Two-dimensional compounds

Compounds that belong to the two-dimensional category undergo polarization reversal due to atomic displacement in a plane that contains a polar axis. The displacement can be imagined as the rotation of atomic groups around an axis that is perpendicular to a reflection plane. Typical examples of two-dimensional compounds include BaMF4 type compounds, where M = Mg, Mn, Fe, Co, Ni, Zn. 

Much evidence of this phenomenon has been presented [12-17], It is due to a real surface reaction, leading to the formation of a two-dimensional compound with well defined physico-chemical characteristics. The most conspicuous ones are electronic effects [18-24] a shift of about 0.5 eV towards higher binding energies, of the Mo 3d3 2 and 3d5 2 levels, has been reported. 

Two-dimensional compounds such as the ET salts. Here there is often significant dimerization along the chains, but metallic behavior is maintained by strong interchain interactions (e.g., sulfur-sulfur interactions) in one perpendicular direction, giving rise to two-dimensional conducting sheets. 

One characteristic feature of the behavior of Xs(T) for organic metals is illustrated in Fig. 4. In contrast to ordinary metals, Xs(X) increases quite substantially with temperature from (say) 60 to 300 K. This increase is strongest for the most one-dimensional compound, TTF-TCNQ [53], and becomes progressively weaker for (TMTSF)2C104 [54], (3-(BEDT-TTF)2I3 (a genuine two-dimensional compound) [25,26], and the more three-dimensional compound (TSeT)2Cl [18] (also, unpublished results of M. Mil-jak and B. Hilti). For HMTSF-TCNQ [33] such a discussion is complicated by the presence of Landau-Peierls diamagnetism from small pockets of electrons and holes, although estimates of Xs(T) have been made by Soda... 

The observation of the ladder-like chain motif in one-dimensional compound 3, two-dimensional compounds 10-11 and three-dimensional open framework compounds 8-9 demonstrate that the Cu3(hedp)2 anionic chain may serve as a useful building block in constructing a number of novel copper phosphonates with layer or open framework structures. 

We mentioned earlier (see Section V,C,1) that for the two-dimensional compounds of formula (catliMnCrCoxlg], with cat+ standing for a monovalent cation, each metal site of a given chirality (A or A) is surrounded by three metal sites of the opposite chirality (A or A), so that within a layer all the Mn(II) sites have the same chirality and all the Cr(III) sites have the other chirality. If the Mn(II) and Cr(III) sites had the same chirality, the hexagons of the honeycomb structure could no longer be closed, and the structure would be three- instead of two-dimensional. This structure as a whole would obviously be chiral 86). 

Layered dichalcogenides are typically two-dimensional compounds. Their structure is built up with [XMX] slabs (X = S, Se, Te) structural units. Within these units are strong ionocovalent or metallic bonds, whereas they are separated by rather large distances (generally of the order of the radii of closest approach) in agreement with weak interslab bondings. 

Although the construction of Fig. 4a gives the two-dimensional compound parabolic concentrator, or CPC, rotating the profile about the axis of symmetry gives the three-dimensional CPC with diameter A at the entrance and A2 at the exit. The two-dimensional CPC is an ideal concentrator, that is, it works perfectly for all rays within the acceptance angle Oq. The three-dimensional CPC is very close to ideal. The flat absorber case is a natural candidate for rotating about the axis because the ratio of diameters (sin 0) agrees with the ratio of maximum skew. Other absorber shapes, such as circular cross-sections (Fig. 4d) (cylinders in two-dimensional, spheres in three-dimensional), do not have this correspondence because the area of the sphere is ttA, whereas the entrance aperture area is 7tA /A. 

In this review we have made an attempt to compile and discuss the various layer-type compounds in relation to the Periodic System. As is demonstrated by the existence of two- and three-dimensional modifications for the same substance, e.g. A1(0H)3 and TiBrs, it may be tedious to find necessary and sufficient conditions which separate the fields of existence of the various types. In any case this investigation does not provide safe criteria to predict the occurrence of two-dimensional compounds. 

Electron-diffraction studies revealed that these phenomena are coupled with the occurrence of superstructures which are caused by charge-density waves. Theoretical work suggests that these two-dimensional compounds are likely to be susceptible to Fermi-surface-driven instabilities. The two-dimensional character of the Fermi surface implies planar surfaces normal to the layers. Large parallel sections of Fermi surface, spanned by a vector qo, lead, in real-space potential, to an oscillatory component of wavelength 1/qo- This must introduce a periodic... 

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