Topological representations of Jahn-Teller distortions

Topological representations (top-reps), which originally were developed to model molecular polyhedral isomerization processes, can be extended to depict the relationships between the polygons and polyhedra involved in Jahn-Teller (JT) distortions. Using this approach, the top-rep of the E ? (b _1g + b _2g) distortion of square planar molecules to rectangle and rhombus isomers becomes a rhombus in which the vertices alternately represent distortions to the rectangle and rhombus isomers. Similarly the top-rep of the E ? e distortion of regular O_h octahedra to elongated D_4h tetragonal bipyramids becomes a triangle in which the vertices represent the three distinct tetragonal bipyramids from a given octahedron and the edge midpoints represent lower symmetry rhombic D_2h intermediates. A regular octahedron can be used as a top-rep for the T ? (e + t ₂) distortions of regular octahedra if the 6 vertices represent distortions to D_4h tetragonal bipyramid isomers, the 8 face midpoints represent distortions to D_3d trigonal antiprism isomers, and the 12 edge midpoints represent lower symmetry rhombic D_2h intermediates. In the case of Jahn-Teller T ? h distortions of regular I_h icosahedra, the corresponding top-rep becomes a regular icosahedron in which the 12 vertices represent distortions to pentagonal D_5d isomers, the 20 face midpoints represent distortions to trigonal D_3d isomers, and the 30 edge midpoints represent D_2h intermediates. A 4-dimensional analogue of the tetrahedron (i.e. the 4-simplex) can be used as a top-rep for the G ? g problem and the H ? g component of the H ? (g + 2h) problem for JT distortions of regular icosahedra. In this case the 5 vertices and the 10 edge midpoints correspond to T_h isomers and D_3d intermediates, respectively.     

Jahn-Teller distortions of icosahedral complexes

Classification of the stable symmetries of Jahn-Teller systems ofI _h parent group is elaborated for triply, fourfold and fivefold degenerate electronic states with integer spin and for fourfold and sixfold degenerate ones with non-integer spin. The results produced by the epikernel principle and by the method of step-by-step descent in symmetry connected with degenerate states splitting are compared. The results obtained on the basis of epikernel principle are incomplete and may even be incorrect.     

Jahn-Teller distortions for carbon interstitialcy configurations in diamond

Jahn-Teller distortions are calculated by the Extended Hückel Theory for the carbon 100 split and bond-centered interstitial in the diamond lattice. Previous calculations, which did not take into account such distortions, predicted these interstitialcies to be the energetically most favorable ones, with the 100 split interstitial being lowest. The present calculations maintain the essence of that prediction, because the energy gain due to the distortions is about 0.5 eV for both interstitials.Work supported in part by the Office of Naval Research under Contract No. 00014-70-C-0296.John Simon Guggenheim Memorial Fellow.     

Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO3

LaTiO3 is known as a Mott insulator which orders antiferromagnetically at T(N)=146 K. We report on results of thermal expansion and temperature dependent x-ray diffraction together with measurements of the heat capacity, electrical transport measurements, and optical spectroscopy in untwinned single crystals. At T(N) significant structural changes appear, which are volume conserving. Concomitant anomalies are also observed in the dc resistivity, in bulk modulus, and optical reflectivity spectra. We interpret these experimental observations as evidence of orbital order.     

Models of solid-state influences in systems with Jahn-Teller distortions

A system with the Jahn-Teller distortion, namely the complex CuCl ₆ ^4– , has been investigated in various surroundings represented by [CuCl ₆ ^4– ...n Na⁺] ^q clusters. The degree of the tetragonal distortion has been calculated using the quantum-chemical CNDO method and compared with values produced by the point-charge model.Presented at the International Symposium on Solid State Chemistry, October 28–31, 1986, Karlovy Vary, Czechoslovakia.     

Charge state dependent Jahn-Teller distortions of the e-center defect in crystalline Si

The atomic and electronic structures of a lattice vacancy trapped next to an As impurity (the E-center defect) in crystalline Si are investigated using ab initio pseudopotential total energy calculations. Jahn-Teller distortions and energies, reorientation barriers, defect wave function characters, and hyperfine coupling parameters associated with (-) and (0) charge states of the E center are calculated using a combination of real-space cluster and plane wave supercell methods. For the first time in the theoretical study of this defect, the senses of the Jahn-Teller distortions in the two charge states are found to be opposite, changing from a large pairing type in (0) to a large resonant-bond type distortion in the (-) charge state, in agreement with experimental data.     

Electric-field and interface effects on the orientation of Jahn-Teller vacancy distortions in semiconductors

Energy variation of various Jahn-Teller vacancy configurations in thin near-surface layers of a semiconductor is considered. The calculations are made within a model in which each dangling bond near a vacancy is described by the wave function of an electron localized at a defect with a zero-range potential. The Jahn-Teller stabilization energy was assumed to be much smaller than the crystal-field splitting but substantially larger than the energy variations introduced by other perturbations. It is shown that vacancy alignment in semiconductor layers with thickness ?100 Å at liquid-nitrogen temperatures and lower can be most effectively caused by the electric field which can exist in such layers due to a surface-state charge. The direct effect of the interface on both the electronic and nuclear energies of various configurations is weak and can become manifest only in layers less than 10 Å thick at liquidhelium temperature.     

