Anion ordering

In addition to long-range ordering of the intralayer cations, there also exists the possibility of anion ordering within the interlayer galleries. This may be 

The structure of a green rust LDH-type material with the formula [Fe 4Fe 2(OH)i2]S04-ca. 8H2O has been determined by Rietveld analysis [157]. The material exists as a one-layer polytype with the interlayers containing two planes of sulfate and water molecules giving a basal spacing of 1.1011 nm. The sulfate ions are oriented with their C3 axes perpendicular to the layers and alternate anions point up and down (as shown earlier in Fig. 14) and form a superlattice with parameter a = UoV5 = 0.5524 nm (Fig. 39). 

2116 nm) respectively. Reprinted with permission from [305]. Copyright Academic Press 

It was assumed that the sulfate oxygen atoms pointing towards the layers are oriented towards Fe cations. This does not necessarily imply long range ordering of the cations, however, because only half of the Fe sites are interacting with sulfate anions in this way and the other half of the sites could 

The effect of ordering of layer cations and/or ordering of interlayer carbonate anions on the crystal chemistry of LDHs has been analyzed [277]. The authors reported the single crystal structure of a mineral with the 

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Table 2.2-1 shows a list of typical cations and anions ordered by their rough price on an industrial scale. 

Table 4.2. Examples ofBFS salts containing non-centrosymmetric anions exhibiting anion ordering transitions...

Note that PFe is centrosymmetric. If the anions were non-centrosymmetric, an additional degree of freedom would be available in the case that such anions order. In fact BFS with non-centrosymmetric anions exhibit aifion ordering below a given temperature, which can be classified as an order-disorder transition. Above the transition temperature Tao anions exhibit random orientations, but for sufficiently low temperatures (low activation energy) they become ordered. 

Examples of these two behaviours are provided by (TMTSF)2Re04 and (TMTSF)2C104. In (TMTSF)2Re04 the anions order at c. 180 K with a reduced wave vector (, and undergo a sharp metal-insulator transition with Ea = 0.16 eV (Jacobsen et al, 1982). The anion ordering transition of (TMTSF)2C104 occurs at 24 K with = (0, 0). This qr value implies that the... 

CIO4 tetrahedra have the same orientation in a given ac-plane while the orientation alternates in the fc-direction. Since the period along a is preserved, no gap opening is expected at T o, as experimentally verihed (Gubser et al, 1982). This is the reason why (TMTSF)2C104 becomes a superconductor in spite of the anion ordering transition. 

The phase transition consists of a cooperative mechanism with charge-ordering, anion order-disorder, Peierls-like lattice distortion, which induces a doubled lattice periodicity giving rise to 2 p nesting, and molecular deformation (Fig. 11c). The high temperature metallic phase is composed of flat EDO molecules with +0.5 charge, while the low temperature insulating phase is composed of both flat monocations... 

Phase transitions. Low-dimensional conductors undergo several types of specific structural phase transitions, such as the Peierls distortion (electron-phonon coupling), the spin-Peierls distortion (spin-phonon coupling), anion-ordering transitions, and so on. These first have to be detected and then measured and understood. However, the foregoing distortions may be very small and difficult to observe, and up to now, only a few lattice distortions have been fully measured and described. 

The situation for (TMTSF)2N03 is somewhat different since the triangular N03 anion adopts a (2, 0, 0) periodicity below TAO = 45 K [74]. This new lattice symmetry implies a folding of the band structure about planes kx = Tt/2a. Consequently, the anion ordering triggers a transition between a quasi-one-dimensional conductor at high temperature and a two-... 

Figure 18 Fermi surface of (TMTSF)2N03 (schematic), in the quasi-one-dimen-sional regime (a) and below the anion ordering transition (b).

The effect of anion ordering on the stability of FISDW phases has been confirmed quantitatively by calculation of the spin susceptibility following the standard approach and including an additional periodic potential with wave vector Q = (0, 5, 0) [135]. After this numerical computation even phases (N = 0, 2) are suppressed, whereas odd phases N = 1,3 are not. The same model also explains the normal-phase reentrance above 17 T. However, the predicted oscillation for Tc versus the magnetic field in the N = 0 phase is still lacking in the experimental data. 

We now turn to a discussion of anion ordering phenomena in organic conductors in order to better understand the structural transitions often associated with them. 

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