Cooperative Jahn-Teller Distortion Research Articles

Crystal and magnetic structure of single-crystal La1-x Sr x MnO 3 (x ≈ 1/8)

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La1-xSrxMnO3 (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (TJT) and monoclinic (P121/c1, M′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (TCO/OO) with a monoclinic (P121/c1, M′′) model because the experimental resolution was insufficient to clearly identify a triclinic $(P\bar {1})$ structure. The refined lattice parameters show an obvious breathing-mode distortion between TCO/OO and TJT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ B/Mn is directed mainly along the b axis in P121/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below Tc = 187(1) K. The magnetization measurements show typical anomalies around TCO/OO and TJT. The measured saturation moment of 3.9(1)μ B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.

Theoretical description of pseudocubic manganites

A comprehensive theoretical model for the bulk manganite system ${\text{La}}_{1\ensuremath{-}x}{(\text{Ca},\text{Sr})}_{x}{\text{MnO}}_{3}$ is presented. The model includes local and cooperative Jahn-Teller distortions and the on-site Coulomb and exchange interactions. The model is solved in the single-site dynamical mean-field approximation using a solver based on the semiclassical approximation. The model semiquantitatively reproduces the observed phase diagram for the doping $0\ensuremath{\le}x<0.5$ and implies that the manganites are in the strong-coupling region but close to Mott insulator/metal phase boundary. The results establish a formalism for use in a broader range of calculations, for example, on heterostructures.

Experimental estimation of the cooperative Jahn-Teller energy in orbitally ordered KCu0.8Mg0.2F3 perovskite

High resolution synchrotron radiation X-ray powder diffraction was used to investigate the melting of the cooperative Jahn-Teller distortion (cJTd) in a perovskite of composition KCu0.8Mg0.2F3. A first order phase transition relaxing the cJTd is observed at T ∼ 600 K. From the transition temperature, an estimation of kT is derived (kT = 0.05 eV) for the cJTd in the doped compound. This is the very first observation of cJTd melting in a compound of the series KCu1−xMgxF3. A structural phase diagram for the Cu rich zone of the series is proposed. In principle, the extension of this experimental investigation could be used to disentangle the orbital order (OO) and cJTd energy scales in the parent compound KCuF3, a test system for OO theories.

Spectroscopic ellipsometry investigations of the thermally induced first-order transition ofRbMn[Fe(CN)6]

We report on detailed spectroscopic ellipsometry investigations on the thermally induced first-order phase transition of rubidium manganese hexacyano-ferrate $(\text{RbMn}[\text{Fe}{(\text{CN})}_{6}])$ pellet sample. This switchable solid shows a cooperative Jahn-Teller distortion coupled with a local charge transfer between Fe and Mn ions. Optical spectra of the real, ${\ensuremath{\epsilon}}^{\ensuremath{'}}$, and imaginary, ${\ensuremath{\epsilon}}^{\ensuremath{''}}$, parts of the complex dielectric constant have been recorded in the interval of temperature, i.e., 250\char21{}350 K, around the transition region and in the optical range of 190\char21{}830 nm. Four (two) optical bands have been identified and assigned in the low-temperature (high-temperature) phase, corresponding to the ${\text{Fe}}^{\text{II}}{\text{-CN-Mn}}^{\text{III}}$ $({\text{Fe}}^{\text{III}}{\text{-CN-Mn}}^{\text{II}})$ state. We performed a quantitative analysis by fitting our spectra with Gauss-Lorentz (GL) line shapes and determined the energy positions, oscillator strengths, and widths of individual transitions, as well as their thermal variation. It has been found that the GL profiles excellently approximate the optical response. Moreover, we observed a significant shift of the metal-ligand charge-transfer band8 upon the thermal transition and correlated it quantitatively to the volume change at the transition. We then derived the coefficient of the volumetric thermal expansion of the title compound and found $\ensuremath{\alpha}=(\ensuremath{\partial}V/\ensuremath{\partial}T)/\overline{V}\ensuremath{\simeq}3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }{\text{K}}^{\ensuremath{-}1}$ in the transition region and ${\ensuremath{\alpha}}_{T}^{LT}\ensuremath{\simeq}4.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }{\text{K}}^{\ensuremath{-}1}$ $({\ensuremath{\alpha}}_{T}^{HT}=8.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }{\text{K}}^{\ensuremath{-}1})$ in the low-temperature (high-temperature) region, in very good agreement with the reported zero thermal-expansion character of this compound. Finally, from the analysis of the integrated absorption spectra, we derived the thermal hysteresis of the high-temperature fraction, which was found in very good agreement with that of magnetic data recorded on the same sample.

