Monoclinic structures

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

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]. 

The usual form of sulfur is known as the rhombic form, which is stable at temperatures up to about 105 °C. Above that temperature, the monoclinic structure is stable. A plastic form of sulfur can be obtained by pouring liquid sulfur into water to cool it quickly, but on standing it is converted to the rhombic form. On a molecular level, the element exists as S8 rings that have the structure... 

Diphenyl-l,3,4-oxadiazole is polymorphic (centrosymmetric monoclinic structure with space group P21/c and monoclinic non-centrosymmetric structure with space group P21/c). DSC investigations showed an irreversible transition from the first to the second form at 97 °C <2003JST219>. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

II.A.1.1. X-Ray Diffraction. The X-ray patterns of silicalite-1 and TS-1 demonstrate a change from the monoclinic structure of the former to orthorhombic when Ti4+ is introduced into the silicalite framework (5). The Rietveld analysis of Millini et al. (75) demonstrates a linear dependence of the lattice parameters and unit cell volume on the extent of Ti substitution in silicalite-1 and constitutes confirmatory evidence for the location of Ti in framework positions. Millini and Perego (77) concluded that the upper limit for incorporation of Ti in the TS-1 framework is about 2.5%. 

Experiments at high pressure have shown that the P-T phase diagram of butadiene is comparatively simple. The crystal phase I is separated from the liquid phase by an orientationally disordered phase II stable in a narrow range of pressure and temperature. The strucmre of phase I is not known, but the analyses of the infrared and Raman spectra have suggested a monoclinic structure with two molecules per unit cell as the most likely [428]. At room temperature, butadiene is stable in the liquid phase at pressures up to 0.7 GPa. At this pressure a reaction starts as revealed by the growth of new infrared bands (see the upper panel of Fig. 25). After several days a product is recovered, and the infrared spectrum identifies it as 4-vinylcyclohexene. No traces of the other dimers can be detected, and only traces of a polymer are present. If we increase the pressure to 1 GPa, the dimerization rate increases but the amount of polymer... 

As a specific example, Li can be intercalated into several oxides with the rutile structure (Murphy, DiSalvo, Caricles and Waszczak, 1978). In Li,jMo02, the structure changes from monoclinic to octahedral and back to monoclinic as x goes from 0 to 1 (Dahn and McKinnon, 1985). In both monoclinic structures. Mo atoms are shifted to form Mo-Mo pairs along the chains (Cox, Cava, McWhan and Murphy, 1982), and these pairs disappear in the octahedral structure at intermediate x (Dahn and McKinnon, 1985). The rutile LijtW02 has been considered as a replacement for metallic Li in electrochemical cells (Murphy et al, 1978). 

Figure 5,54 (A) Cationic occupancies in tetrahedral positions in case of complete disorder (monoclinic structure upper drawing) and complete order (triclinic structure lower drawing). (B) Condition of complete order in microcline and low albite with AliSi = 1 3, compared with cationic ordering in anorthite (AhSi = 2 2). Note doubling of edge c in an-orthite.

Colorless transparent crystal or white powder monoclinic structure density 2.17 g/cm3 decomposes above 100°C soluble in water, 22.49 g/lOOml at 20°C, 60 g/lOOmI at 60°C pH of O.IM aqueous solution 8.2 practically insoluble in alcohol. 

Colorless crystals or white powder monoclinic structure density 3.90 g/cm3 stable at ordinary temperatures melts at 560°C with partial decomposition, releasing oxygen moderately soluble in cold water 4.74 g/lOOmL at 0°C greater solubility in boiling water 32.3 g/lOOmL at 100°C soluble in potassium iodide solution insoluble in alcohol and liquid ammonia... 

The monohydrate is a white crystalline solid monoclinic structure density 2.13 g/cm3 loses its water at about 160°C converts to carbonate when ignited effloresces in warm dry air soluble in water, 33 g/100 mL at 20°C a 0.05m solution of K2C2O4 2H2O has a pH 1.679. 

Colorless crystals or white powder monoclinic structure very hygroscopic melts at 837°C decomposes above 900°C very soluble in water... 

Beta-sulfur is pale-yellow, opaque needle-like crystals monoclinic structure brittle stable between 94.5 to 120°C converts to orthorhombic form on standing density 1.96 g/cm melts at 115.2°C. 

Datolite is caldiun borosilicate with a monoclinic structure (2/m) and space group P2i/c. It consists of superimposed complex sheets of linked oxygen and O, OH tetrahedra around silicon and boron atoms respectively the Si04 and B(0,0H)4 tetrahedra alternate, forming rings of four and eight tetrahedra. The natural datolite in our study consisted of seven samples from a variety of geologic environments. Concentrations of potential luminescence impurities in one sample are presented in Table 4.8. 

In Fig. 2, it is seen also that, when increasing the pressure on the metal, the two enthalpies meet at p s 140 kbar (another estimate gives p = 100 kbars). At that pressure p a transition from the full spin-polarized to a no spin, itinerant phase is predicted to take place. Figure 2 shows that at pJxp = 100 kbar, a transition from ccp to a monoclinic structure takes place, the relative change in volume being however much smaller than that predicted by the theory. 

The monoclinic structure encountered in Am at high pressures is a low symmetry crystallographic form this is a clear indication that the 5 f orbitals have become bonding in the solid. The formation in compression of lower symmetry crystallographic modifications occurs also in the heavier actinides, as we may see from Figs. 1 and 2. 

AS2S3 was deposited at room temperature (27°C) from an acidic (pH 2) thioacetamide bath containing AS2O3 dissolved in concentrated HCl (and in some cases complexed with EDTA) [16]. The terminal thickness (which reached a maximum and then decreased with time) was studied as a function of various deposition parameters. Well-defined XRD peaks were obtained showing the monoclinic structure (notable since this compound has a tendency to be amorphous or nearly so as deposited). A direct bandgap of 2.42 eV (similar to the standard value for AS2S3) was estimated from the optical spectrum. The resistivity was ca. 10 H-cm. 

The two most likely stackings of these layers would appear to be FICP and FCC as discussed in Section 11.2.1.2. Unfortunately the two-layer FICP stacking does not preserve the crystallographic three-fold axis and necessarily leads to an orthorhombic or monoclinic structure. The three-layer FCC stacking permits the space group to be R3 which gives site symmetries of 3 to Cd and... 

Fig. 12.6. Bond graph of CaCrFs (a) tetragonal archetype, (b) observed monoclinic structure.

---