Static JT effect

Stereochemical consequences of the JT theorem are straightforward the higher symmetric polyhedra in electronic degenerate states should distort in order that their electronic ground state may become nondegenerate this is the static JT effect. Some possible distortions are exemplified by ... 

Two different kinds of JT effect are distinguished in the literature the static and dynamic JT effects. In the static JT effect the molecule or complex remains distorted in a particular way long enough for the distortion to be detected experimentally. In the dynamic JT effect, the molecule or complex resonates between two or more equivalent modes of distortion, and the distortion is not directly observable [9]. 

In order to distinguish between kinetic and thermodynamic phenomena it is convenient to refer to the former as the 7tr/ i-effect and the latter as the tra/u-influence or static /ra/u-effect". though this nomenclature is by no means universally accepted. However, it appears that to account satisfactorily for the kinetic /rau.s-effect , both it (kinetic) and a (thermodynamic) effects must be invoked to greater or les.ser extents. Thus, for ligands which are low in the Trans series (e.g. halides), the order can be explained on the basis of a u effect whereas for ligands which arc high in the series the order is best interpreted on the basis of a jt effect. Even so, the relatively high position of H , which can have no rr-acceptor properties, seems to be a result of a a mechanism or some other interaction. 

The temperature dependence of the magnetic complex conformation is known to be one of the distinguishable features of Jahn-Teller (JT) dynamics [1]. Keeping in mind electron paramagnetic resonance (EPR) as one of the most effective methods of studying the JT effect, we shall consider the problem of the transition from the low temperature, static JT situation to the high temperature, dynamic, motional averaged JT situation. The results of such processes are discussed in the literature [1-4]. At the same time, some important features - the nature of the transitions... 

In this work we perform an investigation of cooperative static in the monoclinic phase and dynamic in rhombohedral JT effect of pure LaMn03 using pair interionic potentials in shell model approximation with the direct inclusion of the JT term in crystal energy and dynamic matrix of a crystal. The magnetic and RS properties of the rhombohedral LaMn03 are simulated in the framework of the cooperative dynamical effect approximation. 

The temperature of transition is small relative to the value of splitting of the Mn3+ 5E ground state. That is why we suppose that the transition from monoclinic to rhombohedral phase is a transition from static to dynamic JT effect. Because of the strong correlation in motion of different [Mn06] octahedra we suppose that there is a cooperative dynamical effect. 

To understand the role of the JT effect in C60-based materials, we would like to calculate the modification of the shape of the molecule, the magnitude of the distortion, the splitting it induces among the electronic levels, and the gain of energy associated with the distortion. Other important features such as the dynamic or static nature of the distortion will be considered later. 

Historically, the first experimental evidence of the JT effect was observed by ESR by the splitting of the Lande factor (0-tensor) in 1952 on magnetically diluted Cu2+ salts. Indeed this factor is very sensitive to even small deviation from the cubic symmetry, as will be the case for a static JTD. However, in many cases, such effects could be hidden for C60-based materials by broad linewidths arising from strong electron-spin interactions. It is essential to work with well-separated Cgo ions for this effect to be detectable. 

Discussion. Copper in Krypton. The absorption spectra of copper atoms Isolated in rare gas matrices have been extensively studied (15-25) and the triplet of bands at 310nm attributed to a number of different causes. These include (1) spin orbit splitting and static axial site distortion (17), (2) multiple matrix sites (18), (3) exciplex formation between the metal and a single matrix atom (19), (4) long range metal-metal interactions (2 ), and (5) Jahn-Teller (JT) effect resulting from matrix cage atom vibrations on the excited metal (21,22,23). The MCD of Cu atoms in the rare gas matrices has recently been reported (24,25) and the results interpreted as consistent with either the distorted site or JT hypotheses (39). 

ESR was the experimental technique, which first gave clear evidence for the JT effect and also showed a static or dynamic behavior of the same system at different temperatures In fact the rather long characteristic time of measurement is of the right order of magnitude to reveal both static and dynamic JT effects in dependence on temperature. 

Fig. 3 a, b. Static or dynamic JT effect as a function of temperature or spectroscopic technique. ESR (a) and ligand field (b) spectra of elpasolite-type mixed crystals Ba2Zni, Cu, WO at different temperatures (adapted from Ref. 42)... 

Inspecting Table 2 there is no doubt that electron-electron interactions and high-symmetry contribution of the ligand field have to be taken into account first. The problem arises when one wants to introduce vibronic interactions, which are often greater than or comparable with spin-orbit coupling or low-symmetry static fields. In such cases JT effect cannot be neglected, but has to be treated simultaneously with the other interactions. 

Stretching mode 0 g. The original idea was that static JT distortions could produce the large observed anisotropy however, a simple static treatment showed comparable energies of the zero-point vibration (fiw = 350 cm or 225 cm" ), the JT stabilization (Ejt = 516 cm" ), the spin-orbit interaction = 435 cm" ) and the rhombic field due to the azide ion (Hrj, = 60 cm" ). Therefore all interactions had to be considered simultaneously including dynamic effects. The used Hamiltonian was... 

Fig. 21 STM simulations for systems subject to a static vs. dynamic JT effect. The top row corresponds to the excited state and the bottom to the ground state. In (a), infinitely strong coupling locks the molecule into one particular well. Finite but strong coupling (so that the system jumps between three wells) is shown in (b). Further reduction in localisation leads to essentially free pseudorotation, producing the time-averaged images in (c)...

We can see from Fig. 21 that if the JT effect is dynamic on the time-scale associated with STM capture, then the recorded image takes on a much more symmetrical D h) appearance than when the effect was considered static. However, even if the pseudorotation rate is very fast compared to the response rate of the STM imaging apparatus, there will still be residual effects due to the JT interaction. The most apparent effect is that the circular orbits traced out by the nuclei are imaged by STM in both the ground and excited states. It is interesting that in the image shown in Fig. 21c, the ground state electron density appears to be preferentially localised... 

We have tried to distinguish between static and dynamic JT effects. However, the difference between these two regimes is really the time-scale with which the molecule is observed. Data capture in STM is undoubtedly slow and this must be seen as one drawback of this method of study. For a JT-active molecule, there is usually a set of distorted configurations that are isoenergetic (or, perhaps, nearly isoenergetic if the host surface has a weak effect on them) and interconversion between them is to be expected. The interconversion rate is expected to be rapid on the STM time-scale and so its effect on the recorded STM image needs to be addressed. 

Finally, it has to be recalled that internal strains or crystal packing forces have to be present, in order to stabilize one of several equivalent minima so that the system is seen as statically distorted. It has been stated that even small low-symmetry perturbations can be amplified by vibronic coupling A mechanism of this kind may be more effective in lifting electronic degeneracies than low-symmetry ligand field components. The important point is whether JT contributions are greater or comparable with the low-symmetry ones if it is so, JT terms must be included in to the Hamiltonian and have to be considered before or together with the static low-symmetry crystal field. 

To measure the Seebeck coefficient a, heat was applied to the sample which was placed between the two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature difference (JT <2 E) between the both ends of the sample. The Seebeck coefficient a of the compound was determined from the E/JT. The electrical resistivity p of the compound was measured by the four-probe technique. The repeat measurement was made rapidly with a duration smaller than one second to prevent errors due to the Peltier effect [3]. The thermal conductivity k was measured by the static comparative method [3] using a transparent Si02 ( k =1.36 W/Km at room temperature) as a standard sample in 5x10 torr. 

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