Local Jahn-Teller Distortion Research Articles

Local lattice distortions inLa1−xSrxMnO3studied by pulsed neutron scattering

Pulsed neutron-diffraction measurements were carried out on powder samples of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0l~xl~0.5)$ in order to study the local atomic structure of this system known for its colossal magnetoresistance phenomenon. The results are analyzed in terms of the atomic pair-density function to elucidate the role of local lattice distortion in the magnetic and charge transport properties. It is shown that the Jahn-Teller (JT) distortions are locally present even when the crystallographic structure suggests otherwise, and indicate the formation of small lattice polarons in the insulating phase. Local JT distortions are observed even in the metallic phase up to $x=0.35,$ suggesting that the charge distribution in the metallic phase just above the metal-insulator transition may not be spatially uniform. It is possible that charges are partially confined by spin and lattice resulting in microscopic separation of the charge-rich and charge-poor regions.

Assessing the driving force of a structural distortion by the simulated evolution of the local density of states

In this paper we show that the driving force leading to the metal-semiconductor structural phase transition occurring in epitaxial ${\mathrm{FeSi}}_{2}$ with film thickness, originates from a local Jahn-Teller distortion. This effect can be straightforwardly seen by the analysis of the site-projected density of states during a variable cell molecular dynamics for the bulk configuration. We point out that the evolution of the local density of states is a reliable and powerful tool solely provided by tight-binding molecular dynamics.

Charge Inhomogeneities in the Colossal Magnetoresistant Manganites From the Local Atomic Structure

AbstractWe have measured atomic pair distribution functions (PDF) of La1−xCaxMnO3 using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF We have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve?

Very strong magnetic-field dependence of the oxygen isotope shift of the charge-ordering transition inLa0.5Ca0.5MnO3

Oxygen isotope effect on the charge-ordering transition was studied in La0.5Ca0.5MnO3 . Upon replacing 16 O with 18 O, the charge-ordering temperature increases by 9 K under zero magnetic field. With an increase of the magnetic field, the oxygen isotope shift increases rapidly and reaches a magnitude of about 40 K under a magnetic field of 5.4 T. The present results cannot be explained by the existing theories of the charge-ordering transition in manganites. @S0163-1829~99!00601-3# It is well known that the observation of the oxygenisotope effect in conventional superconductors is important for the development of the microscopic theory of superconductivity. Recently, Zhao and co-workers 1‐3 have initiated the studies of oxygen isotope effects in the colossal magnetoresistive manganese perovskites. The observed strong oxygen isotope effects in the manganites give clear evidence for the existence of small lattice polarons in these systems. The isotope effects along with many other experimental results 4‐7 strongly support those theoretical models 8,9 where a strong coupling of conduction electrons to local Jahn-Teller distortions is considered to be important for the basic physics in manganites.

Jahn-Teller transition ofLiMn2O4studied by x-ray-absorption spectroscopy

The valence states and local structure of ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ have been investigated in comparison with those of ${\mathrm{ZnMn}}_{2}{\mathrm{O}}_{4}$ and $\ensuremath{\lambda}\ensuremath{-}{\mathrm{MnO}}_{2}$ by using x-ray-absorption spectroscopy on the Mn $K$ edge, in order to study the Jahn-Teller (JT) transition in ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ at ${T}_{\mathrm{a}}=280\mathrm{K}.$ In the near-edge spectra, a systematic edge shift in accordance with the formal manganese valence was found, confirming the ${\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}$ mixed states in ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}.$ Extended x-ray-absorption fine structure reveals a structural disorder of ${\mathrm{MnO}}_{6}$ octahedra in the cubic ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ above ${T}_{\mathrm{a}}.$ The local structure is analyzed using a structural model in which ${\mathrm{LiMn}}^{3+}{\mathrm{Mn}}^{4+}{\mathrm{O}}_{4}$ consists of isotropic ${\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ octahedra and anisotropic ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra distorted uniaxially by the JT effect. The magnitude of the local JT distortion, the ratio of the longer Mn-O distance to the shorter one, is found to be 1.17 at 300 K, which is as large as that of tetragonal ${\mathrm{ZnMn}}_{2}{\mathrm{O}}_{4}$ (1.19). The local structure exhibits no change when the temperature is lowered below ${T}_{\mathrm{a}}.$ These results imply that the low-temperature phase is induced by ordering the local JT distortion.

Local Lattice Effects in the Layered ManganiteLa1.4Sr1.6Mn2O7

The pair density function analysis of pulsed neutron diffraction data for the two-layer manganite ${\mathrm{La}}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ indicates the presence of a large local Jahn-Teller (JT) distortion of comparable magnitude to that found in the perovskites, while the crystallographic structure shows a very small JT effect. A model that includes JT displacements provides a better representation of the local structure than the existing crystallographic one. While the local distortion is reduced at lower temperatures its change corresponds more to the increase in the transport in the $\mathrm{ab}$ plane than to the magnetic transition, reflecting the 2D nature of the crystal.