Structural and electronic phase transitions in Sr 1−xCe xMnO 3 perovskites

The structures of eight members of the series Sr 1− x Ce x MnO 3 with 0.075⩽ x⩽0.4 have been established using synchrotron X-ray powder diffraction. These exhibit the sequence of structures cubic Pm 3 ¯ m ⟶ x ≈ 0.08 tetragonal I 4 / mcm ⟶ x ≈ 0.35 orthorhombic Imma with increasing Ce content. Unusual peak broadening due to domain wall scattering is observed near the composition-induced first-order cubic–tetragonal transition. The temperature dependence of the structures for three examples x=0.1, 0.2 and 0.35 is described. For the x=0.1 and 0.35 samples evidence for a first-order loss of the orbital ordering associated with a cooperative Jahn–Teller distortion is presented. The onset of the orbitally disordered state is accompanied by a discontinuous reduction in cell volume.

Electronic, Magnetic, and Structural Properties of Sr2MnRuO6 and LaSrMnRuO6 Double Perovskites

The perovskites Sr2MnRuO6 and LaSrMnRuO6 have been investigated and their properties compared with those of the broader family of A2MRuO6 double perovskites (A=Sr, La; M=Cr, Mn, Fe, Co, Ni). Neutron powder diffraction shows that both phases lack long‐range Mn/Ru order. Sr2MnRuO6 exhibits a cooperative Jahn–Teller distortion, conductivity that proceeds via variable‐range hopping, and antiferromagnetic (AFM) order (C‐type structure, TN≅200K, 2.05(1) μB per transition metal). LaSrMnRuO6 exhibits nearly undistorted octahedra, conductivity that proceeds via diffusion‐assisted small‐polaron hopping, and ferromagnetic (FM) order (TC≅220K, 1.60(4) μB transition metal). Band structure calculations show that AFM Mn–O–Ru coupling optimizes polarization of the Ru t2g orbitals, while FM coupling favors delocalization of the Ru t2g electrons. The transition from antiferromagnetism to ferromagnetism is linked to a loss of orbital order and stabilized over competing glassy states by delocalization of Ru t2g electrons.

Thickness dependent transport properties of compressively strained La0.88Sr0.12MnO3 ultrathin films

Thickness dependent magnetic and transport properties of compressively strained La0.88Sr0.12MnO3 thin films grown on single crystalline SrTiO3 (100) substrates have been studied. All films exhibit a large enhancement of ∼130 K in TC/TIM as compared to that of the bulk target (TC∼175 K). This has been explained in terms of suppression of the cooperative Jahn–Teller distortion due to in-plane compressive strain. The TC/TIM of the 5 nm film is 315 K/318 K and slightly increases for film thicknesses ≤25 nm. At higher film thicknesses, ∼60 nm, the TC/TIM starts decreasing. At T>TIM, the electrical transport is due to thermally activated hopping of small polarons. The activation energy is found to be sensitive to film thickness and shows a minimum at around L∼15–25 nm, which corresponds to the maximum of TC/TIM. All films possess large magnetoresistances (MRs) in the vicinity of room temperature. Low as well as high field MRs are observed to nearly double as the film thickness increases from 5 nm (MR∼28%/3 T) to 60 nm (MR%∼50%/3 T). Thicker films (L≥25 nm) are found to exhibit sufficiently large temperature coefficients of resistivities, ∼4%–5%/K, which could be suitable for bolometric applications.

