Monoclinic structure and

Christ et al. (1969) found that X-ray diffraction powder data for palygorskite samples show both orthorhombic and monoclinic structures and suggested that the variations in symmetry reflect variations in chemical composition. The present data suggest the most likely difference is octahedral Fe. 

Titanium trisulfide [12423-80-2], TiS3, a black crystalline solid having a monoclinic structure and a theoretical density of 3230 kg/m3, can be prepared by reaction between titanium tetrachloride vapor and H2S at 480—540°C. The reaction product is then mixed with sulfur and heated to 600°C in a sealed tube to remove residual chlorine. Sublimation may be used to separate the trisulfide (390°C) from the disulfide (500°C). Titanium trisulfide, insoluble in hydrochloric acid but soluble in both hot and cold sulfuric acid, reacts with concentrated nitric acid to form titanium dioxide. 

Factor group analysis [58] indicates that 429 IR active modes and 432 Raman active modes are expected for the monoclinic structure and 323 IR active modes and 432 Raman active modes are expected for the orthorhombic form. Only 16 components in the IR specrum and 11 components in the Raman spectrum are observed because of the superimposition of many of the expected fundamentals. However, a careful observation of the spectra, with the help of analysis of the perturbations arising from isomorphic substitution and with the aid of derivative spectra, showed the presence of a great number of very weak components (shoulders) in the spectra. 

Structure (i) was originally assigned to all three compounds, but as the result of later studies the following structures have been proposed CoAsS, (ii), NiAsS, (iii), and NiSbS, (i). It would seem that the greatest reliance may be placed on the later work on CoAsS, which shows that the apparently cubic structure is a polysynthetic twin of a monoclinic structure, and it is not impossible that other structures in this family are in fact superstructures of lower symmetry. As a result of the difference between Ni-Sb, 2-57 A, and Ni-S, 2-34 A, the symmetry of NiSbS has dropped to the enantiomorphic crystal class 23 the absolute structure has been determined. ... 

The experimental evidence from diffraction indicates that solid TeFe has two solid phases, a low-temperature phase with an orientationally-ordered, base-centered monoclinic structure and C h symmetry, and a higher-temperature phase with a body-centered cubic structure and cubic Oh symmetry. The experiments with clusters of TeFs, done by electron diffraction, show both of these and at temperatures between the regions of stability of these phases, a monoclinic phase with only partial orientational order as well. Simulations of the clusters show that the transition from body-centered cubic to monoclinic involves both rotational and translational motion of the motion, while the lower-temperature transition, between monoclinic and base-centered monoclinic, requires only coupling of the molecular rotational motions.Furthermore the transition at higher temperature in clusters shows dynamic coexistence of two phases in equilibrium over a range of at least a few degrees of temperature, which immediately implies that this transition involves two local minima in the... 

Nadella et al. [204] found that cold-drawing of polypropylene caused voids and a decrease of crystallite size and greatly enhanced lattice strains and defects. Annealing the fibers at 140°C restored a well-formed monoclinic structure and healed the voids. 

Fig. 13.33 A sketch of the model showing the organization of the crystalline lamellae in very thin films of Nylon-6 forming the discoids of the TEM micrographs of Fig. 13.31 (a) an end-on view of lamellae radiating out from a common center, showing the principal directions of the monoclinic structure and (b) the arrangement of crystalline order in a lamella, relating it to the monoclinic structure (after Muratoglu et al. (1995b) courtesy of Elsevier).

Sheet structure may be present in a disorder model (namely a monoclinic model with sheet-like distortions), including sheets running parallel to the (1 iO) but also sheets running parallel to the (110) planes (144). Computer simulation shows that for PIPD fibers, two possible crystal structures with hydrogen bonding networks may coexist a sheet-like network for the triclinic structure and a two-dimensional network for the monoclinic structure, and there are negligible energy differences between them (142). 

Aluminium hydroxide occurs in a number of modifications which differ by the mode of superposing the brucite-like layers. Bayerite, a-Al(OH)3, is the stable modification of pure Al(OH)3. Hydrargillite, y-Al(OH)3, always contains some alkali ions. It occurs with a monoclinic structure and a triclinic superstructure (gibbsite). 

CrS has a unique monoclinic structure intermediate between NiAs and PtS types. 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

The only known compounds that contain trivalent metals are heptaluoroniobates (TV) of lantanides - LnNbF7. Such compounds crystallize in monoclinic syngony, and have an LnZrF7 type structure. 

In the case of lithium orthoniobate, Li3Nb04, no meta-stable phase was found that had a rock-salt crystal structure with disordered cation distribution [268]. Nevertheless, solid solutions Li2+xTii-4xNb3x03, where 0 < x < 0.22, have a monoclinic structure at low temperatures and undergo transformation to a disordered NaCl type structure at high temperatures [274]. 

For example, the difference between the monoclinic CaC2 phases is the presence of only one type of acetylide ion in phase II and two distinct C2 species in phase III, as concluded from their crystal structures and from C-NMR studies (Fig. 8.3) [8]. The transformation between phases II and III is induced by heating phase II above 150°C until the metastable phase. III, is formed. Phase III remains stable even when being cooled down to room temperature. However, when the metastable phase III is ground in a mortar at room temperature, it transforms back into phase II. 

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