Dynamic Jahn-Teller Distortion Research Articles

Intrinsic EPR in La2-xSrxCuO4: Manifestation of Three-Spin Polarons

Electron-paramagnetic resonance (EPR) measurements on ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ provide experimental evidence of a three-spin polaron, consisting of two ${\mathrm{Cu}}^{2+}$ ions and one $p$ hole. The symmetry properties and the peculiar temperature dependence of the $g$ values of the EPR line indicate the presence of dynamical Jahn-Teller distortions and formation of a collective mode of polarons and surrounding strongly correlated Cu ions (bottlenecked regime).

An electron paramagnetic resonance investigation of a macrobicyclic cage complex of Ag(II). Dynamic and static Jahn-Teller distortions

EPR studies of pure [Ag(di(amH)sar)](NO 3) 4 and the same doped into [Zn(di(amH)-sar)](NO 3) 4 for temperatures of 5–300 K show that both exhibit dynamic and static Jahn-Teller effects above and below their respective Jahn-Teller transition temperatures. Spin Hamiltonian parameters were obtained by computer simulation of experimental spectra. Moreover, pure and doped lattices exhibit cooperative and localized Jahn-Teller effects respectively. From the g-tensors of the pure polycrystalline sample, there is clear evidence of rapid electronic exchange between two inequivalent Ag(II) sites.

Comparison of the temperature-dependent electronic structure of the perovskites La0.65A0.35MnO3 (A=Ca,Ba).

The electronic band structure and the local screening effects of the transition-metal perovskites ${\mathrm{La}}_{0.65}{A}_{0.35}\mathrm{Mn}{\mathrm{O}}_{3}$ ($A=\mathrm{Ba}\mathrm{and}\mathrm{Ca}$) have been examined across the coupled magnetic-metallic phase transition using the techniques of angle-resolved photoemission, resonance photoemission, and inverse photoemission. Temperature-dependent band shifts of the ${e}_{g}$ and ${t}_{2g}$ bands of ${\mathrm{La}}_{0.65}$${\mathrm{Ca}}_{0.35}$Mn${\mathrm{O}}_{3}$ have been observed. These changes in the observed electronic structure correlate with the phase transition, and are in qualitatively agreement with the predicted behavior associated with double exchange coupled with a dynamic Jahn-Teller distortion. Similar shifts were not observed for ${\mathrm{La}}_{0.65}$${\mathrm{Ba}}_{0.35}$Mn${\mathrm{O}}_{3}$. The changes in local screening across the Curie temperature were substantially larger for ${\mathrm{La}}_{0.65}$${\mathrm{Ca}}_{0.35}$Mn${\mathrm{O}}_{3}$ than for ${\mathrm{La}}_{0.65}$${\mathrm{Ba}}_{0.35}$Mn${\mathrm{O}}_{3}$. This is consistent with the less itinerant band structure of ${\mathrm{La}}_{0.65}$${\mathrm{Ca}}_{0.35}$Mn${\mathrm{O}}_{3}$, as compared to the highly dispersive bands of ${\mathrm{La}}_{0.65}$${\mathrm{Ba}}_{0.35}$Mn${\mathrm{O}}_{3}$. These results suggest that there is less hybridization in ${\mathrm{La}}_{0.65}$${\mathrm{Ca}}_{0.35}$Mn${\mathrm{O}}_{3}$, as compared to ${\mathrm{La}}_{0.65}$${\mathrm{Ba}}_{0.35}$Mn${\mathrm{O}}_{3}$. The results point to greater electron localization in the ${\mathrm{La}}_{0.65}$${\mathrm{Ca}}_{0.35}$Mn${\mathrm{O}}_{3}$ compound.

