Orthorhombic lattice distortion

In region II the resistive behavior of x = 0.4 and 0.5 show a small fraction of the 72K material and a new transition near 29K. Similarly the X-ray diffraction patterns show the major phases present have a much smaller orthorhombic lattice distortion. Small amounts of La2-x ax u 4-x/2+5 could be responsible for the transition near 30K, but this phase was not observed in the diffraction patterns. The susceptibility data indicate that these samples may be largely bi- or triphasic depending on the amount of non-superconducting oxide present. 

The nonstoichiometric monohydrides formed by V, Nb and Ta have structures that are determined by T and the H content . Order-disorder transitions involving the H atoms lead to structural complexity in the ordered phases the bcc metal lattice distorts to tetragonal, orthorhombic or monoclinic. In addition, V and Nb, but not Ta, at >0.1 MPa Hj form dihydrides that have the fluorite structure. Hence, the phase diagrams of these elements with H are complex. 

Detailed crystal structures of only a few of the other heavy metal pseudohalides are available in the literature. Among them are, cuprous azide which has a relatively simple tetragonal lattice and cupric azide, mercuric fulminate and a-lead azide which have increasingly complex orthorhombic lattices. a-Lead azide has four types of anion sites of varying amounts of as5nnmetry (33) while cupric azide (35) and mercuric fulminate (72) have two such sites. The structure of cupric azide which is built up of distorted octahedra of asymmetric Ns ions about the central cupric ion is analogous to that of a transition metal complex. 

These Fermi resonance phenomena have found detailed mathematical treatment first tackled by Snyder et al. [138, 139] for single aM-trans chains and later reformulated by Abbate et al. for (i) single chains, (ii) an orthorhombic lattice [140], (iii) a -trans, and (iv) conformationally distorted chains [141, 142]. The most interesting features to be accounted for are the Raman scattering of polymethylene chains in the frequency regions centered near 1450 and 2900 cm . ... 

We need now to translate the above theoretical results into practice. First, let us start from an orthorhombic lattice with two molecules per unit cell and lower the symmetry to a supermolecular organization in which the cell can be considered as isolated and decoupled from its neighbors. This situation can be achieved in the following case (i) isotopic solid solutions (ii) molecule as a guest in a chemically different host lattice (e.g., urea clathrates) [143] (iii) liquid crystalline-like systems and (iv) systems with conformationally distorted chain ends, but with an ordered bulk [103]. The following changes of the Raman spectrum are expected (and observed) ... 

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