Features of high-frequency ultrasound propagation in the temperature range of structural and magnetic phase transitions in La1−x SrxMnO3 (x=0.175) manganite

The propagation of ultrasonic waves at a frequency of 770 MHz in a La_0.825Sr_0.175MnO₃ single crystal is investigated in the temperature range 350–150 K. It is found that the velocity, attenuation, and mode composition of ultrasonic waves change at temperatures of 315–280 and 220 K. These changes correlate with the structural and magnetic phase transitions and can be explained in terms of the Jahn-Teller distortions of the crystal lattice.     

Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

准一维电荷密度波材料Ta2NiSe7 的转角及变温拉曼光谱研究

近年来, 低维过渡金属硫族化合物Ta2NiSe7 因其独特的电荷密度波和特殊的拓扑能带结构而引起广泛关注. 本文利用拉曼光谱技术对准一维电荷密度波材料Ta2NiSe7 的声子模式进行了系统研究. 角度依赖的偏振拉曼光谱实验表明室温下可以观测到19 个拉曼峰, 且所有观察到的拉曼模式强度随样品旋转呈周期性变化, 这意味着Ta2NiSe7 具有较高的面内各向异性. 通过群论分析, 我们确定了拉曼峰的具体模式为Ag 和Bg 两种原子振动模式. 在温度依赖的偏振拉曼光谱测量中, 我们在低温下还观察到了新的声子峰, 推测与电荷密度波结构调制引起的晶格畸变有关. 本研究提供了对Ta2NiSe7 声子振动的全面理解, 这可能为进一步研究电荷密度波与声子振动之间的关联提供参考意义.     

Jahn-teller dynamics in charge-ordered manganites from raman spectroscopy

The Raman spectra of the charge-ordered manganite Pr0.65Ca0.35MnO3 were studied as functions of temperature and excitation energy and compared to magnetic moment and electrical conductivity behaviors. Both the charge ordering ( T(co) approximately 225 K) and the antiferromagnetic transitions ( T(N) approximately 175 K) affect the spectral shape and intensity, indicating strong charge-lattice and spin-lattice couplings. Below T(co) a transition from dynamic Jahn-Teller distortions to a collective static distortion takes place. A change of the spectra is observed on increasing the excitation energy above 2.5 eV and it is attributed to a resonant polaron excitation.     

Density functional studies of LaMnO3 under uniaxial strain

We study the electronic and magnetic properties of tetragonal LaMnO₃ (LMO) under uniaxial strain, appropriate for epitaxially grown LMO heterostructures, from density functional calculations. The optimized tetragonal structure without strain has volume, magnetic order, and Jahn-Teller distortions similar to the bulk LMO, which forms in the orthorhombic structure. Strain affects the relative magnitudes of these distortions and changes the magnetic and conduction properties. While unstrained LMO is a type A antiferromagnet and insulating, we find that it changes to a ferromagnetic metal under tensile strain condition (c-axis stretched). The latter is the result of a diminishing magnitude of the Jahn-Teller distortion with strain, which in turn reduces the splitting of the Mn-e_g orbitals, eventually leading to a metallic state, and finally to a ferromagnet due to the double exchange interaction. The calculated Poisson's ratio from geometry optimization agrees with the experimental values for the bulk.     

A possible correlation between a Jahn-Teller coupling and a Raman scattering

It has been shown how the Raman line of a degenerate vibration can be caused by a vibronic coupling in a degenerate electronic excited state. Such a vibronic coupling is known to cause a distortion in the equilibrium conformation of the molecule (Jahn-Teller distortion) from the symmetrical conformation, and the Raman scattering tensor is found to be calculated by the use of the amount (δ) of this distortion as an empirical parameter. It has been suggested that some of the Raman lines for the degenerate stretching vibrations, which become stronger with the exciting frequency, in some molecules, are caused by such Jahn-Teller couplings. For the intensity of the Raman line at 887 cm^?1 of the degenerate stretching vibration of chromate anion, a slightly more detailed examination has been made, and the amount of the Jahn-Teller distortion in a $\text{B}\text{?}$ (T₂) electronic state has been estimated to be 0.2₀ Å amu^1/2 along the normal coordinate of this vibration.     

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.     

Reverse polarized highly intense Raman transitions in Jahn-Teller and non-Jahn-Teller modes

Intensity enhancement effects in vibrational Raman spectra of electronically degenerate states are investigated. It is shown that the fundamental and overtone Raman transitions in Jahn-Teller active modes are strongly intensified via enhancement effects involving the symmetric and anti-symmetric Raman polarizability tensors. Such enhancements ensue via intensity borrowing from the Rayleigh scattering process. It is explicitly shown that the isotropic Raman polarizability tensor associated with these transitions is not subjected to enhancement effects. Thus the intense fundamental and overtone Raman transitions in the Jahn-Teller active modes will be reverse polarized (or depolarized) rather than polarized. Moreover, fundamental and overtone Raman transitions associated with vibrations which couple with Jahn-Teller active modes via mixed cross quadratic nuclear potential interactions may also be intensified and reverse polarized. The present work modifies and extends the selection rules which were established long ago by M. S. Child and H. C. Longuet-Higgins [Philos. Trans. R. Soc. London A 254, 259–294 (1961)].     