Lattice effect in the metal-insulator transition in CMR manganites

The atomic pair density function (PDF) of La 1− x Sr x MnO 3 determined by pulsed neutron diffraction shows that the local structure significantly deviates from the average structure. In particular, the Jahn-Teller (JT) distortion locally persists even when the crystallographic structure shows no evidence of JT distortion. The existence of the local JT distortions is directly related to the formation of lattice polarons.

Local Jahn-Teller distortion inLa1−xSrxMnO3observed by pulsed neutron diffraction

The atomic pair-density function of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0<~x<~0.4)$ obtained by pulsed neutron diffraction indicates that their local atomic structure significantly deviates from the average structure, and that the local Jahn-Teller (JT) distortion persists even when the crystallographic structure shows no JT distortion. In the paramagnetic insulating phase doped holes form one-site small polarons, represented by the local absence of JT distortion. The polarons become more extended at low temperatures, but local distortions are found even in the metallic phase. The role of polarons in the phase transitions in transport and magnetic properties are discussed.

Oxygen isotope effects in the manganites and cuprates: Evidence for polaronic charge carriers

We present various oxygen-isotope effects in the manganites from magnetization and thermal-expansion measurements. We found that the ferromagnetic transition temperature T c , the magnitude of the jump in the thermal-expansion coefficient Δβ(T c ) at T c all depend strongly on the oxygen mass. Upon replacing 16O with 18O, T c can be lowered by as much as 20 K, and Δβ(T c ) can be raised by about 20%. The observed novel isotope effects clearly demonstrate that charge carriers in these materials are strongly coupled to the local Jahn-Teller distortions, so that polarons are formed. We also report the experimental evidence for polaronic supercarriers in the cuprate superconductors La 2− x Sr x CuO 4. From magnetization and thermal-expansion measurements, we can quantitatively determine the oxygen-isotope effects on the in-plane penetration depth λ ab (0) and on the carrier density n. We find a negligible isotope effect on n, but a large effect on λ ab (0). Our results show that polaronic charge carriers exist and condense into Cooper pairs in the cuprate superconductors.

Structure studies of La1.85Sr0.15CuO4 doped with mg at high doping level

Ceramic cuprates of La1.85Sr0.15Cu1-yMgyO4 with 0.00≤y≤0.30 are synthesized. The resistivity dependence of temperature is obtained up to room temperature. The metal-insulator transition at the range of higher doping level (y> 0.10) is observed. There is no impurity phase detected in the entire Mg-doped region. The structure of these Mg-doped samples is characterized by X-ray diffraction studies. The atomic structural parameters are obtained by Rietveld refinements for the Mg-doped samples with y≤0.30. No phase transition is detected within the accuracy of our experiments. The Mg doping reduce the local Jahn-Teller distortion of the CuO6 octahedra. Some meaningful bond distances are determined according to the refined results. According to the variations of the lattice parameters with y, the whole doping range could be divided into two regions: low doping level (LDL) and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO6 octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO6 octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and decreases in the HDL.

XAFS Characterization of Li Deintercalation in Rechargeable Lithium Battery Materials, LiCoO2-LiNiO2

In situ measurements of the Ni and Co K-edge XAFS spectra of Li1-xNiO2 and Li1-xCoO2 as a function of x were successfully carried out for the first time using an electrochemical cell. It is observed that a local Jahn-Teller distortion of the NiO6 octahedron in LiNiO2 was reduced with deintercalation of Li which caused oxidation of the Jahn-Teller d7Ni3+ ion to d6Ni4+ ion. A substitution of Co for Ni in LiNiO2 also reduced the local distortion of the NiO6 octahedron. XANES spectra of LiNiO2 shifted to higher energy side with increasing x while an abrupt shape change was observed in the XANES spectra of Li1-xCoO2 corresponding to phase transition.

Evidence for a local lattice distortion in Ca-doped LaMnO3.

We have found that a significant change in the local structure of ${\mathrm{La}}_{\frac{2}{3}}{\mathrm{Ca}}_{\frac{1}{3}}\mathrm{Mn}{\mathrm{O}}_{3}$ occurs between 80 K and temperatures above the Curie temperature, ${T}_{c}=270$ K. Near-edge x-ray-absorption spectra exhibits changes between 80 K and above ${T}_{c}$. X-ray-absorption fine structure reveals differences between the nearest-neighbor Mn-O bond distributions at 80 K and above ${T}_{c}$. The single-site distribution at low temperature becomes a complex, multi-site distribution above ${T}_{c}$. The observed change is consistent with proposed small polaron related local Jahn-Teller distortions for $T>{T}_{c}$.

Chemical switching of magnetic properties through topotactic lithium exchange in manganese(III) arsenate hydrate

The switch in local Jahn–Teller distortions of the MnIIIO6 octahedra caused by the topotactic ion-exchange reaction of MnAsO4·H2O to give LiMnAsO4(OH) results in a novel switch from antiferromagnetic to ferromagnetic order within infinite Mn–O–Mn chains in the framework, although in both materials the overall order is antiferromagnetic.