From Weakly to Strongly Correlated Electrons in the Alkali Fulleride Family

We discuss the effect of strong electron-vibron interaction and static or dynamic Jahn-Teller distortion on the electromc properties of the AnC60 family, where A is alkali metal and 0 < n < 6. Repulsive electron-electron correlations are expected for even n, but attractive ones for odd n. Such a model seems able to make compatible weak electron correlations for n = 3, fairly strong correlations for n = 1 and an insulating behavior for n = 2 or 4. An important due for understanding the electronic properties of the Alkali Fulleride fam- ily is the answer to the following question how large are the electron-electron correlations compared to the one-electron energy bandwidth II~? The C60 molecule forms, with the alkali metal series, salts with chemical formula AnC60 where A is an alkali metal and n an integer (0 < n < 6). In icosahedral synimetry, the LUMO of the C60 molecule is a three-fold degen- erate tiu orbital, giving rise, in the crystal, to a narrow baud of about o-a-o-a eV. If there is a general agreement about this order of magnitude for W, the value of the intraniolecular electron-electron repulsion (U in the Hubbard model) is rather controversial. The experimen- tal estimations are indirect and range from more than eV (from photoemission) iii to values much smaller than the bandwidth (from analysis of Fermi hquid properties). This question is conceptually important since the relevant theoretical description of the Alkali fullerides could be either a weakly correlated Fermi hquid or a strongly correlated one, or even a (possibly doped) Mott insulator, depending on the strength of the ratio Uliv. As far as we know, there is no reliable ab mitio calculation of U, taking into accourt many- electron eflects. The effective value of U is certainly smaller than the bare carbon atom on-site electron-electron Coulomb repulsion, typically a few eV's. It is certainly strongly reduced by electronic screening and by the delocalization of the electron wave function over the 60 atoms. However, given the order of magnitude of W, it seems diflicult to beheve, at first sight, that U cari be much smaller than the bandwith iv. It is, therefore, important to discuss the possible eflects of electron correlations on the electronic properties. (*) Author for correspondence je-mail: victorof%Ips.u-psud.fr)

Spectral and voltammetric studies on titanium substituted Keggin-type heteropolyanions

Bonding and redox properties of mono- and dititanium substituted 12-heteropolytungsto-phosphates and their complexes with bivalent Mn, Co, Ni and Cu are studied by UV-Vis, ESR and cyclicvoltammetric techniques. The heteropoly blue analogue [TiIIITiIVW10PO40]8− is unstable and its presence in solution is proved by optical and ESR spectroscopy. ESR results reveal that the odd electron is trapped on the titanium atom at 77 K. The optical electronegativity of Ti(III) in this heteropoly blue is estimated to be 2.4 from the intervalence charge transfer (IVCT) bands. X-ray powder diffraction results show that the complexes K5MII[Ti2W10PO40].xH2O(MII = Cu or Mn) are isomorphous with K7[Ti2W10PO40].6H2O. Electronic spectroscopy indicates very high symmetry around the bivalent metal ion M in the K5MII[Ti2W10PO40].xH2O complexes. This high symmetry is confirmed in K5Cu[Ti2W10PO40] by the observation of dynamic Jahn-Teller (JT) distortion at 300 K and static JT distortion at 77 K.

Transition from static to dynamic Jahn-Teller distortion in (P(C 6H 5) 4) 2C 60I

Air stable bulk single crystals of (P(C 6H 5) 4) 2C 60I can be grown by electrocrystallisation. We will prove that this salt is an excellent model system for the ferromagnetic [TDAE]C 60 and furthermore a good tool, to study the dynamics of C 60 mono anions in the solid state. We could directly observe the temperature dependence of the Jahn-Teller distortion of the C 60- and the transition from a static to a dynamic Jahn-Teller distortion at about 140 K. We will present detailed EPR linewidth and g-factor measurements at 9.7 and 34 GHz between 3.5 and 300 K.

Magnetic heat capacity of TDAE-C60

The heat capacity of TDAE-C60 (TDAE: tetrakis(dimethylamino)ethylene) was measured between 5 and 100 K using an adiabatic calorimeter. The magnetic phase transition was detected as a broad heat capacity peak at 12.5 K. The enthalpy and entropy of transition are 0.43 kJmol and 34 JK mol, respectively. Such a large value of the entropy of transition compared with 2R1n 2 = 11.5 JK mol expected for a localized spin-12 system probably reflects the multi-state ofC−60 due to dynamic Jahn-Teller distortion in the high-temperature region above Tc.

Vibrations and Berry phases of charged buckminsterfullerene.

