Tetragonal Jahn-Teller Distortion Research Articles

Structural and elastic properties of tetragonal nano-structured copper ferrite

Abstract The surge of interest in ferrimagnetic materials over the years has been driven by their fascinating multifunctional properties. The elastic properties of these materials are a subject of much interest and vital importance due to their wide range of technological and industrial applications. In the present work, the structural and elastic properties of tetragonal copper ferrite (CuFe2O4) nanoparticles synthesized by a citrate assisted sol-gel auto combustion technique have been investigated. The nanocrystalline powder of copper ferrite was characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and UV–Visible spectroscopic techniques. The X-ray diffraction pattern revealed pure spinel structure of copper ferrite nanoparticles with Jahn–Teller tetragonal distortion. The band gap of copper ferrite nanoparticles was found to be 3.14 ± 0.03 eV using the diffuse reflectance spectra. The values of elastic moduli (Young’s modulus (E) = 186.3 ± 0.5 GPa, bulk modulus (B) = 124.1 ± 0.3 GPa, and rigidity modulus (G) = 74.4 ± 0.2 GPa) and Debye temperature (θ D = 500.5 ± 0.5 K) suggest that the synthesized nano-structured copper ferrite is a promising candidate for high temperature and high pressure applications.

Crystallographic studies of fac- and mer-isomers of Cu[1,4,7,10,13,16-hexaazacyclooctadecane]2+. Influence of counterion and crystallization solvent on crystal structure and dynamic Jahn-Teller effects. Computational studies of structures, energetics, and mechanism of mer-fac isomerization

The syntheses and crystal structures of mer- and fac-isomers of [Cu(18N6)]2+(X−)2 are reported (18 N6 = 1,4,7,10,13,16-hexaazacyclooctadecane; X− = BF4−, ClO4−, NO3−) and compared to a previously reported example (X− = Cl−). Solution structures of the [Cu(18N6)]2+ dication in MeNO2 or MeOH are shown by FT-IR to be almost exclusively mer, and simple evaporation of these solutions or crystallization using Et2O co-solvent also afford the solid mer-salts. However, using C6H6 or CCl4 as crystallization co-solvents afforded the corresponding fac-isomers of the BF4− and ClO4− salts, in which the anions and the symmetrical solvent occupy locations on the crystallographic 3-fold axis. Similar treatment of the less symmetrical NO3− salts affords the mer-isomer without inclusion of C6H6. The solid-state structures of the mer-isomers each exhibit the anticipated tetragonal Jahn-Teller distortions, with two longer Cu-N bonds. In contrast the fac-isomers all show internally identical Cu-N distances, due to a dynamic exchange between Jahn-Teller distorted structures in the crystal. Density Functional Theory (DFT) is used to provide rationalizations of these observations and to elucidate an energetically feasible mechanistic pathway for the facile interconversion of mer and fac isomers in solution.

Raman scattering study and lattice-dynamics calculations in YTiO3: Precursor of the magnetic phase transition in the phonon anomalies

The origin of the low-temperature ferromagnetic instability in YTiO3 driven by the interplay between lattice, orbital, spin, and charge degrees of freedom is on the background of lattice dynamics. We present a comprehensive study of temperature-dependent polarized Raman scattering in a nearly stoichiometric orthorhombic YTiO3 single crystal. The lattice-dynamics calculations authenticate the assignment for the observed Ag- and B2g-symmetry modes in terms of the atoms vibrational patterns. The obtained results suggest that trigonal GdFeO3-type distortions become unstable against tetragonal Jahn-Teller distortions, leading to the consequent orbital structure refinement and ferromagnetic ground state in YTiO3.

Possible absence of trimeron correlations above the Verwey temperature in Fe3O4

The origin of the metal to insulator transition in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ remains a challenge due to the complexity of the system: it is a mixed valent, strongly correlated system where many interactions such as Jahn-Teller distortion, exchange, and phonons are very close in energetics. A recent interpretation of the Verwey transition as an ordering of a three-site magnetic polaron, the trimeron, has been put forward. Here we investigate the existence of the trimeron correlations in the high-temperature phase of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ using high-resolution iron $2p3d$ resonant inelastic scattering magnetic linear dichroism. Guided by theoretical simulations, we reveal that the polarization dependence of the low-energy spin-orbital excitations is incompatible with tetragonal Jahn-Teller trimeron-type distortion. We conclude that the lowest-energy state of the high-temperature phase of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ arises from an intricate interplay between trigonal crystal-field, exchange, and spin-orbit interactions.