KCrF3: Electronic structure and magnetic and orbital ordering from first principles

The electronic, magnetic and orbital structures of KCrF3 are determined in all its recently identified crystallographic phases (cubic, tetragonal, and monoclinic) with a set of ab initio local spin density approximation (LSDA) and LSDA+U calculations. The high-temperature undistorted cubic phase appears as a metal from LSDA, but it is a Mott insulator with a gap of 1.72 eV at the LSDA+U level. The tetragonal and monoclinic phases of KCrF3 exhibit cooperative Jahn-Teller distortions concomitant with staggered 3x2−r2/3y2−r2 orbital order. We find that the energy gains due to the Jahn-Teller distortion are 82 and 104 meV per chromium ion in the tetragonal and monoclinic phases, respectively. These phases show A-type magnetic ordering and have a band gap of 2.48 eV. In this Mott insulating state, KCrF3 has a substantial conduction bandwidth leading to the possibility for the kinetic energy of charge carriers in electron- or hole-doped derivatives of KCrF3 to overcome the polaron localization at low temperatures, in analogy with the situation encountered in the colossal magnetoresistive manganites.

Magnetic nanocheckerboards with tunable sizes in the Mn-doped CoFe2O4 spinel

Formation of magnetically ordered array of two types of rectangular nanorods, ∼300nm in length and a few nanometers in size, is achieved in the Mn-doped CoFe2O4 spinel through chemical phase separation mediated by cooperative Jahn-Teller distortions. At room temperature, the magnetic nanorods, with composition close to CoFe2O4, interlace with the paramagnetic counterparts and form a highly organized checkerboard pattern in the cross section. The checkerboard size in the range of ∼13.8×7.9–∼17.3×14.0nm2 is tunable with composition and, particularly, with the isothermal annealing time. These three-dimensional nanocheckerboards exhibit a nearly ideal configuration for the patterned perpendicular recording medium.

Orbital Order inZnV2O4

In view of recent controversy regarding the orbital order in the frustrated spinel ZnV2O4, we analyze the orbital and magnetic groundstate of this system within an ab initio density functional theory approach. While LDA+U calculations in the presence of a cooperative Jahn-Teller distortion stabilize an A-type staggered orbital order, the consideration of relativistic spin-orbit effects unquenches the orbital moment and leads to a uniform orbital order with a net magnetic moment close to the experimental one. Our results show that ab initio calculations are able to resolve the existing discrepancies in previous theories and that it is the spin-orbit coupling alongwith electronic correlations which play a significant role in determining the orbital structure in these materials.

Oxygen Stoichiometry-Structure-Property Correlations in Li2 ∕ 3 [ Mn2 ∕ 3Ni1 ∕ 3 ] O2 − δ with O3 Structure

The layered rock salt material with the structure loses up to of its oxygen reversibly at high temperature. It transforms initially to a second layered structure, , that may be associated with cooperative Jahn-Teller distortion of ions and considerable interlayer cation mixing. By continuous disordering and further oxygen loss, it finally transforms into a simple cubic rock salt structure above 1000°C, as shown by thermogravimetry and high-temperature X-ray powder diffraction. X-ray absorption near-edge structure results confirm that oxygen deficiency induces reduction of mainly Mn and to a small extent Ni, commencing with the fully oxidized stoichiometry . This correlates with oxygen contents determined by thermogravimetry and Rietveld refinement of powder X-ray diffraction data, with oxidation states determined by chemical titration and with electrochemical behavior during cycling. The cycling results show that samples with the structure have better performance and capacity than those with and especially structures and that the , capacity decreases with increasing oxygen deficiency δ.

The emergence of resonating valence bond physics in spin–orbital models

We discuss how orbital degeneracy, which is usually removed by a cooperative Jahn–Tellerdistortion, could under appropriate circumstances lead rather to a resonatingvalence bond spin–orbital liquid. The key points are: (i) the tendency to formspin–orbital dimers, a tendency already identified in several cases; (ii) mappingonto quantum dimer models, which have been shown to possess resonatingvalence bond phases on the triangular lattice. How this programme can beimplemented is explained in some detail starting from a microscopic model ofLiNiO2.