Selection rules and Raman spectrum of a molecule with a degenerate ground state and zero spin-orbit coupling: VCl4

Two interesting features can normally be expected in the Raman spectrum of a molecule with a degenerate electronic ground state (i) the contribution of asymmetric or even antisymmetric tensors to the Raman intensity (ii) lifting of the electronic degeneracy due to Jahn-Teller distortion. The experimental data and discussion on VCl₄, presented here, show that this molecule does not show any deviations in the Raman selection rules. Further, its Raman spectrum closely resembles that of TiCl₄ which also has T_d geometry but a nondegenerate singlet ground state. This suggests the absence of any static Jahn-Teller distortion in VCl₄. However, we have observed a broadening of the ν₂ band of VCl₄ over TiCl₄ and we attribute this to a possible dynamic Jahn-Teller splitting.     

Lattice,electrical, and magnetic effects in lanthanum manganites La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175)

The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La_1−x Sr _x MnO₃ (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.     

Mechanisms of off-center ion behavior in oxygen-octahedral perovskite-type ferroelectrics

A mechanism of the off-center impurity-ion behavior induced by the quadratic Jahn-Teller effect on polar distortions and (or) joint action of the local electrostriction and linear Jahn-Teller effect on nonpolar distortions is proposed. The off-center behavior of Ni⁵⁺ and Fe⁴⁺ ions in the incipient ferroelectric KTaO₃ is substantiated. It is shown that, in the limit of neglecting the correlation between off-center Li⁺ ions, their displacements from KNbO₃ lattice sites decrease, whereas, under the conditions of strong correlation, a critical increase in their displacements from lattice sites occurs with approaching the Curie point.     

Peierls instabilities in one-dimensional systems with Jahn-Teller-active monomers

A one-dimensional system is investigated which consists of monomers with Jahn-Teller-active vibrational coordinates. Peierls distortions in the ground states of the half- and quarter-filled systems are studied as a function of the electron-lattice and Jahn-Teller coupling constants. For both degrees of filling and for all the values of the coupling constants the band structure of the ground state is doubly degenerate. The interrelation is discussed between the present system and systems where the active internal degrees of freedom are not of the Jahn-Teller type.     

Behavior of the atomic and magnetic structure of La0.35Pr0.35Ca0.30MnO3 at a metal-insulator phase transition

The evolution of the structural and magnetic properties of the CMR (colossal-magnetoresistance) compound La_0.35Pr_0.35Ca_0.30MnO₃ as the temperature changes from 10 to 293 K is investigated by means of neutron diffraction. It is shown that the changes in the transport and magnetic properties are directly related with the rearrangement of the atomic structure. A phase transition to the metallic state occurs together with simultaneous ferromagnetic ordering of the manganese moments and is accompanied by a jump in volume. The static distortions of the oxygen octahedra which are observed to occur prior to the magnetic phase transition and which are practically absent at room temperature and in the FM phase attest to the orbital ordering of oxygen atoms on the bonds, with freezing-in of the Jahn-Teller modes. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 672–677 (10 May 1998)     

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.     

拉曼光谱技术在脂质过氧化反应中的应用

本实验以亚油酸为材料,分别采用过氧化产物检测法和拉曼光谱技术对亚油酸的自氧化过程进行监测,从氧化反应的中间产物和底物的结构变化两个角度,对亚油酸的自氧化过程进行分析。为不饱和脂肪酸的过氧化机理研究提供实验证据。结果表明:拉曼光谱技术能够用来研究单分子物质的结构变化,随着自氧化过程的进行,亚油酸的拉曼光谱在谱型和峰强方面都有所变化,能够反映分子内部结构的变化。其时间效应与采用TBA和共轭二烯法测定时的时间效应基本一致。     

压力对La0．3Bi0．2Ca0．5MnO3中晶格畸变的影响

利用同步辐射X射线衍射技术,对La0.3Bi0.2Ca0.5MnO3中存在的Jahn-Teller畸变进行了原位的高压研究。实验表明,外加压力能有效地影响到晶格中Mn－O键长和Mo-O-Mn键角的变化。当压力为1．8GPa时,在晶格中存在的两种不同畸变模式之间的相互作用下,导致了位于a-b基面上的Q2畸变模式的消失。     

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     

Raman scattering spectra of ceramics, films, and superlattices of ferroelectric perovskites: A review

Raman investigations of the crystal lattice dynamics in classical ferroelectric perovskites SrTiO₃, PbTiO₃, and BaTiO₃ have been analyzed. The specific features revealed in the behavior of soft modes during the phase transitions occurring in ceramics and powders of these compounds, as well as in several related solid solutions, have been described. Particular attention has been paid to the investigations of ferroelectric thin films and superlattices in which the sequences of structural distortions can be radically different from those known for the initial bulk materials.     

Jahn-Teller solitons, structural phase transitions, and phase separation

It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable Q-ball-like nontopological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudorotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn-Teller coupling approaches this critical value, the size of the soliton diverges signaling an incipient structural phase transition. The soliton phase mimics features commonly attributed to phase separation in complex solids.     

Investigations of the local lattice distortions and the EPR parameters for the orthorhombic Pt3+ center in YAG

The local lattice distortions and the EPR parameters (anisotropic g factors and the hyperfine structure constants) for the orthorhombic Pt³⁺ center in YAG are theoretically investigated from the perturbation formulas of these parameters for a 5d⁷ ion in an orthorhombically elongated octahedron. The [ PtO₆] ^9- cluster on Al³⁺ site is found to experience the axial elongation of about 0.01 Å along Z axis, and the planar impurity-ligand bond lengths suffer the relative variation of about 0.11 Å? along X and Y axes due to the Jahn-Teller effect. As a result, the above local lattice distortions produce significant orthorhombic deformation, whereas the original slight trigonal distortion of the host [ AlO₆] ^9- octahedron is entirely depressed by the Jahn-Teller effect. The calculated EPR parameters based on the above lattice distortions show good agreement with the experimental data, and the local structure of the impurity center is also discussed.