Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Bromine metallization studied by X-ray absorption spectroscopy

The experimental data carried out by M?ssbauer and magnetic resonances investigations of the structural phase transitions in K₂ZnCl₄ crystals are discussed by a simple electrostatic model, calculating, the lattice contributions to the local electric potential V(r), electric field intensity E(r) and electric field gradient tensor, (r) and taking into account both the fractional electric point charges and rigid lattice approximations. The validity of the model is proved by a good fit of the computing results and experimental data of quadrupole splitting parameters at K sites obtained by ³⁹K-NMR methods in high temperature incommensurate phase ( Pnam symmetry). The experimental results obtained by M?ssbauer and EPR methods in commensurate phase (Pna2₁ symmetry) of iron and copper doped K₂ZnCl₄ crystals are explained by relaxing the rigid lattice approximation. The insertion of probe ions appear to be done on not-exactly-Zn²⁺ site. Received 3 February 1999 and Received in final form 4 May 1999     

Stabilization of A-type layered antiferromagnetic phase in LaMnO3 by cooperative Jahn-Teller deformations

It is shown that the layered antiferromagnetic order in stoechiometric cannot be understood purely from electronic interactions. On the contrary, it mainly results from strong cooperative Jahn-Teller deformation. Those involve a compression of the Mn-O octahedron along the c-axis (mode Q ₃ < 0), while alternate Jahn-Teller deformations occur in the ab-plane (mode Q₂). These deformations stabilize a certain type of orbital ordering. The resulting superexchange couplings are calculated by exact diagonalization, taking into account both e_g and t_2g orbitals. The main result is that antiferromagnetic (ferromagnetic) coupling along the c-direction (ab-planes) can be understood only if the Jahn-Teller energy is much larger than the superexchange couplings, which is consistent with experiments. This mechanism contrasts with that based on weak Jahn-Teller coupling which instead predicts elongation along the c-axis (Q ₃ > 0). The crucial role of the deformation anisotropy is also emphasized. Received 24 January 2000     

Observation of phase transition of cesium manganese hexacyanoferrates by X-ray absorption spectroscopy

Cesium manganese hexacyanoferrates exhibit an interesting phenomenon of temperature-induced phase transition accompanied by a variation in the magnetic susceptibility. We observed the variation in the electronic state of Mn during the phase transition by using X-ray absorption spectroscopy. The results of the analyses showed that the content ratio of Fe^II-CN-Mn^III and Fe^III-CN-Mn^II systematically varied during the phase transition. However, the ratio of Fe^II-CN-Mn^II remained constant at almost all temperatures. These results suggest that the charge transfer between Fe and Mn ions in the Fe^III-CN-Mn^II or the Fe^II-CN-Mn^III bond produces the phase transition.     

Persistence of Jahn-Teller distortion up to the insulator to metal transition in LaMnO3

High pressure, low temperature Raman measurements performed on LaMnO3 up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that LaMnO3 is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that LaMnO3 becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.     

Pressure-induced quenching of the Jahn-Teller distortion and insulator-to-metal transition in LaMnO(3)

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.     

Ferroelastic phase transition in CaCl2 studied by Raman spectroscopy

CaCl₂ undergoes a second-order proper ferroelastic phase transition from the tetragonal rutile type to the orthorhombic calcium-chloride type structure atT _c 490 K. The transition is of the optical type and induced by an order parameter ofB _1g symmetry. An underdamped soft mode exists above and belowT _c , the frequency and intensity of which have been measured by Raman spectroscopy. Due to a strong coupling to strain the softening is incomplete and a transverse acoustical mode is predicted to become soft. The frequency of the optical soft mode, _s, exhibits a classical Landau behavior in a remarkably large temperature range. The unusual value of the ratio of _s ² / below and aboveT _c of about –6.5 can be accounted for by appropriate terms of the thermodynamical potential. The Raman active hard modes show no significant anomaly atT _c .     

Structure of cobalt nanoparticles as studied by X-ray absorption spectroscopy

Ab initio modeling is used to analyze the specific near-edge features of the X-ray absorption K spectra of ∼3-nm cobalt nanoparticles. It is established that the observed differences between the spectra of the nanoparticles and those of bulk cobalt samples are due to contribution from atoms of the nanoparticles’ surface layer, the local environment of which is considerably different from that of atoms in the bulk of the nanoparticle. It is also established that the local environment of cobalt atoms in the nanoparticle bulk corresponds to the FCC structure of bulk metal, and the fraction of such atoms is found to be 65 ± 5%.     

Ferroelectric phase transition in deuterated dihydrogen phosphate,studied by infrared spectroscopy

Abstract

The temperature dependence of polar phonon spectra of K(H_1?X D_X)₂ PO₄ single crystal with x = 0.9 for the electric field polarised parallel and perpendicular to the ferroelectric axis is reported between 7 and 300 K, in the para- and ferroelectric phase.     

Anharmonic effects in a Jahn-Teller phase transition

A mechanism is proposed to explain the double phase transitions found in K₂PbCu(NO₂)₆ and Tl₂PbCu(NO₂)₆. The higher temperature transition is caused by the cooperative Jahn-Teller effect. The lower temperature transition occurs as the fourth order anharmonic interaction restabilizes the lattice against instabilities caused by static distortions in the third order anharmonic interaction. Calculated curves of the order parameter and heat capacity reproduce all the features in the experimental curves.     

Resonant X-ray spectroscopy to study K absorption pre-edges in 3d transition metal compounds

Some general aspects concerning resonant X-ray spectroscopy are discussed and an analysis of the 1s3p RIXS plane in hematite (Fe₂O₃) and anatase titania (TiO₂) is presented. We emphasize the importance of recording a full RIXS plane as opposed to line scans. In a simple system such as hematite it is possible to assign the K absorption pre-edge features to ligand field split orbitals and non-local excitations using spin-selective spectroscopy. The core hole effect separates local and non-local excitations in titania.     

