Typical Jahn-Teller Distortion Research Articles

Microstructure, crystal structure and electrical properties of Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics obtained at different sintering conditions

Details of the formation of Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics under different sintering conditions have been studied by optical microscopy, scanning electron microscopy (SEM), electron probe and energy dispersive spectroscopy (EDX) microanalyses, X-ray diffraction (XRD) and electrical resistivity measurements. Microstructure studies of samples sintered at 1170 °C for 1 h indicated the presence of a secondary phase besides the main spinel phase with modified composition. XRD measurements showed that the spinel phase exhibits a tetragonally distorted spinel structure (space group I4 1/ amd, a=5.9410(5) Å, c=8.4196(15) Å). The secondary phase (solid solution based on NiO) crystallizes with the NaCl-type structure (space group Fm 3m , a=4.1872(3) Å). The content of the secondary phase in ceramics is 10.61 mass%. For NiMn 2O 4 ceramics, prepared under the same sintering conditions, the decomposition with Ni 1− x Mn x O solid solution (NaCl-type structure) and spinel phase formation have been observed. The tetragonal modification of the spinel phase for NiMn 2O 4 ceramics is more preferable (space group I4 1/ amd, a=5.9764(5) Å, c=8.4201(8) Å). The distribution of atoms in the structure has been proposed for both ceramics. According to XRD results the Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramic samples, sintered at 920 °C for 8 h (program 1), at 920 °C for 8 h and at 750 °C for 24 h (program 2), at 920 °C for 8 h, at 1200 °C for 1 h and at 920 °C for 24 h (program 3) and at 920 °C for 8 h, at 1200 °C for 1 h, at 920 °C for 24 h and at 750 °C for 48 h (program 4), contain a single phase with the cubic spinel structure (space group Fd 3m ). Small residuals of the secondary phase for the ceramics, prepared via programs 3 and 4, have been observed by SEM investigations. The structure transformations of the spinel phase for Cu 0.1Ni 0.8Co 0.2Mn 1.9O 4 ceramics sintered at 1170 °C are attributed to a Jahn–Teller-type distortion due to a compositional change as a result of the secondary phase separation.

Ionization potentials, electron affinities, and vibrational frequencies of Gen (n=5–10) neutrals and charged ions from density functional theory

Geometrical and electronic properties of Gen (n=5–10) neutrals, cations, and anions have been investigated using the density functional method of Becke’s three-parameter hybrid functional with the Perdew/Wang 91 expression. Berny structural optimization and frequency analyses are performed with the basis of 6-311G(d) for both the neutrals and charged ions. Cohesive energies, ionization potentials, and electron affinities calculated at the optimized ground-state structures agree satisfactorily with recent experimental values. Frequency analyses indicate that the bicapped antitetragonal prism, which was previously proposed as the ground-state structure of Ge10−, is in fact a first-order stationary point with an imaginary frequency at 95i (b2). The optimized ground-state structure of Ge10− obtained in this work is a distorted, bicapped antitetragonal prism with the symmetry of C1. It is a typical Jahn–Teller distortion. Prominent charge-induced structural changes are also determined for Ge5+, Ge6−, Ge7+, Ge8+, Ge8−, and Ge9−.

Light-induced ESR studies in conjugated polymer-fullerene composites

In this contribution we discuss photo-induced electron transfer in conjugated polymer/fullerene composites and compare this with light induced effects in pure components. Two overlapping LESR lines are observed, from positive polarons on the polymer chains and negative charges on the fullerene moieties. Microwave power saturation studies show different relaxation times for these two spins, giving clear evidence for independent spins. The unusually high relaxation rate of the fullerene monoanionic spins is of intrinsic origin and discussed in terms of a splitting of the T 1u level by a Jahn-Teller type distortion as proposed in the literature. We observed two distinct contributions to LESR signals, a prompt and a persistent one. Excitation light intensity dependence of the prompt contributions into P + and C 60 − ESR signals is of bimolecular type (I 0.5) and implies the mutual annihilation within (P +…C 60 −) pair. The persistent contribution is found to be excitation intensity independent and proposed to originate from deep traps due to disorder.