A simple model of electron-vibron interactions in buckminsterfullerene ions is solved semiclassically. Electronic degeneracies of C$_{60}$$^{n-}$ induce dynamical Jahn-Teller distortions, which are unimodal for $n\!\ne\!3$ and bimodal for $n\!=\!3$. The quantization of motion along the Jahn-Teller manifold leads to a symmetric-top rotator Hamiltonian. I find Molecular Aharonov-Bohm effects where electronic Berry phases determine the vibrational spectra, zero point fluctuations, and electrons' pair binding energies. The latter are relevant to superconductivity in alkali-fullerenes.

Electrochemical Generation and Electron Paramagnetic Resonance Studies of C60-, C602-, and C603-

The -1, -2, and -3 anions of C[sub 60] were generated electrochemically in 4:1 toluene-acetonitrile. The visible/near-IR spectra were monitored in situ. Square wave voltammograms and electronic spectra showed that after bulk electrolysis to produce the anions, the following percentages of the starting C[sub 60] concentration could be reoxidized to neutral C[sub 60] in typical data sets: -1 anion, 75%; -2 anion, 66%; -3 anion, 46%. Continuous wave (CW) electron paramagnetic resonance (EPR) spectra of the -1 and -3 anions exhibited broad signals that were temperature dependent. The averaging of anisotropy of the signals between 15 and 60 K is attributed to dynamic Jahn-Teller distortion. Electron spin-echo and saturation recovery EPR data for the -1 anion at 10 and 16 K showed that the electron spin relaxation rates are dependent upon position in the spectrum, which is attributed to variations in the Jahn-Teller distortion. For both the -1 and -3 anion above about 70 K the CW line shape of the EPR signal is electron spin-lattice relaxation rate determined. The EPR data indicate that the -2 anion is diamagnetic. 53 refs., 14 figs.

Optical study of spin-flip transitions at Fe3+ in InP.

The 0.5-eV (4000-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) emission band in InP:Fe is studied in detail by Fourier-transform infrared photoluminescence (PL), time-resolved PL, and photoluminescence excitation (PLE) spectroscopy. We resolve new fine structures in both the zero-phonon (ZP) lines and in the corresponding phonon sideband. The transient PL signal follows an exponential decay with a time constant of 1.1\ifmmode\pm\else\textpm\fi{}0.3 ms. The PLE shows that the 0.5-eV band can be pumped via hole capture of ${\mathrm{Fe}}^{2+}$ and even more efficiently by exciting resonantly the charge-transfer states [${\mathrm{Fe}}^{2+}$${(}^{5}$${\mathit{T}}_{2}$),${\mathit{h}}_{\mathit{b}}$], where the hole is bound to ${\mathrm{Fe}}^{2+}$. The results from PLE and the long decay time strongly support the interpretation of the 0.5-eV band as spin-flip transitions $^{4}$${\mathit{T}}_{1}$${\ensuremath{\rightarrow}}^{6}$${\mathit{A}}_{1}$ of ${\mathrm{Fe}}^{3+}$. We explain the fine structure of the ZP lines by a dynamic Jahn-Teller distortion in the excited state $^{4}$${\mathit{T}}_{1}$. The decay of the charge-transfer states [${\mathrm{Fe}}^{2+}$${(}^{5}$${\mathit{T}}_{2}$),${\mathit{h}}_{\mathit{b}}$] leaves the ${\mathrm{Fe}}^{3+}$ ion in an excited state (${\mathrm{Fe}}^{3+}$${)}^{\mathrm{*}}$, which pumps the 0.5-eV emission.

EPR of dynamic Jahn-Teller distortion in CuII doped magnesium Tutton's salt

The dynamic Jahn-Teller distortion in single crystals of CuII doped magnesium Tutton's salt, MgK2(SO4)2·6H2O was studied by EPR spectroscopy. The directions of the g tensor axes were found to coincide with those of A at room temperature as well as at 113 K and were correlated with the MgO directions in the lattice. Two spatially distinct sites of copper were detected in the crystal. The g and A values were also calculated from the polycrystalline samples at various temperatures. A dx2-y2 ground state has been assigned for CuII, as in the isomorphous zinc Tutton's salt. The Silver and Getz model was applied to understand the dynamics of Jahn-Teller (JT) activity. The system was found to undergo dynamic JT distortion at all temperatures. At low temperatures, the [Cu(H2O)6]2+ ion has three possible degenerate vibronic states corresponding to three possible directions for the long axis of the octahedron. The energy separations between the three inequivalent JT valleys were estimated as δ1,2 = 90 and δ1,3 = 370 cm−1. The lattice strain parameter V2 and the JT stabilization energy EJT were also estimated as 19,140 and 1030 cm−1 respectively. From the EJT value, the JT distortion present in the system was classified as a case of “strong” coupling.