Effect of Jahn-Teller distortion on the short range magnetic order in copper ferrite

Copper ferrite of spinel crystal structure was synthesized in the form of nano-particles using citrate-gel auto-combustion method. The sample morphology and composition were identified using scanning electron microscopy, X-ray diffraction, and X-ray spectroscopy. The latter technique reveals an inverse spinel structure with Jahn-Teller tetragonal distortion. The static magnetization was measured using vibrating sample magnetometer. Magnetic force microscopy was used in combination with the magnetization data to demonstrate the finite size effect of the magnetic spins and their casting behavior due to the introduction of copper ions in the tetrahedral magnetic sub-lattices, which results in tetragonal distorting the spinel structure of the copper ferrite. The magnetic properties of materials are a result of the collective behavior of the magnetic spins, and magnetic force microscopy can probe the collective behavior of the magnetic spins in copper ferrite, yet providing a sufficient resolution to map the effects below the micrometer size scale, such as the magnetic spin canting. A theoretical study was done to clarify the finite size effect of Jahn-Teller distortion on the magnetic properties of the material. When the particles are in the nano-scale, below the single domain size, their magnetic properties are very sensitive to their size change.

Integration of Ligand Field Molecular Mechanics in Tinker.

The ligand field molecular mechanics (LFMM) method for transition-metal complexes has been integrated in Tinker, an easily available and popular molecular modeling software package. The capability to calculate LFMM potentials has been provided by extending the functional forms of the Tinker package as well as by integrating routines for calculating the ligand field stabilization energy (LFSE), which is central to LFMM. The capabilities of the implementation are illustrated by both static calculations on the two spin states of [Fe(NH3)6](2+) and on [Cu(NH3)m](2+) (m = 4, 5, 6) and dynamic (LFMD) simulations of an FeN6-type spin-crossover compound. In addition to showing that results obtained with the Tinker-LFMM implementation are consistent with those of experiment and other computational methods and programs, we note that whereas LFMM is able to handle the conventional tetragonal Jahn-Teller distortion of the bond distances in [Cu(NH3)6](2+), the LFSE term is also necessary in order to obtain even qualitatively correct coordination geometries for the two lower-coordinate copper complexes.

Ion channeling study of lattice distortions in chromium-doped SrTiO3 crystals

The results of ion channeling studies of lattice distortions in SrTiO3: Cr single crystals are presented. Two types of single crystals containing the same amount of Cr impurities but differing in stoichiometry have been investigated. The single crystals grown by the Verneuil method have the compositions of standard-grown SrTiO3: Cr (0.05 at % Cr), whereas the single crystals grown with a strontium deficiency and a chromium compensating amount have the composition Sr0.9995TiO3 (0.05 at % Cr). Analysis of the angular channeling spectra indicates that, in crystals of both types, the main defects are Cr impurities located in octahedral sites. In the SrTiO3: Cr crystals, impurity atoms manifest themselves as Cr4+ with tetragonal Jahn-Teller distortions of the surrounding lattice. In the Sr0.9995TiO3: Cr crystals grown with a Sr deficiency, the characteristic displacements of Ti ions in the third coordination sphere of the Jahn-Teller center Cr4+ exhibit the effect of interaction of the center with a neighboring vacancy in the Sr sublattice.

Magnetic behavior of Ni and Co doped CuMn2O4 spinels

Mn1.68Co0.24Ni0.48Cu0.6O4 was produced via slip casting and sintering of spray-pyrolysis produced powders. The magnetic properties of this composition were measured for the first time, as a function of the processing temperature (900 °C or 1000 °C sintering), in order to study the effects of Cu and Mn valence and site preference. Quantitative x-ray photoelectron spectroscopy showed that Cu+ site occupancy changed from tetrahedral to a mix of tetrahedral and octahedral with increasing sintering temperature. X-ray diffraction demonstrated that the materials had a cubic spinel structure devoid of tetragonal Jahn-Teller distortion. ac magnetic susceptibility indicated ferrimagnetic behavior below ∼109 K and spin glass behavior below ∼66 to 74 K, depending on the measurement frequency. ac susceptibility freezing temperatures were modeled with the Vogel-Fulcher law and showed intermediate characteristics, between those of canonical spin glasses and cluster glasses.