Computational study of Jahn-Teller type distortions in radical cations of methyl-substituted cyclopropanes

We have studied Jahn-Teller (JT) type distortions in a series of methyl-substituted cyclopropane cations with ab initio molecular orbital techniques. Two sets of cyclopropane cation structures are considered for the parent (1a) and the 1-methyl-substituted (1b), 1,1-dimethyl-substituted (1c), 2,3-dimethyl-substituted (1d (trans), 1e (cis)), and 2,2,3,3-tetra-methyl-substituted (1T) species

The first molecular metals with ordered spin structures, R1R2DCNQI2Cu (R1, R2  CH3, CH3O, Cl, Br) — Janh-Teller distortion, CDW instability and antiferromagnetic spin ordering

Abstract New type of molecular metals, R1R2DCNQI2Cu (R1, R2CH3, CH3O, Cl and Br) exhibiting the cooperative structural phase transitions induced by Jahn-Teller type distortions and CDW instabilities are presented. They transform to antiferromagnetic insulators at low temperature. But in the case of DMeDCNQI2Cu with stable metallic state, the 2pπ conduction electrons and the ordered spins on Cu atoms coexist.

The characterisation of VSe2: A study of the thermal expansion

The thermal expansion of VSe2 from 40-1175K has been determined by X-ray diffraction. The lattice expansion is linear with temperature from 40 to about 300K with alpha a=18.6*10-6K-1 and alpha c=9.8*10-6K-1. The behaviour alpha a> alpha c is anomalous for a layer compound. From the relation between the thermal expansion and the elastic constants it follows that either the elastic constants of VSe2 are anomalous for a layer compound or the elastic constants are normal but there is a large anisotropy in the Gruneisen functions gamma a and gamma c ( gamma a> gamma c), together with a relatively large value of the cross compliance constants s13. The latter possibility is probably true. The physical origin lies in the electronic structure; the exceptionally high ratio c/a (1.823 at 40K, 1.818 at 300K) due to a band Jahn-Teller type distortion, decreases at increasing temperature. The expansion above about 300K is no longer linear; with increasing temperature the expansion of the a axis increases while that of the c axis becomes almost zero. The non-linear behaviour is ascribed to the formation of Frenkel defects; intrasandwich vanadium is transferred to the interstitial sites in the Van der Waals gap.

Self-consistent electronic states for reconstructed Si vacancy models

Vacancy states in Si are investigated using a recently developed self-consistent pseudopotential technique. Three different structural models (ideal and two reconstructions) for a neutral vacancy are considered. Vacancy states are found to exist in the Si thermal gap for each structure. The character of these states is predominantly dangling-bond $p$-like localized on the four atoms surrounding the vacancy. The ideal (unreconstructed) vacancy yields an electronic spectrum, which is unstable with respect to Jahn-Teller type distortions. The two different reconstruction models considered yield Jahn-Teller stable situations.

Anisotropic ESR spectra of orbitally degenerate radicals. The pentachlorocyclopentadienyl radical

ESR spectra of the pentachlorocyclopentadienyl radical substituted in cyclododecane single crystals are reported. Analysis of the spectra give an anisotropic g tensor with g vb = 2.0009 ± 0.0001, g ⊥ = 2.0075 ± 0.0001 and an anisotropic chlorine 35 hyperfine coupling tensor with A vb = 6.58 ± 0.06 gauss, A ⊥ = 1.2 ± 0.2 gauss. For accurate analysis of the spectra, the effects of the most abundant isotopic modifications and the quadrupole coupling were considered. The ESR spectra show a pronounced temperature dependence, which is shown to result from a reorientation process of the radical around its 5-fold symmetry axis. The direction of the latter axis has directly been determined by combination of ESR and X-ray data. By CNDO and INDO calculations a limited study of Jahn-Teller type distortions of the radical from its D sh structure were investigated, which indicate local minima with C 2v symmetry. Furthermore, a synthesis of the chlorine coupling tensor based on the CNDO/INDO calculations leads to good agreement with experimental data. The latter require for a satisfactory interpretation both a dynamic Jahn-Teller effect and a stochastic reorientation process.

