insulator, the anti-ferromagnetic to ferromagnetic transition, and the metal-insulator transition.

ESR and magnetization in Jahn-Teller-distortedLaMnO3+δ: Correlation with crystal structure

We present magnetic susceptibility, x-ray-diffraction, and electron-spin-resonance (ESR) studies in the series of nonstoichiometric compounds ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}},$ for $0&lt;~\ensuremath{\delta}&lt;~0.07.$ The measurements were performed in the paramagnetic regime, especially near the temperature ${T}_{\mathrm{JT}}$ for the Jahn-Teller transition observed in these samples. We observed important anomalies in both ${\ensuremath{\chi}}_{\mathrm{dc}}(T)$ and ESR linewidth $\ensuremath{\Delta}{H}_{\mathrm{pp}}(T)$ near ${T}_{\mathrm{JT}},$ which was found dependent on \ensuremath{\delta}. We explain these observations considering the crucial role of the lattice distortions which strongly affect the magnetic interactions. For the ESR spectra we assume a spin Hamiltonian including single-ion crystal-field interaction as well as isotropic and Dzyaloshinskii-Moriya (DM) antisymmetric spin-spin exchange. We found that the Curie-Weiss temperature and the high-$T$ limit of the ESR linewidth, which are usually constant parameters, are in this case temperature dependent. We have been able to reproduce the behavior of ${\ensuremath{\chi}}_{\mathrm{dc}}(\ensuremath{\delta},T)$ and $\ensuremath{\Delta}{H}_{\mathrm{pp}}(\ensuremath{\delta},T),$ including the anomalies at $\ensuremath{\approx}{T}_{\mathrm{JT}},$ with a simple model where the $T$ dependence of the exchange constants and of the crystal-field interaction are proportional to the orthorhombic strain. By contrast, the DM coefficient is constant for all \ensuremath{\delta} and $T$ and would depend only on the tilting of the ${\mathrm{MnO}}_{6}$ octahedra which is approximately the same for all the samples studied. A structural and magnetic phase-diagram $T$ versus \ensuremath{\delta} is also discussed.

Jahn—Teller effects on the superexchange interactions in LaMnO 3

We present Magnetization and ESR measurements in the paramagnetic phase of ceramic LaMnO 3 with carefully controlled stoichiometry. At T ≈ 600 K, near the Jahn—Teller transition observed in this material, we found an important decrease of the ESR linewidth. We explain this observation considering the crucial role of the lattice distortions which strongly affects the anisotropic and isotropic exchange interactions.

Jahn-Teller distortion and electronic correlation effects in undoped manganese perovskites

The formation of a long-range Jahn-Teller distortion in undoped manganites is studied within the dynamical mean-field theory taking both electron-phonon and electron-electron interactions into account. We find that the observed insulating behavior at temperatures above the Jahn-Teller transition and the bond-length changes below it can most naturally be explained by assuming a strong electronic repulsion and a rather weak electron-phonon coupling.

Phenomenological theory of phase transitions with quantum-mechanical order parameters

The phenomenological Landau theory of phase transitions (PT) is generalized for systems with degenerate atomic states. A new approach is based on the formalism of a matrix density constructed on the wave functions of a degenerate level. The space of the order parameter (OP) consists of density matrix components averaged over the Gibbs statistic assembly. A method to introduce a multicomponent OP is suggested for such systems. The multicomponent OP is determined as the mixing coefficients of the basis functions of the irreducible representation of a crystal group. The OP, introduced in such a way, characterises primarily an electronic spectrum rearrangement at the PT and the variation of the set of properties (atomic displacements, magnetic moment occurrences, polarization, etc.) accompanying the PT is expressed by the type of OPs. The suggested approach describes a wide range of PTs such as Jahn-Teller, magnetic (for magnetics with a finite magnetic spin) and orientation (those of order-disorder type in molecular crystals) transitions. Purely electronic PTs such as the snperconductor-normal metal and those with charge density waves can be ascribed to this approach as well. It is shown that all these phase transitions in systems with an OP of quantum-mechanical origin can be described by the unique scheme of the canonical Landau theory. The specificity of separate systems is revealed in constructing the order parameter from density matrix components.

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO4)2 single crystal are investigated. It is established that a structural phase transition takes place at Tc=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>Tc(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of Tc is investigated for various directions of the field.

Anisotropic Cooperative Jahn-Teller Effect and Metamagnetism inPrCu2

We have studied the origin of the cooperative Jahn-Teller effect and the metamagnetic transition in the orthorhombic compound PrCu 2 . We have determined the anisotropic ferroquadrupole phase diagram by measuring the specific heat and thermal expansion under magnetic fields. With increasing the field, the Jahn-Teller transition temperature increases slightly for the field along a - and c -axes, while it decreases intensively along the b -axis. The phase diagram can be explained quantitatively by including the quadrupolar interaction of - K JT O x y in the crystalline electric field and Zeeman Hamiltonian. The metamagnetic transition, which brings about the conversion between the easy ( a )- and hard ( c )-axes magnetization, has been also studied through the magnetostriction. This phenomenon is also explained quantitatively on the basis of the quadrupolar interaction of - K M O 2 2 . The sign of is found to be negative in the state where the c -axis is the hard one, while it ...

Anisotropic ferroquadrupolar phase in magnetic fields for PrCu 2

We have studied the origin of the cooperative Jahn-Teller effect in PrCu 2. We have determined the anisotropic phase diagram by measuring the thermal expansion and the magnetostriction in magnetic fields up to 15 T. The Jahn-Teller transition temperature is almost constant for the field along the a- and c-axis, while it decreases with increasing the field along the b-axis. The phase diagram can be explained qualitatively by including the quadrupolar interaction for 〈O xy〉 in the crystalline electric field Hamiltonian.

Evidence for electron-lattice coupling in RNiO 3 perovskites

In order to get additional insight into the origin of the metal-insulator transition in RNiO 3 perovskites we have investigated (a) the stability of the magnetic structure against the proximity of the metallic state and (b) the effect of the oxygen isotope mass on the metal-insulator transition temperature T MI. The observation of the same kind of magnetic ordering in the four nickelates studied (PrNiO 3, NdNiO 3 with T MI = T N, and 154SmNiO 3, 153EuNiO 3 with T N < T MI), as well as the unusually large, rare-earth-dependent 16O- 18O isotope shift we found in these materials ( δT MI ≤ + 10.3 K) strongly support the electron-lattice interaction as the driving force for the metal-insulator transition. The possibility of a dynamic to static Jahn-Teller transition is discussed.

Anomalous field dependence of muon Knight shift and line width in the singlet ground state compound PrCu 2

We report on ZF and TF‐studies of PrCu2 above the induced Jahn–Teller transition at 7.3 K. Generally a two‐component signal is found, one showing inhomogeneous the other one homogeneous, temperature dependent line broadening. In ZF the former component is well represented by a Gaussian Kubo–Toyabe function with \varDelta \simeq 6.5\,μs-1 at 7.5 K, corresponding to a field width of 76 G. This is about 30 times larger than what is calculated to arise from the 141Pr‐nuclear dipole moments alone, pointing to strong hyperfine enhanced features. TF‐field scans at 12 K revealed that the enhancement is suppressed in external fields exceeding 1 kG. In parallel the Knight shift drops from very large values well above 10% at 100 G to shifts of the order of 1% above 1 kG. A scaling of the Knight shifts with the corresponding relaxation rates seems to imply that the strange field dependence below 1 kG is associated with the magnetic susceptibility of the muons’s nearest neighbour Pr3+‐ions, a result for which we have no explanation yet to offer.