CuN6 Jahn–Teller centers in coordination frameworks comprising fully condensed Kuratowski-type secondary building units: phase transitions and magneto-structural correlations

The metal-organic framework [Cu(ta)(2)] (Hta = 1H-1,2,3-triazole), containing Jahn-Teller active Cu(II) ions and 1,2,3-triazolate ligands, is prepared under solvothermal reaction conditions. The compound shows a reversible phase transition from the tetragonal crystal system (α-[Cu(ta)(2)]: space group I4(1)/amd (no. 141), a = 11.8447(7) Å, c = 18.9782(13) Å, V = 2662.6(3) Å(3)) to the cubic crystal system (β-[Cu(ta)(2)]: space group Fd3m (no. 227), a = 17.4416(15) Å, V = 5305.9(8) Å(3)) within the temperature range of 120-160 °C. Both [Cu(ta)(2)] polymorphs have identical bonding topologies that might be described as fully condensed Kuratowski-type pentanuclear secondary building units of local T(d) point group symmetry in which four Cu(II) ions occupy the vertices of an imaginary tetrahedron. α-[Cu(ta)(2)], as opposed to the high-temperature β-phase, shows a strong tetragonal Jahn-Teller distortion of CuN(6) coordination octahedra. The compounds are characterized by elemental and thermogravimetric analyses, single crystal and powder X-ray diffraction, FTIR-, UV-vis and fluorescence spectroscopy. Magnetic susceptibility investigations reveal two different Cu(II) sites at a ratio of 1 : 2, in agreement with the solid state structure of [Cu(ta)(2)]. At low temperatures the formation of antiferromagnetically coupled Cu(II) dimers is observed, leading to a spin frustration of roughly 1/3 of all magnetically active Cu(II) sites.

Jahn-Teller stabilization of magnetic and orbital ordering in rocksalt CuO

Self-interaction-corrected local-density-functional calculations show that rocksalt-structure CuO spontaneously becomes insulating and magnetically ordered by tetragonal Jahn-Teller distortion of Cu-centered octahedra. The equilibrium structure is axially expanded by 9%, has a charge-transfer gap of 1.1 eV, and antiferromagnetic ordering-II with a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ spin-polarized hole and a Ne\'el temperature of 530 K within the anisotropic Ising model.

How to Build Molecules with Large Magnetic Anisotropy

Predicting single-molecule magnets? Magnetic anisotropy, a property that plays a key role in single-molecule magnets (SMMs), has been analyzed by using theoretical methods. Mononuclear complexes and the dependence of the magnetic anisotropy on their geometrical and electronic structure, as well as how such mononuclear complexes must be combined as building blocks to obtain polynuclear complexes with large anisotropy (see figure) are considered.The magnetic anisotropy of mononuclear transition-metal complexes has been studied by means of electronic structure calculations based on density functional theory. The variation of the zero-field splitting (ZFS) parameters has been analyzed for the following characteristic distortions: a tetragonal Jahn-Teller distortion, the Bailar twist, the Berry pseudorotation, and the planarization of tetrahedral complexes. Finally, the coupling of mononuclear building blocks in polynuclear complexes to obtain a large negative magnetic anisotropy necessary to improve their single-molecule-magnet (SMM) behavior has been studied.

Application of Symmetry Operation Measures in Structural Inorganic Chemistry

This paper presents an application of the recently proposed symmetry operation measures to the determination of the effective symmetry point group of coordination polyhedra in inorganic solids. Several structure types based on octahedra are found to present distinct distortion patterns each, not strictly attached to the crystallographic site symmetry. These include the (NH4)2[CuCl4], CdI2 (brucite), FeS2 (pyrite), TiO2 (rutile), CaCl2, GdFeO3, PbTiO3,LiNbO3, BiI3, CrCl3, Al2O3, and NiWO4 structures. It is shown that a similar analysis can be applied to the Bailar and tetragonal Jahn-Teller distortions of molecular transition metal complexes, as well as to solids based on tetrahedra, such as the ZnCl2, FeS, BeCl2, SiS2, and KFeS2 structure types.

Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds, and crystal structure of KCuBr3

AbstractVibrational spectroscopic and force field studies have been performed of 15 related copper(II) chloride and copper(II) bromide compounds, including hydrated salts crystallizing in ternary aqueous systems with alkali and ammonium halides. For halocuprates with distorted octahedral coordination characteristic stretching Raman wavenumbers, corresponding to symmetric stretching CuIIX modes in the equatorial plane, were found in the ranges 247–288 cm−1 for X = Cl, and 173–189 cm−1 for X = Br, while the low‐wavenumber stretching modes for the weaker axial CuX interactions varied considerably. The tetrahedral coordination for Cs2CuCl4 and Cs2CuBr4 leads to somewhat lower CuX symmetric stretching wavenumbers, 295 and 173 cm−1, respectively. The assignments of the copper–ligand stretching vibrations were performed with the aid of normal coordinate calculations. Correlations between force constants, averaged CuX stretching wavenumbers and bond distances have been evaluated considering the following aspects: (1) Jahn–Teller tetragonal distortion (axial elongation) of the octahedral copper(II) coordination environment, (2) differences between terminal and bridging halide ligands (3) effects of coordinated water and the influence of outer‐sphere cations. Force constant ratios for terminal and bridging metal–halide bonds reveal characteristic differences between planar and tetrahedrally coordinated M2X6 species. In the hydrated copper(II) halide complexes, the halide ligands are more strongly bound than coordinated water molecules. The crystal structure of KCuBr3 (K2Cu2Br6), which was determined to provide structural information for the force field analyses, contains stacks of planar dimeric [Cu2Br6]2− complexes held together by weak axial CuBr interactions. Copyright © 2007 John Wiley & Sons, Ltd.

Inhomogeneous lattice distortions in a Zn1 − x CrxSe crystal

The structure of a Zn1−xCrxSe semiconductor crystal with x = 0.0029 was studied using thermal neutron diffraction. It was detected that the diffraction patterns of the crystal contain regions of diffuse scattering concentrated in the vicinity of the strong Bragg reflections. It was shown that the diffuse scattering was caused by transverse displacements of lattice atoms induced by chromium impurities, which undergo static tetragonal Jahn-Teller distortions in ZnSe.

Synthesis and superoxide dismutase-like activity of new manganese(III) complexes based on tridentate N 2O ligands derived from histamine

We have obtained two new manganese(III) complexes based on tridentate ligands bearing imidazole and phenol moieties. The structure of the chosen ligands favored the Mn(III) oxidation state due to the compatibility of the geometry of the complexes with a Jahn–Teller tetragonal distortion, as was shown by X-ray diffraction for one of the complexes. This induced a lowering of the oxidation potential for the Mn(III)/Mn(II) couple, which can be correlated to the superoxide dismutase-like (SOD-like) activity of the complexes, compared with a previously published Mn(III) complex.

Orbital ordering and one-dimensional magnetic correlation in vanadium spinel oxides [formula omitted

We present our theoretical results on the mechanism of two transitions in vanadium spinel oxides AV 2 O 4 ( A = Zn , Mg, or Cd) in which magnetic V cations constitute a geometrically frustrated pyrochlore structure. We have derived an effective spin-orbital-lattice coupled model in the strong correlation limit of the multiorbital Hubbard model, and applied Monte Carlo simulation to the model. The results reveal that the higher-temperature transition is a layered antiferro-type orbital ordering accompanied by tetragonal Jahn–Teller distortion, and the lower-temperature transition is an antiferromagnetic spin ordering. The orbital order lifts the magnetic frustration partially, and induces spatial anisotropy in magnetic exchange interactions. In the intermediate phase, the system can be considered to consist of weakly coupled antiferromagnetic chains lying in the perpendicular planes to the tetragonal distortion.

Orbital and magnetic transitions in geometrically frustrated vanadium spinels: Monte Carlo study of an effective spin-orbital-lattice coupled model

We present our theoretical and numerical results on thermodynamic properties and the microscopic mechanism of two successive transitions in vanadium spinel oxides $A{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ ($A=\mathrm{Zn}$, Mg, or Cd) obtained by Monte Carlo calculations of an effective spin-orbital-lattice model in the strong correlation limit. Geometrical frustration in the pyrochlore lattice structure of V cations suppresses development of spin and orbital correlations, however, we find that the model exhibits two transitions at low temperatures. First, a discontinuous transition occurs with an orbital ordering assisted by the tetragonal Jahn-Teller distortion. The orbital order reduces the frustration in spin-exchange interactions and induces antiferromagnetic correlations in one-dimensional chains lying in the perpendicular planes to the tetragonal distortion. Second, at a lower temperature, a three-dimensional antiferromagnetic order sets in continuously, which is stabilized by the third-neighbor interaction among the one-dimensional antiferromagnetic chains. Thermal fluctuations are crucial to stabilize the collinear magnetic state by the order-by-disorder mechanism. The results well reproduce the experimental data, such as transition temperatures, temperature dependence of the magnetic susceptibility, changes of the entropy at the transitions, and the magnetic ordering structure at low temperatures. Quantum fluctuation effect is also examined by the linear spin-wave theory at zero temperature. The staggered moment in the ground state is found to be considerably reduced from saturated value and reasonably agrees with the experimental data.