High temperature orbital order melting in KCrF3 perovskite

The effect of high temperatures on the structural properties of the ternary transition metal fluoride KCrF3 has been investigated by high-resolution synchrotron X-ray powder diffraction. The tetragonal room temperature structure (a>c/√2) exhibits large cooperative Jahn–Teller distortions of the CrF6 octahedra which are driven by the antiferrodistortive orbital ordering of the 3d3x2−r2 and 3d3y2−r2 orbitals in the ab plane. At TJT ≈ 973 K, KCrF3 undergoes a first-order structural phase transition to an orbitally disordered cubic phase comprising equivalent Cr–F bond lengths and regular CrF6 octahedra. The onset of the orbitally disordered state is accompanied by an abrupt volume collapse, δV/V = 0.44%, which arises from the more efficient packing of the CrF6 octahedra in the orbital liquid state.

Cooperative Jahn-Teller Distortion, Phase Transitions, and Weak Ferromagnetism in the KCrF3 Perovskite

At ambient conditions, the ternary fluoride with formula KCrF3 adopts a perovskite-type structure and incorporates the Jahn-Teller active Cr2+ (d4) whose electronic configuration and magnetic response are analogous to those of Mn3+ (d4) and Cu2+ (d9). These ingredients make it an attractive system to study owing to the striking similarities with LaMnO3 and the expected strong interplay between spin, orbital, and structural ordering phenomena. Indeed, probing the properties of KCrF3 as a function of temperature (5 < T < 300 K) has revealed a structurally and magnetically far richer phase diagram than hitherto supposed. We found that KCrF3 exhibits large cooperative Jahn-Teller distortions which are driven by orbital ordering, a series of temperature induced complex structural transitions, and weak ferromagnetism which is reminiscent of what is observed in LaMnO3.

Ultrasonic study of Jahn–Teller effect in charge‐ordered Nd1−xCax MnO3 (x = 0.4–0.8) manganites

AbstractThe ultrasonic, magnetic and transport properties of charge‐ordered manganites Nd1−x Cax MnO3 (x = 0.4, 0.5, 0.6, 0.7 and 0.8) were studied between 15 and 300 K. Resistivity and magnetization measurements show that all the samples undergo a charge‐ordering transition at low temperature. Upon cooling from 300 K, an obvious softening of sound velocity above the charge‐ordering transition temperature (T CO) and dramatic stiffening below T CO were observed for all samples. This feature gives clear evidence for strong electron–phonon coupling in these compounds. The Jahn–Teller coupling energy E JT increases with increasing Ca concentration x, and reaches the maximum at around x = 0.6, which is similar in characteristics to the T CO. The results imply that the charge‐ordering in Nd1−x Cax MnO3 is dominated by the Jahn–Teller effect. Our analyses suggested that the type of cooperative Jahn–Teller distortion be Q 2‐mode for x ≤ 0.6, and change to Q 3 mode at around x = 0.6 in the CO state. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hole doping into the metastable LuMnO 3 perovskite

Single-phase samples of (Lu 1− x Ca x )MnO 3 of the orthorhombic perovskite structure have been realized through solid-state synthesis under high pressures for the Ca contents 0 ≤ x ≤ 0.3 and at ambient pressure for x = 0.5 . The samples were characterized for basic structural, magnetic and transport properties with respect to x . With increasing x , Mn is gradually oxidized and the cooperative Jahn–Teller distortion appears to diminish. Increasing the concentration of Mn IV at the expense of Mn III was found to enhance the ferromagnetic-like interactions and decrease electrical resistivity. Efforts to dope the orthorhombic LuMnO 3 perovskite phase with holes by other means than the aliovalent Ca II-for- Lu III substitution turned out to be infeasible: the larger alkaline earth element, Sr, did not enter the phase and high-pressure oxygenation treatments resulted in stabilization of the Lu 2Mn 2O 7 pyrochlore phase.