Laser-driven switching dynamics in phase change materials investigated by time-resolved X-ray absorption spectroscopy

Time-dependent X-ray absorption spectroscopy at the Ge L₃ edge is used to study the optical switching dynamics of epitaxial phase change materials grown on Si(111) membranes. Small differences between the static absorption spectra of the crystalline and laser-amorphized phase of GeTe are observed. Furthermore, a new approach for pump-probe studies on phase change materials is presented. However, no time-dependent change in the X-ray absorption > 0.1% has been detected so far. This is mainly attributed to sample deterioration at 3 kHz pump-probe rates.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.     

Comparison of electronic structures of orthorhombic and hexagonal manganites studied by X-ray absorption spectroscopy

Electronic structures of orthorhombic perovskite-type manganites (OM) and hexagonal manganites (HM) are compared by using X-ray absorption near edge structure spectroscopy. This study reveals that the environments around Mn and O ions in both the systems are different. These are reflected primarily in the lifting of degeneracy of the Mn 3d orbitals. Interestingly, the unoccupied Mn 3dx²−y² orbital in HM is strongly hybridized with O 2p states and appears to be responsible for both long range orderings: ferroelectric ordering at high temperatures and ferromagnetic ordering at low temperatures.     

The ferromagnetic to paramagnetic phase transition of Fe studied by x-ray photoelectron spectroscopy

Valence band x-ray photoelectron spectra from Fe(100) have been measured as a function of temperature to above the Curie temperature, T_c. The room temperature data can be reconciled with the theoretical one-particle density of states (DOS). At T = 1.034T_c, the data do not resemble the paramgnetic DOS of Fe as calculated in the disordered-local-moment limit.     

Antiferromagnetic stripe ordering at the tetragonal-orthorhombic transition in LnOFeAs probed by X-ray absorption spectroscopy

Measurements of the Fe K-edge X-ray absorption near edge (XANES) spectra of LnOFeAs high T_c superconductors exhibit small but measurable changes across the tetragonal to orthorhombic phase structural phase transition. Calculations, based on the local structure centered at the Fe site obtained by crystallographic investigations, reproduce the observed changes in the spectra under the change of the crystal structure, indicating variations in the Fe local unoccupied electronic states. Calculated Fe density of states including stripe spin ordering in the orthorhombic phase show significant modifications with respect to Fe 3d density of states in the tetragonal phase or the density of states in the orthorhombic phase calculated ignoring spin ordering. This implies that the striped spin ordering in the orthorhombic phase is necessary to describe its electronic properties not only in the ground state but also for excited states above the Fermi energy.     

Orbital ordering in LaMnO3 investigated by resonance Raman spectroscopy

Orbital ordering leads to an unconventional excitation spectrum that we investigate by resonance Raman scattering using incident photon energies between 1.7 and 5.0 eV. We use spectral ellipsometry to determine the corresponding dielectric function. Our results show resonant behavior of the phonon Raman cross section when the laser frequency is close to the orbiton-excitation energy of 2 eV in LaMnO3. We show an excellent agreement between theoretical calculations based on the Franck-Condon mechanism activating multiphonon Raman scattering in first order of the electron-phonon coupling and the experimental data of phonons with different symmetries.     

Jahn-Teller phase transition in CsCuCl3 and CsCrCl3: A neutron scattering study

The diffuse scattering above T_c was studied in CsCuCl₃ and CsCrCl₃ by neutron scattering technique. These compounds undergo phase transitions associated with the Jahn-Teller effect. Their structures are characterized by isolated chains of face sharing CuCl₆ and CrCl₆ octahedra, respectively. The diffuse scattering shows two main features: it does not occur at the superllatice points of the low temperature phase, but is centred around the strong Bragg points, and its shape changes from one Bragg point to another. This behaviour, which is incompatible with critical scattering from intra-chain correlations of Jahn-Teller distorted octahedra, can be interpreted as “dense impurity” Huang scattering.     

Electronic structure of carbyne studied by X-ray photoelectron spectroscopy and X-ray emission spectroscopy

The electronic structure of amorphous carbyne has been investigated by X-ray photoelectron spectroscopy and X-ray emission spectroscopy. Carbyne band structure has been calculated semiempirically and the experimental data have been interpreted on the basis of the calculation results. The valence band width was found to be about 20 eV which is the same as that in all other condensed carbon structures. The fine satellite structure near the 1s line of carbon has been studied. It is shown that the energy bands in carbyne are arranged in a mirror-like way relative to the Fermi level. The real carbyne structure is susceptible to conformations which affect primarily the π-subband structure.     

Thermal phase transition of RbMnFe(CN)6 observed by X-ray emission and absorption spectroscopy

The thermal phase transition of RbMnFe(CN)₆ has been observed by Mn and Fe 3p-1s X-ray emission spectroscopy (XES) and 1s X-ray absorption spectroscopy (XAS). The thermal variations of the spin states and the valences of Mn and Fe were determined to be Mn²⁺(S=5/2)-NC-Fe³⁺(S=1/2) for the high-temperature (HT) phase and Mn³⁺(S=2)-NC-Fe²⁺(S=0) for the low-temperature (LT) phase. These transitions are thus caused by charge transfer between Mn and Fe. The temperature dependences of Mn and Fe 3p-1s XES and 1s XAS were observed as the composition of the spectra of the HT and LT phases. The ratios of the HT component in each spectrum show good agreement with the thermal transition curves observed with magnetic susceptibility measurements.