Distorted sites

We must note that recently Ellison Warrens (1987), using Al NMR spectroscopy, have found evidence for the existence of aluminium in pentacoordination in asymmetric or distorted sites using previously established assignments (Kirkpatrick et al. 1986 Risbud et ai, 1987 Cruikshank et al, 1986). 

When the dislocation density corresponds to about 10 cm the number of atoms in the highly modified core region of dislocations will be about 10 cm . These atoms are present in crowded, rarefied or distorted sites. The average extra energy possessed by atoms present in such sites can be as high as 20-40 kJ mol These high energies... 

The crystal structure of this protein has been shown to depend on the salt solution from which it is crystallized.45 When crystallized from (NH4)2S04, the insulin hexamer is held together in part by two Zn2+ ions. These can be visualized as being at either end of a cylinder, and each Zn2 has as a ligand one histidine imidazole nitrogen atom from one of three chains. Thus a histidine (his B.10) of each of the six chains is coordinated to Zn2+, three at each Zn2+. The Zn2+ ions occupy octahedral (trigonally distorted) sites overall. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

Figure 2.9 Relative energies of 3d orbital energy levels of a transition metal ion in low-symmetry distorted sites, (a) Regular octahedron (e.g., periclase) (b) trigonally distorted octahedron (e.g., corundum, spinel, approx, olivine M2 site) (c) tetragonally distorted octahedron (e.g., approx, olivine Ml site) (d) highly distorted six-coordinated sited (e.g., pyroxene M2 site) (e) regular cube (/) distorted cube (e.g., triangular dodecahedral site of garnet).

Electronic entropy. As a result of unequal electron occupancies of degenerate t2g orbitals, the Ti3+ and Fe2+ ions in octahedral or tetrahedral sites, for example, may have large electronic entropies compared to zero values for Cr3. Electronic entropies decrease at elevated temperatures and are smaller when cations are located in distorted sites. 

Since Fe2+ ions are concentrated in the acentric M4 sites of cummingtonite, the intense band around 10,000 cm-1 in the p spectrum (fig. 5.19a) arises from absorption by Fe2+ ions in this very distorted site. The increased intensity and width of absorption bands in the a and y spectra of grunerite and the broadening of the intense band in the P spectra result from increased occupancies of Fe2+ ions in the more regular Ml, M2 and M3 octahedral sites. 

The values of Fe2+ CFSE s that are derived throughout this chapter by assuming baricentric splittings of t2g and eg orbitals in low-symmetry or distorted sites are listed in table 5.16 (see also fig. 7.6). For minerals hosted by Al3+ ions, the CFSE values are seen to decrease in the order... 

At elevated temperatures, the entropy difference between regular and distorted sites is reduced due to thermal population of higher energy orbitals by the sixth 3d electron of Fe2+. The probabilities, P0 and P, of this sixth electron occupying the lowest and next-lowest orbitals separated by an energy 8, at temperature Tis given by the Maxwell-Boltzmann equation,... 

As the temperature rises, the probability of the sixth electron occupying orbital energy levels above the lowest level increases, and so too does the Sel of Fe2+ located in a distorted site, eq. (2.27). However, when Fe2+ is present in a regular octahedron, its sixth 3d electron remains delocalized over the three equivalent orbitals so that Sel remains constant at 9.13 J/(deg. g.ion). Therefore,... 

Mn2+ ions occupy either a distribution of octahedral and tetrahedral sites or distorted non-centrosymmetric sites. The fraction of non-centrosymmetric, tetrahedral or distorted sites increases with decreasing number of nonbridging oxygens so that tetrahedral [Mn04] clusters occur in orthosilicate glasses ... 

The EPR spectrum of the blue copper protein plastocyanin (Figure 3C) has gu > g > 2.00, and thus the copper site must have a dx2 y2 ground state. First, we are interested in determining the orientation of the dx2 y2 orbital relative to the distorted tetrahedral geometry observed in the protein crystal structure. Single crystal EPR spectroscopy allowed us to obtain this orientation and located the unique (i.e., z) direction in this distorted site (29). Plastocyanin crystallizes in an orthorhombic space group with four symmetry related molecules in the unit cell. The orientation of the plastocyanin copper sites in the unit cell are shown in... 

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). 

The interstices occupied by hydrogen in the hydrides of the AB compounds are those in which the coordination is predominantly to hydride-forming elements. In ZrNiHj and ZrCoHj " the H atoms are in two sites, T sites coordinated by three Zr and one Ni atom, and five-coordinated sites of four Zr and one Ni. In the monohydride of TiFe, the coordination is distorted 0, to four Ti and two Ni. However, in all three cases, the H atoms are 9-25% closer to the non-hydride-forming element (Ni or Co) than to the hydride-forming element (Zr or Ti). In the dihydride of FeTi there are four types of distorted sites in three the coordination is similar to that in the monohydride. However, in the fourth site, the H atoms are coordinated to four Ni and two Ti. The last type of site is difficult to fill (because of weak H—Fe bonding) and explains why stoichiometric TiFeHj cannot be attained. In the monohydride of ZrNi, the H atoms are T j coordinated to four Zr atoms . 

The variety of emission lines in the gahnite-like glass-ceramics and the possibility to ascribe these lines to various Cr " centers can be a useful tool in following the mechanisms of the nucleation of glass-ceramics doped by Cr(III). The high quantum efficiency of Cr(III) emission in glass-ceramics and the proximity of T2 and levels in the distorted sites may have importance in designing tunable lasers. 

Figure 31. XANES and EXAFS analysis of the surface structure in 1.9-nm nanoparticles of T102. Top XANES analysis focussing on the pre-edge region. Intensification of the features in this region are consistent with distorted or reduced Ti coordination. Bottom EXAFS analysis shows reduced peak area and coordination in the same samples with XANES intensification. Both analyses point to 5-coordinated Ti on particle surfaces. Capping the particles with ascorbic acid removes the distorted sites. After Chen et al. (1999).

Even smaller but still measurable changes (typically 2 ppm) in the Si chemical shifts result from the presence of crystallographic distortions in Si environments which are otherwise similar, for example, the Q (3A1) site in the mineral natrolite (- 87.7 ppm) which beomes — 86.3 and — 89.1 in the distorted sites of scolecite (Table 4.1). Thus, the Si chemical shifts can provide information about a considerable range of perturbations in the Si environment. 

Figure 8.38. A selection of Ba MAS NMR spectra of inorganic compounds, illustrating Ba in highly symmetric sites (as in BaO, BaZrOa and BaTi03> and in more distorted sites, some of which show sufficient second-order quadrupolar lineshape to be simulated (as in BaAl204 and BaCOs). From MacKenzie and Meinhold (2(XX)), by permission of the copyright owner.

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