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Polyhedral Distortion (polyhedral + distortion)
Selected AbstractsOn the crystal chemistry of olivine-type germanate compounds, Ca1,+,xM1,,,xGeO4 (M2+ = Ca, Mg, Co, Mn)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2008Günther J. Redhammer Germanate compounds, CaMGeO4 with M2+ = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298,K and adopt the olivine structure. The M2 site is exclusively occupied by Ca2+, while on M1 both Ca2+ and M2+ cations are found. The amount of Ca2+ on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1,atoms per formula unit) and the largest in the Mn compound (0.20,atoms per formula unit), while in Ca2GeO4, also with olivine structure, both sites are completely filled with Ca2+. When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b has essentially the same values, regardless of the tetrahedral cation, meaning that b is independent of the tetrahedral cation. Structural variations on the octahedrally coordinated M1 site are largely determined by the size of the M1 cation, the average M1,O bond lengths being identical in Ca silicate and Ca germanate olivine. Increasing the size of the M1 cation induces an increasing polyhedral distortion, expressed by the parameters bond-length distortion, octahedral angle variance and octahedral quadratic elongation. However, the Ca germanate olivine compounds generally have more regular octahedra than the analogous silicates. The octahedrally coordinated M2 site does not exhibit large variations in structural parameters as a consequence of the constant chemical composition; the same is valid for the tetrahedral site. [source] Synthetic aenigmatite analog Na2(Mn5.26Na0.74)Ge6O20: structure and crystal chemical considerationsACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2008Günther J. Redhammer Disodium hexamanganese(II,III) germanate is the first aenigmatite-type compound with significant amounts of manganese. Na2(Mn5.26Na0.74)Ge6O20 is triclinic and contains two different Na positions, six Ge positions and 20 O positions (all with site symmetry 1 on general position 2i of space group P). Five out of the seven M positions are also on general position 2i, while the remaining two have site symmetry (Wyckoff positions 1f and 1c). The structure can be described in terms of two different layers, A and B, stacked along the [011] direction. Layer A contains pyroxene-like chains and isolated octahedra, while layer B is built up by slabs of edge-sharing octahedra connected to one another by bands of Na polyhedra. The GeO4 tetrahedra show slight polyhedral distortion and are among the most regular found so far in germanate compounds. The M sites of layer A are occupied by highly charged (trivalent) cations, while in layer B a central pyroxene-like zigzag chain can be identified, which contains divalent (or low-charged) cations. This applies to the aenigmatite-type compounds in general and to the title compound in particular. [source] Review of polyhedral distortions as a multi-scale minimization of the electric polarization and their correlations with physical propertiesACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2006Abderrahim Benabbas The Jahn,Teller (JT) and non-Jahn,Teller polyhedral distortions are reviewed within the same context, based on a multi-scale minimization of the electric polarization by handling formal ionic valences and valence electron density. This model is applied to tetragonal distortions of octahedra, particularly in K2NiF4 structures with different formula types, along with doping. The predictions are always in good agreement with the observed data. In particular, the ferrodistortive order of JT distortions is obtained from formal charge polarizations, while the antiferrodistortive one is adopted when only the valence electron density is involved. The correlations between physical properties and octahedral elongations through the crystal structures on one side and chemical compositions on the other side are discussed according to this model for high- Tc cuprate superconductors and CMR manganites. [source] | ||||||||||