Effect of Jahn-Teller distortions on the structural and magnetic ordering in perovskite-type transition-metal oxides

The effect of local and cooperative distortions of oxygen octahedra containing Jahn-Teller ions on the elastic, electric, magnetic, and optical properties of lightly doped lanthanum strontium manganites and irondoped lithium niobate ferroelectric has been experimentally investigated. The formation of nano-and microscale clusters and domains around, respectively, Mn³⁺ and Fe²⁺ Jahn-Teller ions at phase transitions and in magnetic or electric fields were studied. A model is proposed to consistently explain the results of the investigations of Jahn-Teller ions in magnetic and/or electrically ordered materials.     

Static distortions induced by the Jahn-Teller effect in tetrahedral tetrachlorocuprates(II) a treatment based on the angular overlap model

Absorption and EPR spectra of tetrahedral tetrachlorocuprates(II), are reexamined taking into account simultaneously ligand field and Jahn-Teller effects on the basis of a parametrization method previously proposed by the author.     

Geometric simulation of perovskite frameworks with Jahn-Teller distortions: applications to the cubic manganites

A new approach is presented for modeling perovskite frameworks with disordered Jahn-Teller (JT) distortions and has been applied to study the elastic response of the LaMnO3 structure to defects in the JT ordering. Surprisingly, antiphase domain boundary defects in the pattern of ordered JT octahedra, along the [110] and [110] bonding directions, are found to produce 1D stripe patterns rotated 45 degrees along a* directions, similar to stripe structures observed in these systems. Geometric simulation is shown to be an efficient and powerful approach for finding relaxed atomic structures in the presence of disorder in networks of corner-shared JT-distorted octahedra such as the perovskites. Geometric modeling rapidly relaxes large supercells (thousands of octahedra) while preserving the local coordination chemistry, and shows great promise for studying these complex systems.     

The Jahn-Teller effect in manganic acetylacetonate

The infra-red spectra of the tris-acetylacetonates of chromium, managanese, iron and aluminium have been measured as far as 400 cm^-1. The nature of the bands due to vibrations involving motion of the oxygen atoms relative to the metal is consistent with the operation of the Jahn-Teller mechanism in the manganic compound.     

Nb二维原子薄片中的Jahn-Teller效应

使用基于密度泛函理论的第一性原理平面波赝势法，研究了Nb二维单层原子薄片的结构稳定性和电子结构性质.对其所有的二维晶格结构的计算表明，由于Jahn-Teller效应，对称性较高的正方和六角晶格都是不稳定的二维结构.而稳定的二维结构是由对称性较高的六角晶格畸变后形成的对称性较差的斜方和中心长方结构.Nb单层原子薄片不能形成长方晶格结构.通过计算电子结构和Jahn-Teller效应，进一步讨论了这些结构的相对稳定性以及各二维晶格结构的电子能带和态密度等性质. 关键词： Nb原子薄片 Jahn-Teller效应 电子结构 从头计算     

Shape memory and pseudoelasticity in metal nanowires

Structural reorientations in metallic fcc nanowires are controlled by a combination of size, thermal energy, and the type of defects formed during inelastic deformation. By utilizing atomistic simulations, we show that certain fcc nanowires can exhibit both shape memory and pseudoelastic behavior. We also show that the formation of defect-free twins, a process related to the material stacking fault energy, nanometer size scale, and surface stresses is the mechanism that controls the ability of fcc nanowires of different materials to show a reversible transition between two crystal orientations during loading and thus shape memory and pseudoelasticity.     

Coexistence of tetragonal with orthorhombic or trigonal Jahn-Teller distortions in an Oh complex

The quadratic Jahn-Teller interaction of a triply degenerate T electronic term in an O_h complex, with E_g and T_2g modes, has been investigated. The resulting adiabatic potential-energy surfaces show tetragonal and orthorhombic or trigonal minima coexisting for reasonable values of the appropriate parameters. The effect of the spin-orbit interaction in a ³T term has also been considered.     

Supershell structure in alkali metal nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.     

Pu+4的几何构型和Jahn-Teller效应

在Pu原子的相对论有效原子实势下,用密度泛函B3LYP方法计算得到Pu +4分子离子的6种稳定的几何构型.计算表明Pu+4的Td构型不能稳定存在,详细讨论了 Td构型的Jahn-Teller效应,讨论结果显示它们的各种畸变方式都符合群的分解原理, 从而证明Pu+4分子离子的Td构型的Jahn-Teller效应非常明显.D4h 构型也存在Jahn-Teller效应.     

Jahn-Teller effect at exciton self-trapping

In the adiabatic approximation two limiting cases of exciton self-trapping have been considered for a degenerate atomic term creating the bands of Frenkel excitons, which interact with all crystal deformations admitted by symmetry: a case of a large radius excitation on the self-trapping barrier and a small-radius case. In the first case wave functions and deformations have been found which correspond to the barrier, in the second case the adiabatic surface of self-trapping excitons have been obtained. It is shown that the interaction with non-totally symmetric deformations leads to a lowering of the crystal point symmetry in a state corresponding to the lowest self-trapping barrier, that is the manifestation of the Jahn-Teller effect applied to a self-trapping exciton.