Tetragonally distorted octahedral sites

Figure 1.24 Energy splitting of antibonding MOs in tetragonally distorted octahedral sites (left) elongation along axis z (right) compression along axis z.

Table 2.6. Stabilization energies of transition metal ions in tetragonally distorted octahedral sites in oxides... 

Figure 3.13 Crystal field states and electronic configurations of Fe2+ ions in regular octahedral and tetragonally distorted octahedral sites. The tetragonally distorted octahedron is elongated along the tetrad axis.

Analogous expressions apply to crystal field states for electronic configurations involving these orbitals (cf. fig. 3.13). These energy separations are shown in fig. 3.21. All three parameters, Dq(eq)y Dt and Ds, may be determined experimentally provided that energies of three absorption bands originating from transition metal ions in tetragonally distorted octahedral sites occur in the crystal field spectra. Equation (3.15) yields the Dq( a) parameter, so that the mean Dq parameter, Dq(m) may be calculated from... 

The interaction of cupric ions with alumina supports has subsequently been studied more extensively as a function of the support surface area, metal loading, and calcination temperature (93,279) by means of EXAFS and X-ray absorption-edge shifts, in conjunction with XRD, EPR, XPS, and optical reflectance spectroscopy. These techniques, each sensitive to certain structural and electronic aspects, allow a unified picture of the phases present and the cation site location. Four Cu2 + ion sites are distinguished in the catalysts. In low concentrations (typically below about 4 wt. % Cu/100 m2/g support surface area) Cu2 + ions enter the defect spinel lattice of the A1203 support. The well-dispersed surface copper aluminate has Cu2+ ions predominantly occupying tetragonally (Jahn-Teller) distorted octahedral sites, although... 

Rutile has a tetragonal lattice in which Ti ions occur coordinated to ions in slightly distorted octahedral sites, one-half of the octahedral sites being empty. The most stable crystal face appears to be the (110), and in Fig. 8.16 is shown the (110) surface that results from breaking the smallest number of cation-anion bonds (after Henrich, 1983, 1987). Two kinds of Ti cation are present on this surface, one with five ligands and one with all six ions, as in the bulk material. The local environment of the five-coordinated cation is similar to that of the surface cation in MgO. 

Fig. 2. Structures for the solid (a) fee Cco, (b) fee MCco, (c) fee M2C60 (d) fee MsCeo, (e) hypothetical bee Ceo, (0 bet M4C60, and two structures for MeCeo (g) bee MeCeo for (M= K, Rb, Cs), and (h) fee MeCeo which is appropriate for M = Na, using the notation of Ref [42]. The notation fee, bee, and bet refer, respectively, to face centered cubic, body centered cubic, and body centered tetragonal structures. The large spheres denote Ceo molecules and the small spheres denote alkali metal ions. For fee M3C60, which has four Ceo molecules per cubic unit cell, the M atoms can either be on octahedral or tetrahedral symmetry sites. Undoped solid Ceo also exhibits the fee crystal structure, but in this case all tetrahedral and octahedral sites are unoccupied. For (g) bcc MeCeo all the M atoms are on distorted tetrahedral sites. For (f) bet M4Ceo, the dopant is also found on distorted tetrahedral sites. For (c) pertaining to small alkali metal ions such as Na, only the tetrahedral sites are occupied. For (h) we see that four Na ions can occupy an octahedral site of this fee lattice.

Although Fe "and Mn " have similar ionic radii, Mn " does not fit as readily into the goethite structure as does Fe " because owing to its four d electrons, Mn " has a tetragonally distorted coordination sphere on an octahedral site Jahn-Tdler effect). 

A converse situation exists whereby the two oxygen ions along the z axis may move closer to the Mn3+ ion (fig. 2.8 >). This results in the stabilization of the dx2 y2 orbital relative to the dz2 orbital, and shorter Mn-O distances along the z axis compared to the x-y plane. In either of the tetragonally distorted environments shown in fig. 2.8 the Mn3+ ion becomes more stable relative to a regular octahedral coordination site. In most minerals, however, the Mn3+ ion occurs in an axially elongated octahedron (see table 6.1). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

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