Phase structural: spin-Peierls

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. 

At some critical temperature, a structural distortion to dimerized chains occurs together with the opening of a gap in the magnetic excitations. Such a distortion is attributable to a 2kF spin-Peierls transition, just as in MEM(TCNQ)2 (cf. Section III.A.7). A 2kF transition results into chain tetramerization for p = 5 and into chain dimerization for p = 1, as also shown in Fig. 5. For instance, the spin-Peierls transition takes place at T sp = 395 K in KTCNQ and at TsP = 381 K in RbTCNQ [67] (a second form, called RbTCNQ II, also exists for the Rb salt [69], but it is not considered here). By analogy with the case of polymers, the dimerized phase at low temperature is also called a bond-ordered-wave (BOW) phase [47]. 

Mixed-chain charge-transfer complexes are characterized by a regular chain structure when in the quasi-neutral phase at high temperature. Often, but not always, the chains undergo dimerization in the low-temperature quasi-ionic phase, at Tc or below, as a possible consequence of one-dimensional spin-Peierls-like transition [74,76]. 

---