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Register a new userThe cooperative Jahn-Teller ordering investigation by absorption spectra in $KDy(MoO_4)_2$
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Added by
Yuriy Kharchenko M.
Publication date
2014
fields
Physics
and research's language is
English
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Absorption spectra fine structure of $KDy(MoO_4)_2$ in the region of cooperative Jahn-Teller type ordering was studied. Temperature anomalies in the spectra occurring at phase transformation correlate with the ultrasound peculiarities observed earlier. Based on the symmetry approaching, possible activity of the irreducible representations of the rhombic $D_{2h}$ point group was discussed, which lead to the incommensurate phase at cooperative ordering. It was supposed, that coupled $A_u$-type phonon mode may lead to the incommensurate phase existence, which is possible at least in the temperature region 17-12 K.
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We combine infrared reflectivity and EXAFS (Extended X-ray Absorption Fine Structure) techniques to study the solid solution La0.67Ca0.33MnO3 prepared by different methods yielding samples with different insulator-metal transition temperatures (TIM). While the small polaron analysis of the optical conductivity provides a natural description for the higher frequency reflectivity tail of conducting samples, our structural results are in accord with two non-equivalent sites in the insulating phase of good-quality samples. Those sites, one for the Mn3+ Jahn-Teller distorted octahedra and another for the Mn4+ ion, gradually turn into one dynamically averaged below the transition TIM. On the other hand, carriers screening antiresonances near infrared longitudinal optical modes, above TIM, mirror thermal activated small polarons weakly smearing EXAFS oscillations. We associate this to the lack of M3+,M4+ explicit structure in the Mn K-edge absorption band. Extra octahedra, detected by EXAFS below TIM in higher resistivity samples, seem to be excluded of participating in the dynamics of the insulator-metal transition shifting TIM toward lower temperatures.
We consider the superexchange in `frustrated Jahn-Teller systems, such as the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which transition metal ions with doubly degenerate orbitals form a triangular or pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds. We show that this interaction is much different from a more familiar exchange in systems with the 180-degree bonds, e.g. perovskites. In contrast to the strong interplay between the orbital and spin degrees of freedom in perovskites, in the 90-degree exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Due to frustration, the mean-field orbital ground state is strongly degenerate. Quantum orbital fluctuations select particular ferro-orbital states, such as the one observed in NaNiO_2. We also discuss why LiNiO_2 may still behave as an orbital liquid.
The Jahn-Teller (JT) distortion that can remove electronic degeneracies in partially occupied states and results in systematic atomic displacements is a common underlying feature to many of the intriguing phenomena observed in 3d perovskites, encompassing magnetism, superconductivity, orbital ordering and colossal magnetoresistance. Although the seminal Jahn and Teller theorem has been postulated almost a century ago, the origins of this effect in perovskite materials are still debated, including propositions such as super exchange, spin-phonon coupling, sterically induced lattice distortions, and strong dynamical correlation effects. Here we analyze the driving forces behind the Jahn-Teller motions and associated electronic fingerprints in a full range of ABX3 compounds. We identify (i) compounds that are prone to an electronically-driven instabilities (i.e. a pure JT effect) such as KCrF3, KCuF3 or LaVO3 and proceed to relax the structures, finding quantitatively the JTD in excellent agreement with experiment; (ii) compounds such as LaMnO3 or LaTiO3 that do not show electronically driven JTD despite orbital degeneracies, because their strongly hybridized B, d-X, p states supply but too weak JT forces to overcome the needed atomic distortions; (iii) although LaVO3 exhibits similar B, d-X, p hybridizations as LaTiO3, the former compound exhibits a robust electronic instability while LaTiO3 has zero stabilization energy, the reason being that LaVO3 has two electrons t2g2 relative to LaTiO3 with just one t2g1. (iv) We explain the trends in orbital ordering whereby electrons occupy orbitals that point to orthogonal directions between all nearest-neighbor 3d atoms. We thereby provide a unified vision to explain octahedra deformations in perovskites that, at odds with common wisdom, does not require the celebrated Mott-Hubbard mechanism.
By means of in situ synchrotron X-ray diffraction and Raman spectroscopy under hydrostatic pressure, we investigate the stability of the quadruple perovskite LaMn7O12. At 34 GPa, the data unveil a first-order structural phase transition from the monoclinic I2/m symmetry stable at ambient conditions to cubic Im-3 symmetry. Considering that the same structural transition occurs at 653 K upon heating at ambient pressure, we propose a rare scenario of reentrant-type phase transition. In the high-pressure Im-3 phase, the Jahn-Teller distortion of the MnO6 octahedra and the orbital order present in the I2/m phase are suppressed, which is promising to investigate the possibility of pressure-induced Mott insulator-metal transition in the ideal situation of no structural distortions. The observation of a progressive line broadening of almost all Raman modes with pressure suggests that this transition may be incipient above 20 GPa.
We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.
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