Site distortion

In some cases, the CFSE attained by a transition metal ion in a regular octahedral site may be enhanced if the coordination polyhedron is distorted. This effect is potentially very important in most silicate minerals since their crystal structures typically contain six-coordinated sites that are distorted from octahedral symmetry. Such distortions are partly responsible for the ranges of metal-oxygen distances alluded to earlier, eq. (6.6). Note, however, that the displacement of a cation from the centre of a regular octahedron, such as the comparatively undistorted orthopyroxene Ml coordination polyhedron (fig. 5.16), also causes inequalities of metal-oxygen distances. 

In low-symmetry, distorted coordination sites, the 3d orbitals of a transition metal ion are resolved into additional energy levels. As a result, the energy of 

The estimated CFSE s of Fe2+ ions in different sites of individual ferromag-nesian silicates show that Fe2+ ions, although receiving higher CFSE s in the pyroxene Ml site and the amphibole M1+M2+M3 sites, are strongly enriched in the very distorted pyroxene M2 and amphibole M4 sites. Other criteria discussed later ( 6.8.4) have had to be invoked to explain these apparent anomalies. 

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Any deviations from ideal behavior may be attributable to a high site distortion, represented in linear terms by parameter A ... 

The electric quadrupole interaction is generated when an electric field gradient acts on the nucleus.The quadrupole interaction or splitting, AEq, provides information about the site distortion, e. g. by specifically bound ligands it increases with increasing distortion. 

Dinuclear clusters ferrous site distortion, 38 175 spin ladder, 38 182-183 Dinuclear cobalt complex, 45 291-293 Dinuclear complexes osmium, electrochemistry, 37 321-323 quadruply bridged, 40 187-235 axial ligand substitution properties, 40 232-234... 

Armbruster, T., Rothlisberger, F., and Seifert, F. (1990). Layer topology, stacking variation, and site distortion in melilite-related compounds in the system Ca0-Zn0-Ge02-Si02. Amer. Miner. 75, 847-58. 

This is equivalent to the initial Hamiltonian in equation (1) when the extrema are considered, but depends on active one-centre nuclear displacements only. It does not mean, however, that only these local distortions are different from zero in extrema points. Some non-active intra-site distortions, will be non-zero as well. Indeed, using for them the same Van Vleck expansion, with the coefficients a Jkj), and the equilibrim values of phonon coordinates (6), we obtain... 

In attempts to verify the crystal chemistry of Fe3+ ions in these minerals, other correlations have been sought between the two Mossbauer spectral parameters, I.S. or Q.S., and crystal structure parameters based on bond distances and site distortion properties (2,5-9). These correlations are now critically examined. 

Figure 5.5 The crystal structure of garnet (pyrope) projected into a dodecahedron. Note the configuration of the eight-fold coordinated Mg sites (distorted cube) which share edges with [A106] octahedra and [Si04] tetrahedra.

The three criteria for predicting cation ordering, based on ionic radii, crystal field stabilization energy and site distortion, while being useful for qualitatively explaining intersite distribution patterns of transition metals and related elements, have been critically evaluated (e.g., Brown, 1982). These assessments have highlighted additional criteria that need to be taken into consideration in more general interpretations of cation order-disorder behaviour in minerals. These criteria are based on ionicity-covalency relationships. 

Fig. 44. Octahedral-site distortion resulting from spin-orbit ordering of a single ha electron into I>3 orbitals. Ordering of a single hg hole into I>3 orbitals would cause a distortion of opposite sign.

Fig. 53. Tetrahedral-site distortions due to ordering of a single electron. (Ordering of a single hole gives distortions of opposite sign.) (a) Jahn-Teller ordering into a drv orbital, (b) Spin-orbit ordering into the (dyz db idtx) orbitals.

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 lattice site distortion of water molecules is further shown by the relative intensities of Vi and V3 of H2O (D2O) in the Raman spectra. In the case of not (or only little) distorted H2O molecules, the Raman intensities of the antisymmetric stretching modes V3 are very low ... 

Salje E, Devarajan V, Bismayer U, Guimaraes DMC (1983) Phase transitions in Pb3(Pi xAS c04)2 Influence of the central peak and flip mode on the Raman scattering of hard modes. J Phys C 16 5233-5243 Salje E, Kuscholke B, Wrack B, Kroll H (1985) Thermodynamics of sodium feldspar 11 Experimental results and numerical calculations. Phys Chem Minerals 12 99-107 Salje E, Wrack B (1983) Specific-heat measurements and critical exponents of the ferroelastic phase transition in Pb3(P04)2 and Pb3(Pi.j,AS c04)2. Phys Rev B28 6510-6518 Schofield PF, Chamock JM, Cressey G, Henderson CMB (1994) An EXAFS study of cation site distortions through the E2/c-ET phase transition in the synthetic cuproscheelite-sanmartinite solid solution. Mineral Mag 58 185-199... 

Interpretation. The Mossbauer spectra obtained for the IC structure in Fe-akermanite comprise two and only two subspectra over the temperature range down to 4.2 K. In terms of the theoretical model (McConnell 1999), they may be identified with the two symmetries P4 and P2i2i2. The former involves a simple rotation of the tetrahedral site with respect to the Si207 dimers and hence no change to the coordination envirornment, while the latter involves an orthorhombic site distortion. The quadrupole doublets observed in the IC structure spectrum may be characterised as follows ... 

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