Electron spin relaxation times of C −60 in solution

Potassium and calcium salts of C − 60 have been studied in solid and liquid solutions by ESR and optical spectroscopies. Over the temperature interval from 8 to 45 K the electron spin relaxation times T 1 and T M (the phase memory time) were measured by saturation recovery and electron spin echo spectroscopies. For both salts T 1 decreased from a few milliseconds at 8 K to less than 1 μs at 40 K. T M was 1 to 3 μm between 8 and 25 K and decreased rapidly with increasing temperature to become equal to T 1 near 40 or 50 K. Below 40 K the linewidth of the anion signal was about 5.5 G, which is much larger than the linewidth calculated from T M (0.02 G at 10 K). Above about 50 K the linewidth was relaxation time determined and increased rapidly with increasing temperature until reaching a plateau near the melting point of the solution. Analysis of the relaxation rates for C − 60 between 35 K and the melting (softening) point of the solutions gives an Arrhenius activation energy between 2.2 and 2.9 kJ/mol (180–240 cm −1) depending on the solvent. This measured range of activation energies for relaxation is close to and somewhat less than the lowest vibrational mode (≈ 270 cm −1 of C 60; in C − 60 this vibration may couple to the triply degenerate electronic ground state to produce a dynamic Jahn—Teller distortion.

Rotationally resolved laser spectroscopy of the jet-cooled methylcyclopentadienyl radical (CH3–C5H4 and CD3–C5H4)

The rotationally resolved, electronic, excitation spectra of supersonically cooled methylcyclopentadienyl radicals, CH3–C5H4 and CD3–C5H4, have been obtained. Analysis of these spectra characterizes both the ground and electronically excited states involved in this transition. The ground state barrier to internal rotation of the methyl group is very high compared to similar closed-shell species. Additionally it shows a strong isotopic dependence and increases in magnitude further in the excited state. The doubly degenerate 2E″1 ground state of C5H5 is split into two components, X̃ 2B2 and à 2A2. The rotational analysis shows that the cyclopentadienyl ring undergoes a significant, static distortion from a regular pentagon. The measured distortion is consistent with and related to the dynamic Jahn–Teller distortion in C5H5 itself.

Phonon scattering studies of Ni and V centres in GaP and InP

Thermal conductivity measurements have been made from 50 mK to 100 K of GaP and InP samples doped with Ni or V. The GaP samples both show strong resonant phonon scattering at 500 and 380 GHz respectively which is attributed to the T1-T2 tunnelling splitting associated with a dynamic orthorhombic Jahn-Teller distortion of the T1 orbital ground state of Ni2+ and V2+. Low-frequency scattering in both systems is tentatively attributed to these same ions in complex with a defect. The assignment of the scattering to V2+ is supported by studies of photo-induced changes in the V2+ concentration. Uniaxial stress measurements on GaP:V suggest strong coupling to E modes but the strength of the coupling to T2 modes is less clear. The absence of resonance scattering in a n-type sample of InP:Ni suggests the Ni is all in the Ni+ state (2T2) as expected and the absence of scattering in n-type InP:V is consistent with the instability of V2+ in that system.

Neutronenbeugungsmessungen an Li2CrCl4 / Neutron Diffraction Studies of Li2CrCl4

Neutron diffraction studies of Li2CrCl4 at 350 °C are presented. The cubic high-temperature polymorph of Li2CrCl4 (space group Fd3̅m, Z = 8, R = 3.26%) crystallizes in an inverse spinel structure (cF56) with large Frenkel disorder of the tetrahedrally coordinated lithium ions. The occupation of the interstitial octahedral 16c sites by lithium ions is 38%, that of the tetrahedral 8a lattice sites only 24%. This large disorder is obviously the reason of the high ionic conductivity of Li2CrCl4 at elevated temperatures. There are some indications of a dynamic Jahn-Teller distortion of the CrCl6 units.