Synthesis, X-ray structure, polarized optical spectra and DFT theoretical calculations of two new organic-inorganic hybrid fluoromanganates(III): (bpaH2)[MnF4(H2O)2]2 and (bpeH2)[MnF4(H2O)2]2.

Two new fluoromanganates(III) of 1,2-bis(4-pyridyl)ethane (bpa) and trans-1,2-bis(4-pyridyl)ethylene (bpe), LH(2)[MnF(4)(H(2)O)(2)](2) (L = bpa or bpe), have been prepared and their structure have been solved by single-crystal X-ray diffraction. The [MnF(4)(H(2)O)(2)](-) anion displays an octahedral geometry with a strong Jahn-Teller tetragonal distortion along the H(2)O-Mn-OH(2) axis. The equatorial metal-ligand distances (Mn-F 1.827(1)-1.859(2) A) are shorter than the axial ones (Mn-O 2.203(2)-2.234(2) A). Three polarized absorption bands at 22,500, 18,300 and 14,500 cm(-1) are observed in the optical spectra of (bpaH(2))[MnF(4)(H(2)O)(2)](2). Finally, we present theoretical calculations on the equilibrium bond distances as well as the crystal-field electron structure using density functional methods. The calculated Mn-F bond distances (1.85 A) are in agreement with the experimental data but the obtained Mn-O distances (2.53-2.56 A) are higher than the experimental one as usually found in similar Jahn-Teller distorted systems. The calculated d-d transition energies are compared with experimental energies derived from the optical spectra. The variation of the HOMO energy and transition energies against the Mn-O distance is also shown.

Intermediate spin state stabilized by the Jahn-Teller distortion inLa1/2Ba1/2CoO3

The recent high-resolution neutron and synchrotron diffraction measurements by Fauth et al. [Phys. Rev. B 65, 060401(R) (2002)] demonstrated the existence of an intermediate spin state in half-doped ${\mathrm{La}}_{1/2}{\mathrm{Ba}}_{1/2}{\mathrm{CoO}}_{3}$ which is accompanied by a long-range tetragonal Jahn-Teller distortion. In this paper we find that the charge uniform ferromagnetic intermediate spin state strongly couples to the Jahn-Teller effect and is stable only at moderate distortion of $1%.$ While the calculated magnetic moment of this state is in agreement with the experimentally measured one, the corresponding orbital ordering is the nearest-neighboring alternating ${d}_{{3x}^{2}\ensuremath{-}{r}^{2}}{/d}_{{3y}^{2}\ensuremath{-}{r}^{2}},$ instead of the observed uniform ${d}_{{3z}^{2}\ensuremath{-}{r}^{2}}$ orbital ordering. A calculation is carried out using the self-consistent unrestricted Hartree-Fock approximation on a realistic multiband d-$p$ Hubbard Hamiltonian and a generalized Jahn-Teller coupling model, in which all possible spin-, charge-, and orbital-ordered states are considered.

Jahn-Teller effect in low-spin Ni3+ in the LaAlO3 ceramic

This paper reports on an EPR study of LaAl1−xNixO3 solid solutions with x≤0.12 made in the 4.2-to 300-K temperature range. In the X range, the broadening of the single EPR line with geff=2.148 was observed at temperatures below ∼40 K. In the Q range, a slightly anisotropic EPR line with g ⊥ ′ =2.145±0.002 and g ‖ ′ =2.154±0.002 transforms to a rhombic-symmetry spectrum with g1=2.183±0.002, g2=2.143±0.002, and g3=2.118±0.002. It is shown that the observed low-temperature effects are due to the lowered symmetry of the complex under the combined action of the tetragonal Jahn-Teller distortions and of the trigonal component of the crystal field.