Disordered Fluoride Phases

In addition, the following nonstoichiometric phases, usually solid solutions (of variabie composition), have also been obtained, and are also disordered  

2 X-Ray Powder Diffraction, Mossbauer Spectroscopy, Lone Pair Stereoactivity, and Tin Coordination All the phases listed above to be disordered, regardless of whether they are stoichiometric or 

X-ray powder diffraction pattern of the precipitate obtained versus the molar fraction n of Ca(N03)2(aq) added to a solution of SnF2 (a) CaSn2Fe, (b) Cai xSnxF2 (x 0.30), and (c) a mixture of CaSn2F6 and Cai xSnxF2. 

X-ray powder diffraction pattern of a-PbSnF4(aqi) subjected to ball-milling versus ball-milling time (a) not milled, (b) 10s, (c) 20s, (d) 50s, and (e) 60s. 

Repetition of a stereoactive lone pair by the cubic symmetry of disordered fluorite-type systems (a) [MFs] cube in fluorite-type MF2, (b) [SnEFy] polyhedron when the tin lone pair pushes one fluoride ion away, and (c) the mBmIOh local symmetry of the tin site generates a total of eight lone pairs. 

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Disordered Chloride Fluoride Phases 11.7.2.1 Samples Prepared by Precipitation... 

Carbon tetrafluoride. Carbon tetra-fluoride, which undergoes a transition to a plastically crystalline (orientationally disordered) phase, has been investigated by the Parrinello-Rahman molecular dynamics method under constant-pressure conditions (6). A simple intermolecular potential model of the Lennard-Jones form was derived by taking into account the experimen-... 

The discussion of the behavior of eondis crystals of flexibte linear macromolecules and some of the homolc ous oUgcaners covers a witte range of cdisorder-effects and accompanying motion. For many of the macromolecules in the eondis phase chain-extension afto- erystallization with chain folding is possible [polyethylene, polytetrafluOToethylene, poly(vinylidene fluoride), polydilorotrifluoro-ethylene, some aliphatic nylons and polyphosphazenes]. Stx h extended ehain crystals are exceptionally stable and elose to equlibrium. 

Several of the described eondis phases are CD glasses (Sect. 4.2-4 ami 4.6) with different paths into the glassy states. Some of these CD glasses seem to have no corresponding, stabte eondis phase, but are produced as intermediates in the path from melt to stabte erystal [polypropylene]. The ferroelectric effect of poly(vinylidene fluoride), polytrifluOToethylene and its copolymers seems coimected to eondis glass and disorder transitions (Curie temperatures). Condis states seem also to have enhanced chain mobility on mechanical (teformation. 

There are, however, indications for a phase transition in formic acid around 208 K jl10,111. Which kind of structural change actu ally occurs, a transition to structure IV, an order-disorder transition involving structures I and IV, or a significant change in interchain orientation or -distances is still an open question lll-113. An ab initio calculation of vibrational spectra of formic acid chains is currently out of reach. We compiled therefore available Raman results on 0-H frequencies in isolated formic acid, in the cyclic dimer and in the crystal in table 11 in order to enable a comparison with the previously discussed cases of hydrogen fluoride and hydrogen cyanide. 

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