Structural Distortions and Phase Transitions

E. Kaldis, Oxygen nonstoichiometry and lattice effects in YBa2Cu30x. Phase transitions, structural distortions and phase separation 1... 

Phase Transitions, Structural Distortions and Phase Separation... 

Figure 9 (Top) schematic of bistability in 1,3,2-dithiazolyl radicals arising from a solid-solid phase transition between regular and Peierls distorted n-stacks (bottom) free energy diagram of the two structural phases present...

Peierls showed 74 [41,42] that an instability in a one-dimensional chain, with one electron per site, driven by electron-phonon interactions, can lead to a subtle structural distortion and to a first-order Peierls phase transition, at and below a finite temperature TP (the Peierls temperature) [42], For instance, at and below Tp either a dimerization into two sets of unequal interparticle distances d and d" (such that d + d" = 2d) or some other structural distortion must occur. The electronic energy of the metallic chain may also be lowered by the formation of a charge-density wave (CDW) of amplitude p(x) ... 

DTA studies in the temperature range 400 to 2400°C indicate two phase transitions at 1100 and 1700°C in the nonstoichiometric range. These transitions show a large scattering of the DTA peaks ( 50°C) and no structure of these transitions could be determined, Fritzler, Kaldis [2], Fritzler et al. [3], also see Kaldis et al. [4]. Energy-dispersive X-ray diffraction studies on stoichiometric TmSe at room temperature and applied pressures up to 29 GPa (290 kbar) show a continuous valence change of Tm from the intermediate valence state (v 2.7) to a nearly trivalent state at 5 GPa. A distinct structural phase transition is not observed. The valence transition is slowed down or briefly interrupted at --2.5 GPa. A small lattice distortion apparently occurs at 10 GPa, Debray etal. [5], see also Debray etal. [6]. 

The room temperature transformation of the columbite phase to baddeleyite commences at 13-17 GPa 6, with transition pressure increasing linearly with temperature Direct transition from rutile to baddeleyite phase at room temperature and 12 GPa has also been reported 7. The baddeleyite phase undergoes further transition to an as yet undefined high-symmetry structure at 70-80 GPa. The most likely candidate for the high-pressure phase is fluorite, which is consistent with the general pattern of increasing Ti coordination number from 6 in rutile, to 7 in baddeleyite (a distorted fluorite structure), and to 8 in fluorite. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

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