Tetragonal distortions of the

The tetragonal distortion of the ccp structure also leads to the bcc structure. 

The 220/204 reflections and the 312/116 reflections were split, which is consistent with the tetragonal distortion of the crystal lattice49 (Fig. 6.15). Lattice parameters a and c were calculated from X-ray d spacings according to Eq. 6.5,... 

This structure can be described as a tetragonal distortion of the AuCu3-type structure. It may also be considered a variant of the previously described AuCu( I) type (compare with its tP4 pseudo-cell). 

The striking features of the structure (when compared with that of a normal spinel, Fig. 29) are (a) that the Li(2)04 tetrahedra are very tilted, so that the anion array is grossly distorted from the approximate cubic eutaxy observed in the normal spinel structure and (b) that, by contrast, the cation array is almost exactly that of the normal structure, and very regular indeed. [There is a small tetragonal distortion of the unit cell -symmetry P4i22-which has c/( /2 a) = 0.9697, compared with unity for the equivalent ratio of the cubic cell of the normal spinel structure.]... 

The vibrational progression in the Se4+ and Te4+ emission bands indicates clearly the Jahn-Teller effect in the excited state the coupling with the eg vibrational mode points to a tetragonal distortion of the excited state, According to Schmidtke et al. [18], the magnitude of this tetragonal distortion is Az= — 2Ax= — 2Ay=0.lA, a considerable distortion indeed. 

We discuss here two examples of vibronic effects in polynuclear highly symmetrical transition metal clusters. The existence of degenerate and quasi-degenerate molecular orbitals in their energy spectra results in the Jahn-Teller effect or in the vibronic mixing of different electronic states. We show that both quantum-chemical methods and model approaches can provide valuable information about these vibronic effects. In the case of the hexanuclear rhenium tri-anion, the Jahn-Teller effect is responsible for the experimentally observed tetragonal distortion of the cluster. The vibronic model of mixed-valence compounds allows to explain the nature of a transient in the photo-catalytic reaction of the decatungstate cluster. 

It seems natural to suppose that the tetragonal distortion of the tri-anion results from the Jahn-Teller effect. In order to study the problem more thoroughly we undertook recently the DFT calculations of this cluster as well as of several other hexanuclear rhenium chalcohalide clusters. The technical details of these calculations can be found in the original publication [8]. Here we only want to note that the introduction of relativistic corrections for Re atoms is crucial for the correct reproduction of the geometry of clusters. In our calculations, this was done by the zero order regular approximation (ZORA) Hamiltonian [9] within ADF 2000.02 package [10]. 

A ternary compound of cerium with copper and antimony of the stoichiometric ratio 3 3 4 was identified and studied by means of X-ray analysis by Skolozdra et al. (1993). Ce3Cu3Sb4 compound was found to have the Y3Au3Sb4 type with the lattice parameters of a = 0.9721 (X-ray powder diffraction). For experimental details, see the Y-Cu-Sb system. At variance with this data, Patil et al. (1996) reported a tetragonal distortion of the cubic crystal structure Y3Cu3Sb4 for the Ce3Cu3Sb4 alloy which was prepared by arc melting the constituent ele-... 

Figure 6-41. Tetragonal distortion of the regular octahedral arrangement around a d9 ion.

The low spin configuration was earlier found in the deep-coloured, tris chelated amino-tropon-iminates [Mn(LN)3] (for LN = 43). Black [Mn(LN)3], prepared from Mn2(CO)10 and LmH, had a magnetic moment of 3.12 BM whereas the brown-black [Mn(acac)2(LN)J and black [Mn-(acac)(LN)2] both were high spin species with / eff = 4.9BM (in CHC13).141 An X-ray structural analysis of the latter showed a tetragonal distortion of the [MnN204] octahedron.580... 

In 5-TiNo,5o, the random distribution of nitrogen atoms undergoes an ordering process below about 880 °C with a concurrent tetragonal distortion of the metal lattice. The so-formed S -TiNo.s phase is only formed from 5-TiNi by lattice distortion and is not an equilibrium phase (e g. it is not formed in isothermal diffusion couples in the form of a phase band). 

Fig. 9. (a) Qualitative MO diagram for octahedral TeYe , indicating vibronic coupling of ground state and first excited state (arrow, see text) (b) Splitting of the ttu state with tetragonal distortion of the octahedron (17),... 

Owing to the tetragonal distortion of the cupric ions (7), this ion pair must be considered an anisotropic complex as defined by J0rgensen 22)— i.e., an intermediate between an inner- and outer-sphere complex. 

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