Electronic Factors Influencing the Local and Cooperative Jahn-Teller Interactions in Spinels with Copper(II) and Nickel(II)

Optical spectra and structural data of mixed crystals Cu x Zn 1- x Cr 2O 4, Ni x Zn 1- x Cr 2O 4, and Cu 1- x Ni x Cr 2O 4 are reported. The optical results are used as a probe characterizing the d -contributions to the Cu-O bonds. The cooperative Jahn-Teller forces are analyzed, extending the statistical model by Wojtowicz to the tetrahedral sublattice. Thermodynamical and optical data are used to estimate the corresponding cooperative strain energies. A microscopic bonding model is proposed which is based on the bonding properties of the oxygen ligands with respect to Cu 2+ due to the presence of the "contrapolarizing" Cr 3+ cations. This concept makes it possible to understand the enhancement of the local Jahn-Teller distortions with increasing cooperativeness.

The cooperative Jahn-Teller effect in BaFe 2.5Mn 9.5O 19

The distorted magnetoplumbite-type structure of BaFe 2.5Mn 9.5O 19 has been studied by powder neutron diffraction. The compound is orthorhombic, space group Cmcm ( a=5.826 Å, b=10.367 Å, c=23.004 Å, Z=4). Manganese is substituted into all five iron sites of the barium ferrite with population decreasing when going from the centre of the S block (100% of Mn for the 2a site) to the centre of the R block (40% for the 4e( 1 2 ) site). The structure includes two types of local Jahn-Teller distortions — the shortened Mn 3+O 6 octahedral in sites 4f IV and two thirds of sites 12k and the elongated octahedra in the remaining third. Manganese in the tetrahedral site 4f IV is bivalent and is compensated by Mn 4+ ions located preferentially in the octahedral site 2a. Ferrimagnetic arrangement below T c=220 K involves a large deal of static disorder. The ordered moments are aligned along the c-axis.

Transfer of Jahn-teller electron (exciton) and correlation of local Jahn-teller distortions

The electron (exciton) transfer between degenerate states of lattice sites is considered. The relative probabilities of transfer between different components of degenerate levels is shown to depend essentially upon the correlation of local Jahn-Teller distortions.

Local and cooperative Jahn-Teller distortions of nickel(2+) and copper (2+) in tetrahedral coordination

Local and cooperative Jahn-Teller distortions of nickel(2+) and copper (2+) in tetrahedral coordination

Local structural distortion and superconductivity in doped La2CuO4 and YBa2Cu3O7

Abstract The interaction between Cu ions in doped La 2 CuO 4 and YBa 2 Cu 3 O 7−δ via the elastic field is evaluated, assuming that they are in the fluctuating valence state and furthermore that the rate of valence fluctuation is slower than the phonon vibration. It is shown that for Cu 3+ ions in doped La 2 CuO 4 the elastic coupling between the local Jahn-Teller distortion around one ion and the Coulomb contraction around the other results in a strongly anisotropic interaction between the ions, which is attractive for the ions on the same a-b plane. The binding energy is estimated to be about 300K. It is suggested that this pair formation and the exchange interaction between the holes on the paired ions are critical in producing the high temperature superconductivity. A similar mechanism can operate also in YBa 2 Cu 3 O 7−° .

Origin of the noncubic anisotropy in Mn∶ (Y, Gd)3Fe5O12 thin films

Thin Mn∶ YIG films exhibit a large noncubic anisotropy. We show that this anisotropy may be understood on the basis of a strong Jahn-Teller effect of octahedrally coordinated Mn3+ ions. The corresponding mechanism is simple — the strain caused by the film/substrate misfit orders the local Jahn-Teller distortions and, as a consequence, noncubic anisotropy appears. In particular, due to the nonparallel local coordinate systems of octahedral sites, an uniaxial anisotropy is predicted also for the (111) growth plane. It is pointed out that the interaction between the Jahn-Teller centres and/or their interaction with the lattice defects lower the magnitude of the anisotropy. The temperature dependence of the uniaxial anisotropy was measured on the (001) and (111) oriented films using the FMR method. The results are in qualitative agreement with the prediction of theory. Quantitative comparison of theory with the experiment is difficult, as little is known about the distribution of the strains due to the lattice defects and about the interaction between the Jahn-Teller centres.

The Donor Level of V4+ and the Metal-Nonmetal Transition in SrTi1-xVxO3

The electrical conductivity of single crystalline SrTi1-xVxO3 (0.1≦x≦1) was measured down to 4.2 K. The depth of the donor level of V4+ in SrTiO3 was about 0.6 eV. The metal-nonmetal transition occurred at x≃0.6 with σ≃280 Ω-1 cm-1 and the conductivity increased with x up to about 2×104 Ω-1 cm-1 at x=1. In the semiconducting region of x<0.6, the electrical conduction was of the activation type with a constant activation energy in the higher temperature range and with a decreasing activation energy in the lower temperature range. The temperature dependence of the electrical conductivity can be explained by hopping conduction in the donor band. The deep donor level, the metal-nonmetal transition at extremely large concentrations of donors and other results can be systematically explained by assuming the local Jahn-Teller distortion around V4+ ions.