Compression mechanisms of symmetric and Jahn–Teller distorted octahedra in double perovskites: A2CuWO 6 ( A=Sr, Ba), Sr 2CoMoO 6, and La 2LiRuO 6

Synchrotron X-ray powder diffraction was used to study the high pressure dependence of the lattice parameter and structural evolution of the monoclinic La 2LiRuO 6 and tetragonal Sr 2CoMoO 6, Sr 2CuWO 6, and Ba 2CuWO 6 double perovskite phases. The c lattice parameters of Sr 2CuWO 6 and Ba 2CuWO 6 decreased more rapidly than the a lattice parameters and Ba 2CuWO 6 exhibited a more anisotropic compression compared to Sr 2CuWO 6. Based on lower pressure refinements of Ba 2CuWO 6, the anisotropic compression is proposed to be due to the preferential compression of the Cu–O bonds containing cooperative Jahn–Teller distortions aligned parallel to the c-axis, which is in contrast to Sr 2CoMoO 6, where the change in the octahedral tilting and symmetric bond compression is the prevailing compression mechanism. The bulk moduli were obtained from a fit of the volume–pressure data using the second-order Murnaghan equation of state.

Crystalline and magnetic structures of La1−x BixMnO3+δ manganites

The crystalline and magnetic structures and magnetic properties of La{sub 1-x}Bi{sub x}MnO{sub 3+{delta}} (0.4 {<=} x {<=} 0.6, 0 {<=} {delta} {<=} 0.06) manganites have been studied. The solid solutions having the stoichiometric oxygen content are shown to be orbitally ordered A-type antiferromagnets. An increase in the oxygen content above the stoichiometric value is found to cause Mn{sup 4+} ions in the perovskite lattice, to remove the cooperative Jahn-Teller distortions, and to form a long-range ferromagnetic order. This order becomes broken as the concentration of the tetravalent manganese ions increases further. The tendency toward breaking the ferromagnetic order increases with the bismuth content. The magnetic properties are interpreted in terms of superexchange interactions on the assumption of local lattice distortions induced by anisotropy of the 6s{sup 2}(Bi{sup 3+})-2p{sup 6}(O{sup 2-}) chemical bonds.

Antiferroelastic structural transitions in PrAlO3 by means of neutron diffraction

AbstractStructural phase transitions of the perovskite‐type PrAlO3 have been studied by using neutron diffraction. The results show that, on cooling, the structures are successively transformed from R$ \bar 3 $c (Phase II) to Imma (Phase III), Imma (Phase IV), I4/mcm (Phase V) by the 215‐K, 153‐K, and 122‐K transitions. The tilting schemes in Phase III, IV and V are mainly characterized by (ψa = ψc, ψb = 0), (ψa ≠ ψc, ψb = 0), and (ψc, ψa = ψb = 0), respectively, where ψa, ψb and ψc are rotational angles around a‐, b‐, and c‐primitive axes of the AlO6 octahedron. The refreezes of the condensed Rand R optical soft modes occurring at about 215 K and 122 K are interpreted by correlation with the cooperative Jahn–Teller distortion (JTD) of the Pr3+–4f orbitals in the PrO12 polyhedra. The phase IV is characterized as an intermediate state, in which the ψa tilts are continuously reduced by the JTD and disappear just above the 122‐K transition. The easy mobility of the domain walls suggests the PrAlO3 crystal to be of antiferroelastic nature, an unusual property that results from the cooperative displacements of O ions induced by the correlation between the antiphase tilts along primitive axes of the AlO6 octahedra and the JTD around Pr+3 ions in PrO12 polyhedra. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spin state of cobalt ions in NdCoO3

The crystal structure of NdCoO3 is studied by neutron diffraction. It is shown that static cooperative Jahn—Teller distortions are absent up to the temperature of transition to the metallic state. The results are discussed within the model considering the transition of cobalt ions from the low-spin to the intermediate-spin state by gradual thermal excitation without a clearly pronounced critical behavior.