Jahn-Teller distortion in the excited state of tellurium(IV) in Cs 2MCl 6 (M=Zr, Sn)

The luminescence spectra of the diluted systems Cs 2ZrCl 6:Te 4+ and Cs 2SnCl 6:Te 4+ are reported at liquid-helium and room temperature. The shape of the excitation spectrum of the Ten+ luminescence reveals a dynamic Jahn-Teller distortion in the excited state. At low temperatures the Te 4+ emission shows a vibrational progression in the e g mode. The same vibrational mode is derived from the temperature dependence of the splitting of the A excitation band. Vibrational structure is also observed in the B band of the excitation spectrum of Cs 2SnCl 6:Te 4+.

The Temperature-Variable Geometry of the Pseudo-Octahedral CuO6 Entity in Bis(methoxyacetato) diaquo-Cooper(II) - a Single Crystal EPR Study

Abstract The Cu-O bond lengths in the pseudooctahedral bis(methoxvacetato)diaquo-copper(II) com­plex are determined by the Jahn-Teller effect on the one hand and by the bonding properties of the three different oxygen ligator atoms (from a carboxylate. water and ether group) on the other. It is derived from EPR single crystal and powder investigations between 4.2 and 350 K. that the system becomes fluxional above =120 K due to a continuous transition from a static to a (par­tially) dynamic Jahn-Teller distortion. A model is presented, which allows to correlate the EPR results with the Cu-O spacings. if they are calibrated with structural data at two different temperatures. The structure of the ground state potential surface of Cu2+ is derived from the spectroscopic data.

ChemInform Abstract: A NEW SERIES OF HEPTAMOLYBDONICKELATE(IV) AND MANGANATE(IV). INVESTIGATION OF THE UNUSUAL MAGNETIC AND ELECTRONIC SPECTRAL BEHAVIOR OF THE NICKEL(IV) COMPOUND

AbstractDie neuen isomorphen Heteropolymolybdate (I) und (II) mit bislang unbekannter l :7(Ni/Mn : Mo)‐Stöchiometrie werden durch Oxidation wäßriger, Ammoniumheptamolybdat und NiSO4 ‐ 6 H2O bzw. MnSO4 ‐ 4 H2O enthaltender Lösungen mit KzSzOg dargestellt.

A New Series of Heptamolybdonickelate(IV) and Manganate(IV). Investigation of the Unusual Magnetic and Electronic Spectral Behavior of the Nickel(IV) Compound

Abstract Two new heteropolymolybdates of nickel(IV) and manganese(IV) of composition K2H8NiMo7O28·6H2O and K2H8MnMo7O28·4H2O with a hitherto unknown 1 : 7 (Ni/Mn : Mo) stoichiometry are reported. X-Ray powder patterns show that they are isomorphous. The electronic spectrum of the nickel(IV) compound exhibits four absorption bands at 16.4, 17.5, 18.3, and 24×103 cm−1, and the room temperature magnetic moment of the compound is abnormally high, 1.78 μB. The rationale for these observations is that there are Ni3+ impurity which occupy interstitial positions in the crystal lattice and as a consequence of dynamic Jahn-Teller distortion the high-spin 4A2g(4T1g) state of the Ni3+ ion lies close to the low-spin ground state 2A1g(2Eg) to get thermally populated to an appreciable extent leading to significant triplet state contribution.

Methyl-substituted tribenzo[b,f,j][1,5,9]triazacycloduodecine with copper(II) ion: a dynamic Jahn-Teller effect

The tridentate macrocyclic ligand (MeTRI) derived from the Schiff base condensation of 2-amino-5-methylbenzaldehyde forms a bis complex with copper(II) that exhibits a dynamic Jahn-Teller distortion. The complex crystallizes in the cubic space group Pa3 with a = 16.445 (5) A at 105 K and Z = 4. The structures were determined at 298 and 105 K. D/sub 3/ site symmetry at the copper ion is achieved by its coordination to two MeTRI ligands. The immediate coordination sphere of the copper(Cu) ion consists of six symmetry-equivalent nitrogen(N) atoms (Cu-N = 2.058 (6) A). Although no phase transition was observed at low temperature, the coordinated nitrogen atoms exhibited the large thermal anisotropies often associated with a dynamic Jahn-Teller distortion. 2 figures, 4 tables.