Mössbauer resonance of Fe 57 in oxidic spinels containing Cu and Fe

The Mössbauer spectra of Fe 57 in tetragonal and cubic CuFe 2O 4, CU 0.5Zn 0.5Fe 2O 4, and Cu 0.3Cd 0.3Fe 2O 4 have been investigated between 4.2–1000°K and in applied external magnetic fields. The spectra consist of a superposition of two apparent six-line patterns which are attributed predominantly to iron at the B sites (pattern I) and A sites (pattern II) of the crystal structure on the basis of their relative isomer shifts, hyperfine fields, quadrupole interactions, and thermal shifts. However, the intensity ratio of the Lorentzian fits of the two patterns does not correspond to the ratio of the Fe site occupancy factors as determined by diffraction methods. It is found that the patterns, especially I, are of a complex structure and exhibit anomalous line shapes due to various distributions of the neighboring cations and local Jahn-Teller type distortions of crystallographically equivalent sites. Both of these give rise to fluctuations in the A− B type super exchange interactions (resulting in varying hyperfine fields) and, to a lesser extent, to fluctuations in the quadrupole perturbation of the fields at the iron sites. The observed slight differences in the magnitude of the H eff( B) and the differential linewidths of II are interpreted in terms of a spin transfer from the A site cationic 3 d spin moments to the 4 S orbital of the B site Fe 3+. By use of applied magnetic fields the separation of the patterns is increased so that the Lorentzian fits yield approximately correct intensity ratios. In cubic CuFe 2O 4 and in the spinels containing Zn and Cd, the site distortions are such that patterns I and II cannot be resolved in the zero applied field spectra whereas this is possible if external fields are employed.

On the temperature dependence of the tetragonal-to-cubic phase transformations in transition-metal spinels: Part I: Distortions occuring only in the octahedral sites

Wojtowicz's theory on the tetragonal distortion of spinels, in the case that the macroscopic distortion arises as a consequence of Jahn-Teller type distortion in the immediate environment of the octahedral sites, is generalized, taking into account the possibility of migration of ions between the sublattices. The configurational energy, the thermodynamical behaviour and the temperature and composition dependence of several thermodynamic functions are calculated explicitly. The tetragonal-to-cubic-transition is still found to be of the first kind.

Theoretical Model for Tetragonal-to-Cubic Phase Transformations in Transition Metal Spinels

The origin of the large tetragonal distortions which occur in a number of transition metal oxides having the spinel structure has been examined recently by Dunitz and Orgel in terms of the crystal field theory. According to these authors the macroscopic distortions arise as a consequence of a Jahn-Teller type distortion in the immediate environment of certain transition metal ions. Thus, all the observed large distortions in spinels have been correlated with the results of this crystal field treatment on the basis of the spatial ordering of the local distortions.In this communication we investigate the detailed properties of the transformations from tetragonal to cubic phases which are observed at elevated temperatures. An approximate model has been constructed which explicitly takes into account the interactions between local Jahn-Teller distortions about neighboring octahedral site cations. The configurational energy of the model has been derived in a completely general form in terms of occupation variables, and has been used to deduce the structure of the stable low-temperature phase. By the use of the usual methods of statistical mechanics it has proved possible to derive the thermodynamic behavior of the model, and hence to contribute to an understanding of the cooperative nature of these phase transformations. The temperature and composition dependence of the long-range order parameter, the thermodynamic functions, and the lattice parameters are calculated explicitly. The principal result of importance is the demonstration that the transformations from tetragonal to cubic spinel phases are thermodynamic transitions of the first order type. That is, a latent heat, a volume discontinuity, lattice parameter discontinuities, and a lambda anomaly in the heat capacity are to be observed at the transformation temperature. The available experimental evidence supports the conclusions drawn from the theoretical model. The agreement between theory and experiment is found to be semiquantitative in most of the cases considered.

Theoretical Model for Cubic-to-Tetragonal Phase Transformations in Transition Metal Spinels

According to Dunitz and Orgel, the large tetragonal distortions which occur in a number of transition metal oxides having the spinel structure are presumed to arise as a consequence of a Jahn-Teller type distortion in the immediate environment of certain transition metal ions. All the observed large distortions can be correlated with the results of this crystal field treatment on the basis of the spatial ordering of the local distortions. In this communication we investigate the detailed properties of the transformations from tetragonal to cubic phases which are observed at elevated temperatures. An approximate model has been constructed which explicitly takes into account the interactions between local Jahn-Teller distortions about neighboring B-site transition metal cations. By the use of the usual method of statistical mechanics it has proved possible to derive the thermodynamic behavior of this model, and hence to contribute to an understanding of the cooperative nature of these transformations. The principal result of importance is the demonstration that the transformations from tetragonal to cubic spinel phases are thermodynamically of the first order. That is, a latent heat, a volume discontinuity, lattice parameter discontinuities, and a lambda anomaly in the heat capacity are to be observed at the transformation temperature. The available evidence seems to support the conclusions drawn from the model.